The Southern Ocean is a major sink of atmospheric CO2, but the nature and magnitude of its variability remains uncertain and debated. Estimates based on observations suggest substantial variability that is not reproduced by process-based ocean models, with increasingly divergent estimates over the past decade. We examine potential constraints on the nature and magnitude of climate-driven variability of the Southern Ocean CO2 sink from observation-based air-sea O2 fluxes. On interannual time scales, the variability in the air-sea fluxes of CO2 and O2 estimated from observations is consistent across the two species and positively correlated with the variability simulated by ocean models. Our analysis suggests that variations in ocean ventilation related to the Southern Annular Mode are responsible for this interannual variability. On decadal time scales, the existence of significant variability in the air-sea CO2 flux estimated from observations also tends to be supported by observation-based estimates of O2 flux variability. However, the large decadal variability in air-sea CO2 flux is absent from ocean models. Our analysis suggests that issues in representing the balance between the thermal and non-thermal components of the CO2 sink and/or insufficient variability in mode water formation might contribute to the lack of decadal variability in the current generation of ocean models. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and
their redistribution among the atmosphere, ocean, and terrestrial biosphere
in a changing climate is critical to better understand the global carbon
cycle, support the development of climate policies, and project future
climate change. Here we describe and synthesize datasets and methodology to
quantify the five major components of the global carbon budget and their
uncertainties. Fossil CO2 emissions (EFOS) are based on energy
statistics and cement production data, while emissions from land-use change
(ELUC), mainly deforestation, are based on land use and land-use change
data and bookkeeping models. Atmospheric CO2 concentration is measured
directly, and its growth rate (GATM) is computed from the annual
changes in concentration. The ocean CO2 sink (SOCEAN) is estimated
with global ocean biogeochemistry models and observation-based
data products. The terrestrial CO2 sink (SLAND) is estimated with
dynamic global vegetation models. The resulting carbon budget imbalance
(BIM), the difference between the estimated total emissions and the
estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a
measure of imperfect data and understanding of the contemporary carbon
cycle. All uncertainties are reported as ±1σ. For the first
time, an approach is shown to reconcile the difference in our ELUC
estimate with the one from national greenhouse gas inventories, supporting
the assessment of collective countries' climate progress. For the year 2020, EFOS declined by 5.4 % relative to 2019, with
fossil emissions at 9.5 ± 0.5 GtC yr−1 (9.3 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and ELUC was 0.9 ± 0.7 GtC yr−1, for a total anthropogenic CO2 emission of
10.2 ± 0.8 GtC yr−1 (37.4 ± 2.9 GtCO2). Also, for
2020, GATM was 5.0 ± 0.2 GtC yr−1 (2.4 ± 0.1 ppm yr−1), SOCEAN was 3.0 ± 0.4 GtC yr−1, and SLAND
was 2.9 ± 1 GtC yr−1, with a BIM of −0.8 GtC yr−1. The
global atmospheric CO2 concentration averaged over 2020 reached 412.45 ± 0.1 ppm. Preliminary data for 2021 suggest a rebound in EFOS
relative to 2020 of +4.8 % (4.2 % to 5.4 %) globally. Overall, the mean and trend in the components of the global carbon budget
are consistently estimated over the period 1959–2020, but discrepancies of
up to 1 GtC yr−1 persist for the representation of annual to
semi-decadal variability in CO2 fluxes. Comparison of estimates from
multiple approaches and observations shows (1) a persistent large
uncertainty in the estimate of land-use changes emissions, (2) a low
agreement between the different methods on the magnitude of the land
CO2 flux in the northern extra-tropics, and (3) a discrepancy between
the different methods on the strength of the ocean sink over the last
decade. This living data update documents changes in the methods and datasets used in this new global carbon budget and the progress in understanding
of the global carbon cycle compared with previous publications of this dataset (Friedlingstein et al. 2020, 2019; Le
Quéré et al. 2018b, a, 2016, 2015b, a, 2014, 2013). The
data presented in this work are available at https://doi.org/10.18160/gcp-2021 (Friedlingstein et al. 2021).
.

AbstractThe Southern Ocean is an important sink of anthropogenic CO2, but it is among the least well-observed ocean basins, and consequentially substantial uncertainties in the CO2 flux reconstruction exist. A recent attempt to address historically sparse wintertime sampling produced ‘pseudo’ wintertime observations of surface pCO2 using subsurface summertime observations south of the Antarctic Polar Front. Here, we present an estimate of the Southern Ocean CO2 sink that combines a machine learning-based mapping method with an updated set of pseudo observations that increases regional wintertime data coverage by 68% compared with the historical dataset. Our results confirm the suggestion that improved winter coverage has a modest impact on the reconstruction, slightly strengthening the uptake trend in the 2000s. After also adjusting for surface boundary layer temperature effects, we find a 2004-2018 mean sink of −0.16 ± 0.07 PgC yr−1 south of the Polar Front and −1.27 ± 0.23 PgC yr−1 south of 35°S, consistent with independent estimates from atmospheric data.

Throughout Earth’s history, the abundance of oxygen in our atmosphere has varied, but by how much remains debated. Previously, an upper limit for atmospheric oxygen has been bounded by assumptions made regarding the fire window: atmospheric oxygen concentrations higher than 30–40% would threaten the regeneration of forests in the present world. Here we have tested these assumptions by adapting a Dynamic Global Vegetation Model to run over high atmospheric oxygen concentrations. Our results show that whilst global tree cover is significantly reduced under high O2 concentrations, forests persist in the wettest parts of the low and high latitudes and fire is more dependent on fuel moisture than O2 levels. This implies that the effect of fire on suppressing global vegetation under high O2 may be lower than previously assumed and questions our understanding of the mechanisms involved in regulating the abundance of oxygen in our atmosphere, with moisture as a potentially important factor.

Since 1750, land-use change and fossil fuel combustion has led to a 46% increase in the atmospheric carbon dioxide (CO2) concentrations, causing global warming with substantial societal consequences. The Paris Agreement aims to limit global temperature increases to well below 2C above preindustrial levels. Increasing levels of CO2 and other greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O), in the atmosphere are the primary cause of climate change. Approximately half of the carbon emissions to the atmosphere are sequestered by ocean and land sinks, leading to ocean acidification but also slowing the rate of global warming. However, there are significant uncertainties in the future global warming scenarios due to uncertainties in the size, nature, and stability of these sinks. Quantifying and monitoring the size and timing of natural sinks and the impact of climate change on ecosystems are important information to guide policy-makers' decisions and strategies on reductions in emissions. Continuous, long-term observations are required to quantify GHG emissions, sinks, and their impacts on Earth systems. The Integrated Carbon Observation System (ICOS) was designed as the European in situ observation and information system to support science and society in their efforts to mitigate climate change. It provides standardized and open data currently from over 140 measurement stations across 12 European countries. The stations observe GHG concentrations in the atmosphere and carbon and GHG fluxes between the atmosphere, land surface, and the oceans. This article describes how ICOS fulfills its mission to harmonize these observations, ensure the related long-term financial commitments, provide easy access to well-documented and reproducible high-quality data and related protocols and tools for scientific studies, and deliver information and GHG-related products to stakeholders in society and policy.

AbstractThe response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to ‘warm’, ‘present-day’ and ‘cold’ states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively-driven upwelling sustain the AMOC in the warmer (present-day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

The source of oxygen to Earth’s atmosphere is organic carbon burial, whilst the main sink is oxidative weathering of fossil carbon. However, this sink is to insensitive to counteract oxygen rising above its current level of about 21%. Biogeochemical models suggest that wildfires provide an additional regulatory feedback mechanism. However, none have considered how the evolution of different plant groups through time have interacted with this feedback. The Cretaceous Period saw not only super-ambient levels of atmospheric oxygen but also the evolution of the angiosperms, that then rose to dominate Earth’s ecosystems. Here we show, using the COPSE biogeochemical model, that angiosperm-driven alteration of fire feedbacks likely lowered atmospheric oxygen levels from ~30% to 25% by the end of the Cretaceous. This likely set the stage for the emergence of closed-canopy angiosperm tropical rainforests that we suggest would not have been possible without angiosperm enhancement of fire feedbacks.

We present new estimates of the regional North Atlantic (15-80g ¯N) CO2 flux for the 2000-2017 period using atmospheric CO2 measurements from the NOAA long-term surface site network in combination with an atmospheric carbon cycle data assimilation system (GEOS-Chem-LETKF, Local Ensemble Transform Kalman Filter). We assess the sensitivity of flux estimates to alternative ocean CO2 prior flux distributions and to the specification of uncertainties associated with ocean fluxes. We present a new scheme to characterize uncertainty in ocean prior fluxes, derived from a set of eight surface pCO2-based ocean flux products, and which reflects uncertainties associated with measurement density and pCO2-interpolation methods. This scheme provides improved model performance in comparison to fixed prior uncertainty schemes, based on metrics of model-observation differences at the network of surface sites. Long-term average posterior flux estimates for the 2000-2017 period from our GEOS-Chem-LETKF analyses are -0.255¯±¯0.037¯PgC¯yr-1 for the subtropical basin (15-50g ¯N) and -0.203¯±¯0.037¯PgC¯yr-1 for the subpolar region (50-80g ¯N, eastern boundary at 20g ¯E). Our basin-scale estimates of interannual variability (IAV) are 0.036¯±¯0.006 and 0.034¯±¯0.009¯PgC¯yr-1 for subtropical and subpolar regions, respectively. We find statistically significant trends in carbon uptake for the subtropical and subpolar North Atlantic of -0.064¯±¯0.007 and -0.063¯±¯0.008¯PgC¯yr-1¯decade-1; these trends are of comparable magnitude to estimates from surface ocean pCO2-based flux products, but they are larger, by a factor of 3-4, than trends estimated from global ocean biogeochemistry models.

The Southern Ocean plays an important role in the global oceanic uptake of CO2. Estimates of the air-sea CO2 flux are made using the partial pressure of CO2 at the sea surface ((Formula presented.)), but winter observations of the region historically have been sparse, with almost no coverage in the Pacific or Indian ocean sectors south of the Polar front in the period 2004–2017. Here, we use summertime observations of relevant properties in this region to identify subsurface waters that were last in contact with the atmosphere in the preceding winter, and then reconstruct “pseudo observations” of the wintertime (Formula presented.). These greatly improve wintertime coverage south of the Polar Front in all sectors, improving the robustness of flux estimates there. We add the pseudo observations to other available observations of (Formula presented.) and use a multiple linear regression to produce a gap-filled time-evolving estimate of (Formula presented.) from which we calculate the air-sea flux. The inclusion of the pseudo observations increases outgassing at the beginning of the period, but the effect reduces with time. We estimate a 2004–2017 long-term mean flux of −0.02 ± 0.02 Pg C yr−1 for the Southern Ocean south of the Polar Front, similar to comparable studies based on shipboard (Formula presented.) data. However, we diverge somewhat from an estimate which utilized autonomous float data for recent years: we find a small sink in 2017 of −0.08 ± 0.03 Pg C yr−1 where the float-based estimate suggested a small source.

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and
their redistribution among the atmosphere, ocean, and terrestrial biosphere
in a changing climate – the “global carbon budget” – is important to
better understand the global carbon cycle, support the development of
climate policies, and project future climate change. Here we describe and
synthesize data sets and methodology to quantify the five major components
of the global carbon budget and their uncertainties. Fossil CO2
emissions (EFOS) are based on energy statistics and cement production
data, while emissions from land-use change (ELUC), mainly
deforestation, are based on land use and land-use change data and
bookkeeping models. Atmospheric CO2 concentration is measured directly
and its growth rate (GATM) is computed from the annual changes in
concentration. The ocean CO2 sink (SOCEAN) and terrestrial
CO2 sink (SLAND) are estimated with global process models
constrained by observations. The resulting carbon budget imbalance
(BIM), the difference between the estimated total emissions and the
estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a
measure of imperfect data and understanding of the contemporary carbon
cycle. All uncertainties are reported as ±1σ. For the last
decade available (2010–2019), EFOS was 9.6 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.4 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and
ELUC was 1.6 ± 0.7 GtC yr−1. For the same decade, GATM was 5.1 ± 0.02 GtC yr−1 (2.4 ± 0.01 ppm yr−1), SOCEAN 2.5 ±  0.6 GtC yr−1, and SLAND 3.4 ± 0.9 GtC yr−1, with a budget
imbalance BIM of −0.1 GtC yr−1 indicating a near balance between
estimated sources and sinks over the last decade. For the year 2019 alone, the
growth in EFOS was only about 0.1 % with fossil emissions increasing
to 9.9 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.7 ± 0.5 GtC yr−1 when cement carbonation sink is included), and ELUC was 1.8 ± 0.7 GtC yr−1, for total anthropogenic CO2 emissions of 11.5 ± 0.9 GtC yr−1 (42.2 ± 3.3 GtCO2). Also for 2019, GATM was
5.4 ± 0.2 GtC yr−1 (2.5 ± 0.1 ppm yr−1), SOCEAN
was 2.6 ± 0.6 GtC yr−1, and SLAND was 3.1 ± 1.2 GtC yr−1, with a BIM of 0.3 GtC. The global atmospheric CO2
concentration reached 409.85 ± 0.1 ppm averaged over 2019. Preliminary
data for 2020, accounting for the COVID-19-induced changes in emissions,
suggest a decrease in EFOS relative to 2019 of about −7 % (median
estimate) based on individual estimates from four studies of −6 %, −7 %,
−7 % (−3 % to −11 %), and −13 %. Overall, the mean and trend in the
components of the global carbon budget are consistently estimated over the
period 1959–2019, but discrepancies of up to 1 GtC yr−1 persist for the
representation of semi-decadal variability in CO2 fluxes. Comparison of
estimates from diverse approaches and observations shows (1) no consensus
in the mean and trend in land-use change emissions over the last decade, (2)
a persistent low agreement between the different methods on the magnitude of
the land CO2 flux in the northern extra-tropics, and (3) an apparent
discrepancy between the different methods for the ocean sink outside the
tropics, particularly in the Southern Ocean. This living data update
documents changes in the methods and data sets used in this new global
carbon budget and the progress in understanding of the global carbon cycle
compared with previous publications of this data set (Friedlingstein et al.
2019; Le Quéré et al. 2018b, a, 2016, 2015b, a, 2014,
2013). The data presented in this work are available at https://doi.org/10.18160/gcp-2020 (Friedlingstein et al. 2020).
.

AbstractThe ocean is a sink for ~25% of the atmospheric CO2 emitted by human activities, an amount in excess of 2 petagrams of carbon per year (PgC yr−1). Time-resolved estimates of global ocean-atmosphere CO2 flux provide an important constraint on the global carbon budget. However, previous estimates of this flux, derived from surface ocean CO2 concentrations, have not corrected the data for temperature gradients between the surface and sampling at a few meters depth, or for the effect of the cool ocean surface skin. Here we calculate a time history of ocean-atmosphere CO2 fluxes from 1992 to 2018, corrected for these effects. These increase the calculated net flux into the oceans by 0.8–0.9  PgC yr−1, at times doubling uncorrected values. We estimate uncertainties using multiple interpolation methods, finding convergent results for fluxes globally after 2000, or over the Northern Hemisphere throughout the period. Our corrections reconcile surface uptake with independent estimates of the increase in ocean CO2 inventory, and suggest most ocean models underestimate uptake.

AbstractThe contemporary air‐sea flux of CO2 is investigated by the use of an air‐sea flux equation, with particular attention to the uncertainties in global values and their origin with respect to that equation. In particular, uncertainties deriving from the transfer velocity and from sparse upper ocean sampling are investigated. Eight formulations of air‐sea gas transfer velocity are used to evaluate the combined standard uncertainty resulting from several sources of error. Depending on expert opinion, a standard uncertainty in transfer velocity of either ~5% or ~10% can be argued and that will contribute a proportional error in air‐sea flux. The limited sampling of upper ocean fCO2 is readily apparent in the Surface Ocean CO2 Atlas databases. The effect of sparse sampling on the calculated fluxes was investigated by a bootstrap method, that is, treating each ship cruise to an oceanic region as a random episode and creating 10 synthetic data sets by randomly selecting episodes with replacement. Convincing values of global net air‐sea flux can only be achieved using upper ocean data collected over several decades but referenced to a standard year. The global annual referenced values are robust to sparse sampling, but seasonal and regional values exhibit more sampling uncertainty. Additional uncertainties are related to thermal and haline effects and to aspects of air‐sea gas exchange not captured by standard models. An estimate of global net CO2 exchange referenced to 2010 of −3.0 ± 0.6 Pg C/year is proposed, where the uncertainty derives primarily from uncertainty in the transfer velocity.

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo's global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al. 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

AbstractThe North Atlantic Ocean is a region of intense uptake of atmospheric CO2. To assess how this CO2 sink has evolved over recent decades, various approaches have been used to estimate basin‐wide uptake from the irregularly sampled in situ CO2 observations. Until now, the lack of robust uncertainties associated with observation‐based gap‐filling methods required to produce these estimates has limited the capacity to validate climate model simulated surface ocean CO2 concentrations. After robustly quantifying basin‐wide and annually varying interpolation uncertainties using both observational and model data, we show that the North Atlantic surface ocean fugacity of CO2 (fCO2−ocean) increased at a significantly slower rate than that simulated by the latest generation of Earth System Models during the period 1992–2014. We further show, with initialized model simulations, that the inability of these models to capture the observed trend in surface fCO2−ocean is primarily due to biases in the models' ocean biogeochemistry. Our results imply that current projections may underestimate the contribution of the North Atlantic to mitigating increasing future atmospheric CO2 concentrations.

The ability to routinely quantify global carbon dioxide (CO2) absorption by the oceans has become crucial: it provides a powerful constraint for establishing global and regional carbon (C) budgets, and enables identification of the ecological impacts and risks of this uptake on the marine environment. Advances in understanding, technology, and international coordination have made it possible to measure CO2 absorption by the oceans to a greater degree of accuracy than is possible in terrestrial landscapes. These advances, combined with new satellite‐based Earth observation capabilities, increasing public availability of data, and cloud computing, provide important opportunities for addressing critical knowledge gaps. Furthermore, Earth observation in synergy with in‐situ monitoring can provide the large‐scale ocean monitoring that is necessary to support policies to protect ocean ecosystems at risk, and motivate societal shifts toward meeting C emissions targets; however, sustained effort will be needed.

In this work, we use realistic isopycnal velocities with a 3-D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 67°W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20-fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection-diffusion model with constant, but different, zonal velocities east and west of 67°W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation.

Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere-the "global carbon budget"-is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. CO2 emissions from fossil fuels and industry (EFF) are based on energy statistics and cement production data, respectively, while emissions from land-use change (ELUC), mainly deforestation, are based on land-cover change data and bookkeeping models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) and terrestrial CO2 sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1δ. For the last decade available (2007-2016), EFF was 9.4±0.5 GtC yr-1, ELUC 1.3±0.7 GtC yr-1, GATM 4.7±0.1 GtC yr-1, SOCEAN 2.4±0.5 GtC yr-1, and SLAND 3.0±0.8 GtC yr-1, with a budget imbalance BIM of 0.6 GtC yr-1 indicating overestimated emissions and/or underestimated sinks. For year 2016 alone, the growth in EFF was approximately zero and emissions remained at 9.9±0.5 GtC yr-1. Also for 2016, ELUC was 1.3±0.7 GtC yr-1, GATM was 6.1±0.2 GtC yr-1, SOCEAN was 2.6±0.5 GtC yr-1, and SLAND was 2.7±1.0 GtC yr-1, with a small BIM of-0.3 GtC. GATM continued to be higher in 2016 compared to the past decade (2007-2016), reflecting in part the high fossil emissions and the small SLAND consistent with El Ninõ conditions. The global atmospheric CO2 concentration reached 402.8±0.1 ppm averaged over 2016. For 2017, preliminary data for the first 6-9 months indicate a renewed growth in EFF of C2.0% (range of 0.8 to 3.0 %) based on national emissions projections for China, USA, and India, and projections of gross domestic product (GDP) corrected for recent changes in the carbon intensity of the economy for the rest of the world. This living data update documents changes in the methods and data sets used in this new global carbon budget compared with previous publications of this data set (Le Quéré et al. 2016, 2015b, a, 2014, 2013). All results presented here can be downloaded from https://doi.org/10.18160/GCP-2017 (GCP, 2017).

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere – the "global carbon budget" – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. CO2 emissions from fossil fuels and industry (EFF) are based on energy statistics and cement production data, respectively, while emissions from land-use change (ELUC), mainly deforestation, are based on land-cover change data and bookkeeping models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) and terrestrial CO2 sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of our imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the last decade available (2007–2016), EFF was 9.4 ± 0.5 GtC yr−1, ELUC 1.3 ± 0.7 GtC yr−1, GATM 4.7 ± 0.1 GtC yr−1, SOCEAN 2.4 ± 0.5 GtC yr−1, and SLAND 3.0 ± 0.8 GtC yr−1, with a budget imbalance BIM of 0.6 GtC yr−1 indicating overestimated emissions and/or underestimated sinks. For year 2016 alone, the growth in EFF was approximately zero and emissions remained at 9.9 ± 0.5 GtC yr−1. Also for 2016, ELUC was 1.3 ± 0.7 GtC yr−1, GATM was 6.1 ± 0.2 GtC yr−1, SOCEAN was 2.6 ± 0.5 GtC yr−1 and SLAND was 2.7 ± 1.0 GtC yr−1, with a small BIM of −0.3 GtC. GATM continued to be higher in 2016 compared to the past decade (2007–2016), reflecting in part the higher fossil emissions and smaller SLAND for that year consistent with El Niño conditions. The global atmospheric CO2 concentration reached 402.8 ± 0.1 ppm averaged over 2016. For 2017, preliminary data indicate a renewed growth in EFF of +2.0 % (range of 0.8 % to 3.0 %) based on national emissions projections for China, USA, and India, and projections of Gross Domestic Product corrected for recent changes in the carbon intensity of the economy for the rest of the world. For 2017, initial data indicate an increase in atmospheric CO2 concentration of around 5.3 GtC (2.5 ppm), attributed to a combination of increasing emissions and receding El Niño conditions. This living data update documents changes in the methods and data sets used in this new global carbon budget compared with previous publications of this data set (Le Quéré et al. 2016; 2015b; 2015a; 2014; 2013). All results presented here can be downloaded from https://doi.org/10.18160/GCP-2017.
.

Changes of ocean ventilation rates and deoxygenation are two of the less obvious but important indirect impacts expected as a result of climate change on the oceans. They are expected to occur because of (i) the effects of increased stratification on ocean circulation and hence its ventilation, due to reduced upwelling, deep-water formation and turbulent mixing, (ii) reduced oxygenation through decreased oxygen solubility at higher surface temperature, and (iii) the effects of warming on biological production, respiration and remineralization. The potential socio-economic consequences of reduced oxygen levels on fisheries and ecosystems may be far-reaching and significant. At a Royal Society Discussion Meeting convened to discuss these matters, 12 oral presentations and 23 posters were presented, covering a wide range of the physical, chemical and biological aspects of the issue. Overall, it appears that there are still considerable discrepancies between the observations and model simulations of the relevant processes. Our current understanding of both the causes and consequences of reduced oxygen in the ocean, and our ability to represent them in models are therefore inadequate, and the reasons for this remain unclear. It is too early to say whether or not the socio-economic consequences are likely to be serious. However, the consequences are ecologically, biogeochemically and climatically potentially very significant, and further research on these indirect impacts of climate change via reduced ventilation and oxygenation of the oceans should be accorded a high priority. This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

The Surface Ocean CO2 Atlas (SOCAT) is a synthesis of quality-controlled fCO2 (fugacity of carbon dioxide) values for the global surface oceans and coastal seas with regular updates. Version 3 of SOCAT has 14.7 million fCO2 values from 3646 data sets covering the years 1957 to 2014. This latest version has an additional 4.6 million fCO2 values relative to version 2 and extends the record from 2011 to 2014. Version 3 also significantly increases the data availability for 2005 to 2013. SOCAT has an average of approximately 1.2 million surface water fCO2 values per year for the years 2006 to 2012. Quality and documentation of the data has improved. A new feature is the data set quality control (QC) flag of E for data from alternative sensors and platforms. The accuracy of surface water fCO2 has been defined for all data set QC flags. Automated range checking has been carried out for all data sets during their upload into SOCAT. The upgrade of the interactive Data Set Viewer (previously known as the Cruise Data Viewer) allows better interrogation of the SOCAT data collection and rapid creation of high-quality figures for scientific presentations. Automated data upload has been launched for version 4 and will enable more frequent SOCAT releases in the future. Highprofile scientific applications of SOCAT include quantification of the ocean sink for atmospheric carbon dioxide and its long-term variation, detection of ocean acidification, as well as evaluation of coupled-climate and ocean-only biogeochemical models. Users of SOCAT data products are urged to acknowledge the contribution of data providers, as stated in the SOCAT Fair Data Use Statement. This ESSD (Earth System Science Data) "living data" publication documents the methods and data sets used for the assembly of this new version of the SOCAT data collection and compares these with those used for earlier versions of the data collection (Pfeil et al. 2013; Sabine et al. 2013; Bakker et al. 2014). Individual data set files, included in the synthesis product, can be downloaded here: doi:10.1594/PANGAEA.849770. The gridded products are available here: doi:10.3334/CDIAC/OTG.SOCAT-V3-GRID.

©2015. The Authors. The accumulation of carbon within the Weddell Gyre and its exchanges across the gyre boundaries are investigated with three recent full-depth oceanographic sections enclosing this climatically important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual-cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO 2. observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058±0.010PgCyr -1. derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air-to-sea CO 2. flux of 0.002±0.007PgCyr -1. (mean 1998-2011) to 0.012±0.024PgCyr -1. (mean 2008-2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements, reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033±0.021PgCyr -1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deepwater formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long time scales.

Atmospheric CO2 concentrations over glacial-interglacial cycles closely correspond to Antarctic temperature patterns. These are distinct from temperature variations in the mid to northern latitudes, so this suggests that the Southern Ocean is pivotal in controlling natural CO2 concentrations. Here we assess the sensitivity of atmospheric CO2 concentrations to glacial-interglacial changes in the ocean's meridional overturning circulation using a circulation model for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60ppm of atmospheric CO2. We therefore suggest that the direct effect of temperatures on Southern Ocean buoyancy forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO2 concentrations over glacial-interglacial cycles.

The variability in the storage of the oceanic anthropogenic CO2 (Cant) on decadal timescales is evaluated within the main water masses of the Subtropical North Atlantic along 24.5°N. Inorganic carbon measurements on five cruises of the A05 section are used to assess the changes in Cant between 1992 and 2011, using four methods (δC*, TrOCA, ϕCT0, TTD). We find good agreement between the Cant distribution and storage obtained using chlorofluorocarbons and CO2 measurements in both the vertical and horizontal scales. Cant distribution shows higher concentrations and greater decadal storage rates in the upper layers with both values decreasing with depth. The greatest enrichment is obserbed in the central water masses, with their upper limb showing a mean annual accumulation of about 1μmolkg-1yr-1 and the lower limb showing, on average, half that value. We detect zonal gradients in the accumulation of Cant. This finding is less clear in the upper waters, where greater variability exists between methods. In accordance with data from time series stations, greater accumulation of Cant is observed in the upper waters of the western basin of the North Atlantic Subtropical Gyre. In intermediate and deep layers, the zonal gradient in the storage of Cant is more robust between methods. The much lower mean storage rates found along the section (

The Surface Ocean CO2 Atlas (SOCAT), an activity of the international marine carbon research community, provides access to synthesis and gridded fCO2 (fugacity of carbon dioxide) products for the surface oceans. Version 2 of SOCAT is an update of the previous release (version 1) with more data (increased from 6.3 million to 10.1 million surface water fCO 2 values) and extended data coverage (from 1968-2007 to 1968-2011). The quality control criteria, while identical in both versions, have been applied more strictly in version 2 than in version 1. The SOCAT website (http://www.socat.info/) has links to quality control comments, metadata, individual data set files, and synthesis and gridded data products. Interactive online tools allow visitors to explore the richness of the data. Applications of SOCAT include process studies, quantification of the ocean carbon sink and its spatial, seasonal, year-to-year and longerterm variation, as well as initialisation or validation of ocean carbon models and coupled climate-carbon models. © Author(s) 2014. CC Attribution 3.0 License.

The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s-1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s-1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.

The Southern Ocean plays a pivotal role in the global ocean circulation and climate. There, the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange substantial quantities of heat and carbon with the atmosphere. The sensitivity of the upper cell to climatic changes in forcing is relatively well established. However, little is known about how the lower cell responds, and in particular whether small-scale mixing in the abyssal Southern Ocean, an important controlling process of the lower cell, is influenced by atmospheric forcing. Here, we present observational evidence that relates changes in abyssal mixing to oceanic eddy variability on timescales of months to decades. Observational estimates of mixing rates, obtained along a repeat hydrographic transect across Drake Passage, are shown to be dependent on local oceanic eddy energy, derived from moored current meter and altimetric measurements. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerly winds, our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing. © 2014 Macmillan Publishers Limited.

Water column dissolved inorganic carbon and total alkalinity were measured during five hydrographic sections in the Atlantic Ocean and Drake Passage. The work was funded through the Strategic Funding Initiative of the UK's Oceans2025 programme, which ran from 2007 to 2012. The aims of this programme were to establish the regional budgets of natural and anthropogenic carbon in the North Atlantic, the South Atlantic, and the Atlantic sector of the Southern Ocean, as well as the rates of change of these budgets. This paper describes in detail the dissolved inorganic carbon and total alkalinity data collected along east"west sections at 47° N to 60° N, 24.5° N, and 24° S in the Atlantic and across two Drake Passage sections. Other hydrographic and biogeochemical parameters were measured during these sections, and relevant standard operating procedures are mentioned here. Over 95% of dissolved inorganic carbon and total alkalinity samples taken during the 24.5° N, 24° S, and the Drake Passage sections were analysed onboard and subjected to a first-level quality control addressing technical and analytical issues. Samples taken along 47° N to 60° N were analysed and subjected to quality control back in the laboratory. Complete post-cruise second-level quality control was performed using crossover analysis with historical data in the vicinity of measurements, and data were submitted to the CLIVAR and Carbon Hydrographic Data Office (CCHDO), the Carbon Dioxide Information Analysis Center (CDIAC) and and will be included in the Global Ocean Data Analyses Project, version 2 (GLODAP 2), the upcoming update of Key et al. (2004). © Author(s) 2014.

A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered--contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500-500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.

A well-documented, publicly available, global data set of surface ocean carbon dioxide (CO2) parameters has been called for by international groups for nearly two decades. The Surface Ocean CO2 Atlas (SOCAT) project was initiated by the international marine carbon science community in 2007 with the aim of providing a comprehensive, publicly available, regularly updated, global data set of marine surface CO2, which had been subject to quality control (QC). Many additional CO2 data, not yet made public via the Carbon Dioxide Information Analysis Center (CDIAC), were retrieved from data originators, public websites and other data centres. All data were put in a uniform format following a strict protocol. Quality control was carried out according to clearly defined criteria. Regional specialists performed the quality control, using state-of-the-art web-based tools, specially developed for accomplishing this global team effort. SOCAT version 1.5 was made public in September 2011 and holds 6.3 million quality controlled surface CO2 data points from the global oceans and coastal seas, spanning four decades (1968–2007). Three types of data products are available: individual cruise files, a merged complete data set and gridded products. With the rapid expansion of marine CO2 data collection and the importance of quantifying net global oceanic CO2 uptake and its changes, sustained data synthesis and data access are priorities. © 2013 Author(s).

The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star's main sequence lifetime. We describe the time that a planet spends within the HZ as its habitable zone lifetime. The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the classic HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a hybrid HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79×109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets' HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns. © Copyright 2013, Mary Ann Liebert, Inc. 2013.

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters. Some of this deficit is made up by intense near-bottom mixing occurring in restricted 'hot-spots' associated with rough ocean-floor topography, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows. Here we show that diapycnal mixing of mid-depth (∼1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 10 6 m 3 s -1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography. © 2013 Macmillan Publishers Limited. All rights reserved.

The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from O(1×10-10Wkg -1) in the Southeast Pacific to O(1×10-9Wkg -1) in the Scotia Sea, typically reaching 3×10-9Wkg -1 within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottom-generated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m -2 in the Southeast Pacific and 14 mW m-2 in the Scotia Sea. Typically, 10%-30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructure-derived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites. Key Points Turbulence increases by an order of magnitude between SE Pacific & Scotia Sea Lee waves play a pivotal role in Southern Ocean diapycnal mixing and circulation 10-30 % of Southern Ocean predicted lee wave energy is dissipated locally ©2013. American Geophysical Union. All Rights Reserved.

As a response to public demand for a well-documented, quality controlled, publically available, global surface ocean carbon dioxide (CO2) data set, the international marine carbon science community developed the Surface Ocean CO2 Atlas (SOCAT). The first SOCAT product is a collection of 6.3 million quality controlled surface CO2 data from the global oceans and coastal seas, spanning four decades (1968-2007). The SOCAT gridded data presented here is the second data product to come from the SOCAT project. Recognizing that some groups may have trouble working with millions of measurements, the SOCAT gridded product was generated to provide a robust, regularly spaced CO2 fugacity (fCO2) product with minimal spatial and temporal interpolation, which should be easier to work with for many applications. Gridded SOCAT is rich with information that has not been fully explored yet (e.g. regional differences in the seasonal cycles), but also contains biases and limitations that the user needs to recognize and address (e.g. local influences on values in some coastal regions). © 2013 Author(s).

A section across the Atlantic at 24S recorded in March 2009, sampled a tracer plume released in the deep Brazil Basin 13 years earlier. The 1-D diffusion equation was used to model the vertical spread of the tracer, yielding a mean diapycnal diffusivity estimate of approximately 3 × 10 -4 m2/s at 4 km depth. This estimate is similar to that found by surveys of the tracer plume made between 1996 and 2000, within four years of the tracer release and therefore provides strong evidence for the long-term stability of that result. Copyright 2012 by the American Geophysical Union.

The influence of the island mass effect of South Georgia on the seasonal marine carbon cycle was investigated during austral summer (January-February) 2008. South Georgia (54-55°S 36-38°W) lies on the North Scotia Ridge, strongly influencing the passage of the Southern Antarctic Circumpolar Current Front to the south. Surface waters upstream of the island, in the central Scotia Sea, were characterised by relative high-nutrient low-chlorophyll (HNLC) conditions from winter (September) 2007 to summer, as indicated by satellite and shipboard observations. The fugacity of carbon dioxide (fCO 2) was slightly supersaturated and the HNLC waters represented a summertime CO 2 source of 2.6±1.5mmolm -2day -1. Extensive phytoplankton blooms developed in the Georgia Basin, downstream of South Georgia, in October 2007 and persisted until March 2008. The seasonal depletion in dissolved inorganic carbon (DIC) was 94±1μmolkg -1 and the ΔfCO 2(sea-air) was -92±21μatm in the core of the bloom by early February. These conditions created a strong sink for atmospheric CO 2 of -12.9±11.7mmolm -2day -1. In contrast, wintertime mixing into DIC-rich sub-surface waters created a strong CO 2 source of 22.0±14.4mmolm -2day -1. These processes drive substantial seasonal changes in DIC of up to -0.7μmolkg -1day -1 from winter to summer. Similarly to the blooms of Kerguelen and Crozet, the South Georgia bloom is likely to be fuelled by natural iron fertilisation. A DIC deficit of 2.2±0.3molm -2 upstream of South Georgia suggested that the relative HNLC waters were more productive than indicated by satellites. The DIC deficit more than doubled downstream of South Georgia (4.6±0.8molm -2) to create the strongest seasonal carbon uptake in ice-free waters of the Southern Ocean to date. © 2011 Elsevier Ltd.

The ultimate climate emergency is a 'runaway greenhouse': a hot and water-vapour-rich atmosphere limits the emission of thermal radiation to space, causing runaway warming. Warming ceases only after the surface reaches approximately 1400K and emits radiation in the near-infrared, where water is not a good greenhouse gas. This would evaporate the entire ocean and exterminate all planetary life. Venus experienced a runaway greenhouse in the past, and we expect that the Earth will in around 2 billion years as solar luminosity increases. But could we bring on such a catastrophe prematurely, by our current climatealtering activities? Here, we review what is known about the runaway greenhouse to answer this question, describing the various limits on outgoing radiation and how climate will evolve between these. The good news is that almost all lines of evidence lead us to believe that is unlikely to be possible, even in principle, to trigger full a runaway greenhouse by addition of non-condensible greenhouse gases such as carbon dioxide to the atmosphere. However, our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak. We cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one. High climate sensitivity might provide a warning. If we, or more likely our remote descendants, are threatened with a runaway greenhouse, then geoengineering to reflect sunlight might be life's only hope. Injecting reflective aerosols into the stratosphere would be too short-lived, and even sunshades in space might require excessive maintenance. In the distant future, modifying Earth's orbit might provide a sustainable solution. The runaway greenhouse also remains relevant in planetary sciences and astrobiology: as extrasolar planets smaller and nearer to their stars are detected, some will be in a runaway greenhouse state. © 2012 the Royal Society.

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) isconstricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10-8 W kg-1 at abyssal and middepths. The mixing enhancementin the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1×10-4 m2 s-1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smootherbathymetry. Outside of the frontal regions, dissipation rates are O(10-10) W kg-1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean. © 2012 American Meteorological Society.

The oceans are an important sink for anthropogenically produced CO(2), and on time scales longer than a century they will be the main repository for the CO(2) that humans are emitting. Our knowledge of how ocean uptake varies (regionally and temporally) and the processes that control it is currently observation-limited. Traditionally, and based on sparse observations and models at coarse resolution, ocean uptake has been thought to be relatively invariant. However, in the few places where we have enough observations to define the uptake over periods of many years or decades, it has been found to change substantially at basin scales, responding to indices of climate variability. We illustrate this for three well-studied regions: the equatorial Pacific, the Indian Ocean sector of the Southern Ocean, and the North Atlantic. A lesson to take from this is that ocean uptake is sensitive to climate (regionally, but presumably also globally). This reinforces the expectation that, as global climate changes in the future owing to human influences, ocean uptake of CO(2) will respond. To evaluate and give early warning of such carbon-climate feedbacks, it is important to track trends in both ocean and land sinks for CO(2). Recent coordinated observational programmes have shown that, by organization of an observing network, the atmosphere-ocean flux of CO(2) can, in principle, be accurately tracked at seasonal or better resolution, over at least the Northern Hemisphere oceans. This would provide a valuable constraint on both the ocean and (by difference) land vegetation sinks for atmospheric CO(2).

A connection is hypothesized between the physiological consequences of mutualistic symbiosis and life's average long-term impact on certain highly biologically conserved environmental variables. This hypothesis is developed analytically and with a variant of the Daisyworld model. Biological homeostasis is frequently effective due to co-ordination between opposing physiological "rein" functions, which buffer an organism in response to an external (often environmental) perturbation. It is proposed that during evolutionary history the pooling of different species' physiological functions in mutualistic symbioses increased the range of suboptimal environmental conditions that could be buffered against--a mutual tolerance benefit sometimes sufficient to outweigh the cost of cooperation. A related argument is that for a small number of biologically-crucial physical variables (i) the difference between organism interiors and the life-environment interface is relatively low, and (ii) the biologically optimum level of that variable is relatively highly conserved across different species. For such variables, symbiosis tends to cause (at a cost) an increase in the number of environmental buffering functions per unit of selection, which in turn biases the overall impact of the biota on the state of the variable towards the biological optimum. When a costly but more temperature-tolerant and physiologically versatile symbiosis between one black (warming) and one white (cooling) "daisy" is added to the (otherwise unaltered) Daisyworld parable, four new results emerge: (1) the extension of habitability to a wider luminosity range, (2) resistance to the impact of "cheater" white daisies with cold optima, that derive short-term benefit from environmental destabilisation, (3) the capacity to maintain residual, oscillatory regulation in response to forcings that change more rapidly than allele frequencies and (crucially) (4) "succession"-type dynamics in which the tolerant symbiosis colonises and to an extent makes habitable an otherwise lifeless environment, but is later displaced by free-living genotypes that have higher local fitness once conditions improve. The final result is arguably analogous to lichen colonisation of the Neoproterozoic land surface, followed by the Phanerozoic rise of vascular plants. Caution is necessary in extrapolating from the Daisyworld parable to real ecology/geochemistry, but sufficiently conserved variables may be water potential, macronutrient stoichiometry and (to a lesser extent) the temperature window for metabolic activity.

The canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo-proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach of the soft tissue pump is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO 2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude upwelling and mixing of deep water and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there. The glacial states are more sensitive to changes in the circulation and less sensitive to changes in nutrient utilization rates than the interglacial states. © 2010 Author(s).

Recent data suggest the accumulation of anthropogenic carbon dioxide (△Canth) in the subtropical North Atlantic is not occurring at a steady rate throughout the water column. Carbon measurements from three transatlantic cruises along 24.5°N in 1992, 1998, and 2004 were investigated for changes in Canth using both a back-calculation shortcut technique and extended multiple linear regression. For three time periods (1992-1998, 1998-2004, and 1992-2004) we observed spatial and vertical changes in Canth storage, along with a general increase in total concentration. In the surface layers, total dissolved inorganic carbon (TCO 2) and Canth concentrations increased in line with atmospheric CO2 levels: TCO2 +8.8 ± 0.5 μmol kg-1 for 1992-1998 and +8.6 ± 0.5 μmol kg-1 for 1998-2004 and Canth +8.0 ± 0.2 μmol kg-1 for 1992-1998 and +6.8 ± 0.3 mmol kg-1 for 1998-2004. In deeper waters, △Canth was significantly different than zero for all depths above 5000 dbar between 1992 and 2004, while on a subdecadal timescale, significant variability was observed for △Canth at a depth range of 800-1000 dbar. Evidence is presented for the arrival at 24.5°N at depth of freshly ventilated Labrador Sea Water from the subpolar North Atlantic between 1992 and 1998, as well as consistent smaller △Canth signals alongside the Mid-Atlantic Ridge. This is in addition to low-level, stable increases identified in the deep eastern basin between 1992 and 2004, the first time that △Canth has been detected and confirmed by new measurements of carbon tetrachloride and CFC-11 from 2004. These results highlight the importance of the subtropics as a site for long-term C anth storage away from the surface. © 2010 by the American Geophysical Union.

A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy-moisture balance model in which the positive AMOC-MDGrelation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment ΔT that ranges from-3° to 9°C. (An increase in ΔT results in increased sensitivity of density to temperature gradients.) the AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in theAMOCcan be derived from scaling analysis when using the depth of the maximumoverturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation. © 2010 American Meteorological Society.

The effect of sea ice melt on the carbonate chemistry of surface waters in the Weddell-Scotia Confluence, Southern Ocean, was investigated during January 2008. Contrasting concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA) and the fugacity of carbon dioxide (fCO2) were observed in and around the receding sea ice edge. The precipitation of carbonate minerals such as ikaite (CaCO3·6H2O) in sea ice brine has the net effect of decreasing DIC and TA and increasing the fCO2 in the brine. Deficits in DIC up to 12 ± 3 μmol kg-1 in the marginal ice zone (MIZ) were consistent with the release of DIC-poor brines to surface waters during sea ice melt. Biological utilization of carbon was the dominant processes and accounted for 41 ± 1 μmol kg-1 of the summer DIC deficit. The data suggest that the combined effects of biological carbon uptake and the precipitation of carbonates created substantial undersaturation in fCO2 of 95 μatm in the MIZ during summer sea ice melt. Further work is required to improve the understanding of ikaite chemistry in Antarctic sea ice and its importance for the sea ice carbon pump. © 2010 the Authors Tellus B © 2010 International Meteorological Institute in Stockholm.

A climatological mean distribution for the surface water pCO2 over the global oceans in non-El Niño conditions has been constructed with spatial resolution of 4° (latitude) ×5° (longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al. 2002. Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601-1622]. A time-trend analysis using deseasonalized surface water pCO2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO2 over these oceanic areas has increased on average at a mean rate of 1.5 μatm y-1 with basin-specific rates varying between 1.2±0.5 and 2.1±0.4 μatm y-1. A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Niño periods in the equatorial Pacific and those made in coastal zones are excluded from the database. Seasonal changes in the surface water pCO2 and the sea-air pCO2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones. The net air-sea CO2 flux is estimated using the sea-air pCO2 difference and the air-sea gas transfer rate that is parameterized as a function of (wind speed)2 with a scaling factor of 0.26. This is estimated by inverting the bomb 14C data using Ocean General Circulation models and the 1979-2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N-14°S) is the major source for atmospheric CO2, emitting about +0.48 Pg-C y-1, and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of -0.70 Pg-C y-1 for the northern and -1.05 Pg-C y-1 for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO2 sink area on the basis of per unit area, with a mean of -2.5 tons-C month-1 km-2. This is due to the combination of the low pCO2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50°-62°S), the mean annual flux is small (-0.06 Pg-C y-1) because of a cancellation of the summer uptake CO2 flux with the winter release of CO2 caused by deepwater upwelling. The annual mean for the contemporary net CO2 uptake flux over the global oceans is estimated to be -1.6±0.9 Pg-C y-1, which includes an undersampling correction to the direct estimate of -1.4±0.7 Pg-C y-1. Taking the pre-industrial steady-state ocean source of 0.4±0.2 Pg-C y-1 into account, the total ocean uptake flux including the anthropogenic CO2 is estimated to be -2.0±1.0 Pg-C y-1 in 2000. © 2008 Elsevier Ltd.

Here we present monthly, basin-wide maps of the partial pressure of carbon dioxide (pCO2) for the North Atlantic on a 1° latitude by 1° longitude grid for years 2004 through 2006 inclusive. The maps have been computed using a neural network technique which reconstructs the non-linear relationships between three biogeochemical parameters and marine pCO 2. A self organizing map (SOM) neural network has been trained using 389 000 triplets of the SeaWiFS-MODIS chlorophyll-a concentration, the NCEP/NCAR reanalysis sea surface temperature, and the FOAM mixed layer depth. The trained SOM was labelled with 137 000 underway pCO2 measurements collected in situ during 2004, 2005 and 2006 in the North Atlantic, spanning the range of 208 to 437 μatm. The root mean square error (RMSE) of the neural network fit to the data is 11.6 μatm, which equals to just above 3 per cent of an average pCO2 value in the in situ dataset. The seasonal pCO2 cycle as well as estimates of the interannual variability in the major biogeochemical provinces are presented and discussed. High resolution combined with basin-wide coverage makes the maps a useful tool for several applications such as the monitoring of basin-wide air-sea CO2 fluxes or improvement of seasonal and interannual marine CO2 cycles in future model predictions. The method itself is a valuable alternative to traditional statistical modelling techniques used in geosciences.

The oceans are a major sink for atmospheric carbon dioxide (CO2). Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO2 sink for those ocean basins that are well covered by shipping routes.

We examine observations from 1990 to 2006 from four voluntary observing ships and two time-series stations in the North Atlantic, fitting a sinusoidal annual cycle and linear year-on-year trend at all locations where there are sufficient data. Results show that in the subtropical regions, sea-surface fCO2 has closely followed the increasing trend in atmospheric fCO2. In contrast, farther north, sea-surface fCO2 has increased faster than fCO2 in the atmosphere. The resulting ΔfCO2, driving air-sea flux of CO2, has therefore decreased in the North Atlantic, particularly at higher latitudes, as has the annual mean air-sea flux. Several underlying causes may have led to the observed changes in sea-surface fCO2. Low-frequency modes, such as the North Atlantic Oscillation, lead to changes in the sea-surface temperature, in sea-surface circulation and in vertical mixing, affecting sea-surface fCO2 through biogeochemical processes. A comparison with measurements covering a longer time period shows that the sea-surface fCO2 rise has accelerated since 1990 in the northern North Atlantic. © 2008 Elsevier Ltd.

Open ocean deep postconvection contributes to the formation of the dense waters that fill the global deep ocean. The dynamics of postconvective vortices are key to understanding the role of convection in ocean circulation. Submesoscale coherent vortices (SCVs) observed in convective regions are likely to be the anticyclonic components of hetons. Hetons are dipoles, consisting of a surface cyclone and a weakly stratified subsurface anticyclone, that can be formed by convection. Here, key postconvective processes are investigated using numerical experiments of increasing sophistication with two primary goals: 1) to understand how the ambient hydrography and topography influence the propagation of hetons and 2) to provide a theoretical context for recent observations of SCVs in the Greenland Sea. It is found that the alignment of hetons is controlled by ambient horizontal density gradients and that hetons self-propagate into lighter waters as a result. This provides a mechanism for transporting convected water out of a cyclonic gyre, but the propagation is arrested if the heton meets large-amplitude topography. Upon interaction with topography, hetons usually separate, and the surface cyclone returns toward denser water. The anticyclone usually remains close to topography and may become trapped for several hundred days. These findings may explain the observed accumulation and longevity of SCVs at the Greenland Fracture Zone, on the rim of the Greenland Sea gyre. The separation and sorting of cyclones from anticyclones have likely implications for the density and vorticity budgets of convective regions. © 2008 American Meteorological Society.

We report the results of an experiment in the Northeast Atlantic in which sulphur hexafluoride (SF6) was released within an eddy and the behaviour of trace gases, nutrients and productivity followed within a Lagrangian framework over a period of 24 days. Measurements were also made in the air above the eddy in order to estimate air-sea exchange rates for some components. The physical, biological and biogeochemical properties of the eddy resemble those of other eddies studied in this area, suggesting that the results we report may be applicable beyond the specific eddy studied. During a period of low wind speed at the start of the experiment, we are able to quantitatively describe and balance the nutrient and carbon budgets for the eddy. We also report concentrations of various trace gases in the region which are similar to those observed in other studies and we estimate exchange rates for several trace gases. We show that the importance of gas exchange over other loss terms varies with time and also varies for the different gases. We show that the various trace gases considered (CO2, dimethyl sulphide (DMS), N2O, CH4, non-methane-hydrocarbons, methyl bromide, methyl iodide and volatile selenium species) are all influenced by physical and biological processes, but the overall distribution and temporal variability of individual gases are different to one another. A storm disrupted the stratification in the eddy during the experiment, resulting in enhanced nutrient supply to surface waters, enhanced gas exchange rates and a change in plankton community, which we quantify, although overall productivity was little changed. Emphasis is placed on the regularity of storms in the temperate ocean and the importance of these stochastic processes in such systems. © 2008 Elsevier Ltd. All rights reserved.

Despite great advances in understanding of the earth's climate, our estimate of the global temperature rise due to a doubling of atmospheric CO 2 has not greatly changed in a hundred years, and the estimate of the uncertainty on that number has actually increased. This is because while the basic mechanism of greenhouse-gas forcing of climate is well understood, the multiple, mostly positive, feedback loops that amplify this effect are not. The combined effect of many of these feedbacks can be seen in the record of past climate, and analysis of these suggests that our present models tend to under-predict the eventual, equilibrium climate change due to a given increase in atmospheric CO2. In the foreseeable future (next 20 years) climate modelling research will probably not materially decrease the uncertainty on predictions for the climate of 2100. The uncertainty will only start to decrease as we actually observe what happens to the climate. The best use of climate models at present is via ensembles of predictions that give a probabilistic description of the range of uncertainty involved in future climate. Recent studies suggest a skewed probability distribution, with a tail stretching out to high climate sensitivities. Combined with estimates of the likely economic impact of climate change, this strongly suggests that research should be concentrated on trying to reduce the uncertainty represented by this tail of low probability, but high impact, scenarios. © 2007 Springer Science+Business Media B.V.

The first generation of open-ocean iron enrichments (1993 to 2005) have all had broadly the same design. Enrichment of patches of ocean was typically on a 10 km length-scale, and experiments were of a duration of weeks. These scales were dictated by what could conveniently be achieved from research vessels, using tracers to track Lagrangian patches. The extrapolation of experimental findings to the larger scales required for carbon sequestration by ocean iron fertilization (OIF) leaves many uncertainties, to answer which, longer duration (i.e. months) and larger scale observations (100 to 200 km length-scale) are required. However, to extrapolate to a timescale of decades and to the scale of ocean basins, such observations must be conducted in parallel (and where possible assimilated into) detailed models of the physics and biogeochemistry of the fertilized waters. Our present understanding suggests that any carbon sequestration will occur as the net result of changes in the air-sea flux integrated over millions km2 and many years, and can only realistically be assessed by modelling. A central role of the observational studies will be to make such models as accurate as possible in their simulations and predictions. We present a scheme for the design of a second generation of ocean iron-enrichments and discuss the challenges that are evident in linking the modelling and observational components of such studies. © Inter-Research 2008.

It is premature to sell carbon offsets from ocean iron fertilization unless research provides the scientific foundation to evaluate risks and benefits.

Structurally complex life and intelligence evolved late on Earth; models for the evolution of global temperature suggest that, due to the increasing solar luminosity, the future life span of the (eukaryote) biosphere will be "only" about another billion years, a short time compared to the approximately 4 Ga since life began. A simple stochastic model (Carter, 1983) suggests that this timing might be governed by the necessity to pass a small number, n, of very difficult evolutionary steps, with n < 10 and a best guess of n = 4, in order for intelligent observers like ourselves to evolve. Here I extend the model analysis to derive probability distributions for each step. Past steps should tend to be evenly spaced through Earth's history, and this is consistent with identification of the steps with some of the major transitions in the evolution of life on Earth. A complementary approach, identifying the critical steps with major reorganizations in Earth's biogeochemical cycles, suggests that the Archean-Proterozoic and Proterozoic-Phanerozoic transitions might be identified with critical steps. The success of the model lends support to a "Rare Earth" hypothesis (Ward and Brownlee, 2000): structurally complex life is separated from prokaryotes by several very unlikely steps and, hence, will be much less common than prokaryotes. Intelligence is one further unlikely step, so it is much less common still.

Oceanic phytoplankton, absorbing solar radiation, can influence the bio-optical properties of seawater and hence upper ocean physics. We include this process in a global ocean general circulation model (OGCM) coupled to a dynamic green ocean model (DGOM) based on multiple plankton functional types (PFT). We not only study the impact of this process on ocean physics but we also explore the biogeochemical response due to this biophysical feedback. The phytoplankton-light feedback (PLF) impacts the dynamics of the upper tropical and subtropical oceans. The change in circulation enhances both the vertical supply in the tropics and the lateral supply of nutrients from the tropics to the subtropics boosting the subtropical productivity by up to 60 gC m-2 a-1. Physical changes, due to the PLF, impact on light and nutrient availability causing shifts in the ocean ecosystems. In the extratropics, increased stratification favors calcifiers (by up to ∼8%) at the expense of mixed phytoplankton. In the Southern Ocean, silicifiers increase their biomass (by up to ∼10%) because of the combined alleviation of iron and light limitation. The PLF has a small effect globally on air-sea fluxes of carbon dioxide (CO2, 72 TmoIC a-1 outgassing) and oxygen (O2, 46 TmolO2 a-1 ingassing) because changes in biogeochemical processes (primary production, biogenic calcification, and export production) highly vary regionally and can also oppose each other. From out study it emerges that the main impact of the PLF is an amplification of the seasonal cycle of physical and biogeochemical properties of the high-latitude oceans mostly driven by the amplification of the SST seasonal cycle. Copyright 2008 by the American Geophysical Union.

In summer 1996, a tracer release experiment using sulphur hexafluoride (SF6) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996-2002 period documented by ∼10 shipboard surveys. The tracer marked "Greenland Sea Arctic Intermediate Water" (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002. For the first 18 months, the tracer remained mainly in the Greenland Sea. Vigorous horizontal mixing within the Greenland Sea gyre and a tight circulation of the gyre interacting slowly with the other basins under strong topographic influences were identified. We use the tracer distributions to derive the horizontal shear at the scale of the Greenland Sea gyre, and rates of horizontal mixing at ∼10 and ∼300 km scales. Mixing rates at small scale are high, several times those observed at comparable depths at lower latitudes. Horizontal stirring at the sub-gyre scale is mediated by numerous and vigorous eddies. Evidence obtained during the tracer release suggests that these play an important role in mixing water masses to form the intermediate waters of the central Greenland Sea. By year two, the tracer had entered the surrounding current systems at intermediate depths and small concentrations were in proximity to the overflows into the North Atlantic. After 3 years, the tracer had spread over the Nordic Seas basins. Finally by year six, an intensive large survey provided an overall synoptic documentation of the spreading of the tagged GSAIW in the Nordic Seas. A circulation scheme of the tagged water originating from the centre of the GS is deduced from the horizontal spread of the tracer. We present this circulation and evaluate the transport budgets of the tracer between the GS and the surroundings basins. The overall residence time for the tagged GSAIW in the Greenland Sea was about 2.5 years. We infer an export of intermediate water of GSAIW from the GS of 1 to 1.85 Sv (1 Sv = 106 m3 s-1) for the period from September 1998 to June 2002 based on the evolution of the amount of tracer leaving the GS gyre. There is strong exchange between the Greenland Sea and Arctic Ocean via Fram Strait, but the contribution of the Greenland Sea to the Denmark Strait and Iceland Scotland overflows is modest, probably not exceeding 6% during the period under study. © 2008 Elsevier Ltd.

To determine the exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait is one of the most important aspects, and one of the major challenges, in describing the circulation in the Arctic Mediterranean Sea. Especially the northward transport of Arctic Intermediate Water (AIW) from the Nordic Seas into the Arctic Ocean is little known. In the two-ship study of the circulation in the Nordic Seas, Arctic Ocean - 2002, the Swedish icebreaker Oden operated in the ice-covered areas in and north of Fram Strait and in the western margins of Greenland and Iceland seas, while RV Knorr of Woods Hole worked in the ice free part of the Nordic Seas. Here two hydrographic sections obtained by Oden, augmented by tracer and velocity measurements with Lowered Acoustic Doppler Current Profiler (LADCP), are examined. The first section, reaching from the Svalbard shelf across the Yermak Plateau, covers the region north of Svalbard where inflow to the Arctic Ocean takes place. The second, western, section spans the outflow area extending from west of the Yermak Plateau onto the Greenland shelf. Geostrophic and LADCP derived velocities are both used to estimate the exchanges of water masses between the Nordic Seas and the Arctic Ocean. The geostrophic computations indicate a total flow of 3.6 Sv entering the Arctic on the eastern section. The southward flow on the western section is found to be 5.1 Sv. The total inflow to the Arctic Ocean obtained using the LADCP derived velocities is much larger, 13.6 Sv, and the southward transport on the western section is 13.7 Sv, equal to the northward transport north of Svalbard. Sulphur hexafluoride (SF6) originating from a tracer release experiment in the Greenland Sea in 1996 has become a marker for the circulation of AIW. From the geostrophic velocities we obtain 0.5 Sv and from the LADCP derived velocities 2.8 Sv of AIW flowing into the Arctic. The annual transport of SF6 into the Arctic Ocean derived from geostrophy is 5 kg/year, which is of the same magnitude as the observed total annual transport into the North Atlantic, while the LADCP measurements (19 kg/year) imply that it is substantially larger. Little SF6 was found on the western section, confirming the dominance of the Arctic Ocean water masses and indicating that the major recirculation in Fram Strait takes place farther to the south. © 2008 Elsevier Ltd. All rights reserved.

A time series of observations from merchant ships between the U.K. and the Caribbean is used to establish the variability of sea surface pCO2 and air-to-sea flux from the mid-1990s to early 2000s. We show that the sink for atmospheric CO2 exhibits important interannual variability, which is in phase across large regions from year to year. Additionally, there has been an interdecadal decline, evident throughout the study region but especially significant in the northeast of the area covered, with the sink reducing >50% from the mid-1990s to the period 2002-2005. A review of available observations suggests a large region of decrease covering much of the North Atlantic but excluding the western subtropical areas. We estimate that the uptake of the region between 20°N and 65°N declined by ∼0.24 Pg C a-1 from 1994/1995 to 2002-2005. Declining rates of wintertime mixing and ventilation between surface and subsurface waters due to increasing stratification, linked to variation in the North Atlantic Oscillation, are suggested as the main cause of the change. These are exacerbated by a contribution from the changing buffer capacity of the ocean water, as the carbon content of surface waters increases. Copyright 2007 by the American Geophysical Union.

Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.

The oceanic overturning circulation has a central role in the Earth's climate system and in biogeochemical cycling, as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean. Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment - an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage - to measure the rates of mixing and upwelling in the current's intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the tracer reveals rapid upwelling along density surfaces and intense mixing across density surfaces, both occurring at rates that are an order of magnitude greater than rates implicit in models of the average Southern Ocean overturning. These findings support the view that deep-water pathways along and across density surfaces intensify and intertwine as the Antarctic Circumpolar Current flows over complex ocean-floor topography, giving rise to a short circuit of the overturning circulation in these regions. ©2007 Nature Publishing Group.

Marine productivity is often higher downstream than upstream of islands. This so-called island mass effect was tested and quantified with respect to biological carbon uptake and air-sea exchange of carbon dioxide (CO2) at the Crozet Plateau between November 2004 and January 2005 during two CROZEX cruises. The remote plateau is situated at 45.5-47.0°S 49.0-53.0°E, south of the Subantarctic Front (SAF) in the Polar Frontal Zone (PFZ). Surface waters upstream (south) of the plateau had high nutrient and low chlorophyll (HNLC) concentrations. The fugacity of carbon dioxide (fCO2) in surface water was just below the atmospheric value and oceanic CO2 uptake was small (0.2±0.1 mol m-2) throughout CROZEX. The mixed-layer concentration of dissolved inorganic carbon (DIC) decreased by 15 μmol kg-1 from November to January in these HNLC waters, indicating significant biological carbon uptake. Extensive phytoplankton blooms occurred downstream (north) of the plateau in austral spring. These reduced surface water fCO2 by 30-70 μatm and DIC by 30-60 μmol kg-1 and created an important oceanic sink for atmospheric CO2 of 0.6-0.8±0.4 mol m-2, corresponding to a total uptake of 1.3±0.8 Tg C (1 Tg=1012 g). The reduction of DIC in the upper 100 m was much larger downstream (2-3 mol m-2) than upstream (1 mol m-2) of the plateau in January, further confirming the existence of the island mass effect for the Crozet Archipelago. An additional finding is the sizeable DIC deficit in the HNLC waters upstream (south) of the plateau, suggesting that some HNLC waters of the PFZ are more productive than commonly thought. Deep mixed layers of 60-90 m may hide such sustained, modest marine productivity from detection by satellite. © 2007 Elsevier Ltd. All rights reserved.

The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability. The largest chemical transition was the 'Great Oxidation', approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10(-5) of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved, but the causes of this delay and of the Great Oxidation itself remain uncertain. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10(-5) PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 x 10(-3) PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10(-5) PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis and for the evolution of multicellular life.

The Lagrangian Southern Ocean Iron Release Experiment (SOIREE) allowed study of a gradually evolving iron-mediated phytoplankton bloom in water labelled with the inert tracer sulfur hexafluoride, SF6. This article describes a pelagic carbon budget for the mixed layer in SOIREE and assesses the extent to which closure of the budget is achieved. Net community production (NCP) converted 837 mmol m-2 of inorganic carbon to organic carbon in 12.0 d after the first iron addition. A large fraction (41%) of NCP remained as particulate organic carbon in the mixed layer of the iron-enriched patch, while 23% was lost by horizontal dispersion and 0-29% was exported. The closure of the carbon budget is hampered by the lack of measurements of dissolved organic carbon (DOC), by a major uncertainty in carbon export, and by use of empirical conversion factors in estimates of carbon biomass and metabolic rates. Lagrangian carbon-budget studies may be improved by direct measurement of all major carbon parameters and conversion factors. Carbon cycling in the SOIREE bloom resembled that in 'natural' algal blooms in the open Southern Ocean in some respects, but not in all. Daily NCP in the SOIREE bloom (70 mmol m-2 d-1) was higher than in natural blooms, partly because other studies did not account for horizontal dispersion, were for longer periods or included less productive areas. The build-up of POC stock and carbon export as a fraction of NCP in SOIREE were in the lower range of observations elsewhere. © 2006 Elsevier Ltd. All rights reserved.

Decreased ventilation of the Southern Ocean in glacial time is implicated in most explanations of lower glacial atmospheric CO2. Today, the deep (>2000 m) ocean south of the Polar Front is rapidly ventilated from below, with the interaction of deep currents with topography driving high mixing rates well up into the water column. We show from a buoyancy budget that mixing rates are high in all the deepwaters of the Southern Ocean. Between the surface and ∼2000 m depth, water is upwelled by a residual meridional overturning that is directly linked to buoyancy fluxes through the ocean surface. Combined with the rapid deep mixing, this upwelling serves to return deep water to the surface on a short time scale. We propose two new mechanisms by which, in glacial time, the deep Southern Ocean may have been more isolated from the surface. Firstly, the deep ocean appears to have been more stratified because of denser bottom water resulting from intense sea ice formation near Antarctica. The greater stratification would have slowed the deep mixing. Secondly, subzero atmospheric temperatures may have meant that the present-day buoyancy flux from the atmosphere to the ocean surface was reduced or reversed. This in turn would have reduced or eliminated the upwelling (contrary to a common assumption, upwelling is not solely a function of the wind stress but is coupled to the air-sea buoyancy flux too). The observed very close link between Antarctic temperatures and atmospheric CO2 could then be explained as a natural consequence of the connection between the air-sea buoyancy flux and upwelling in the Southern Ocean, if slower ventilation of the Southern Ocean led to lower atmospheric CO2. Here we use a box model, similar to those of previous authors, to show that weaker mixing and reduced upwelling in the Southern Ocean can explain the low glacial atmospheric CO2 in such a formulation. Copyright © Blackwell Munksgaard, 2005.

Using about 138 000 measurements of surface pCO2 in the Atlantic subpolar gyre (50-70°N, 60-10°W) during 1995-1997, we compare two methods of interpolation in space and time: a monthly distribution of surface pCO2 constructed using multiple linear regressions on position and temperature, and a self-organizing neural network approach. Both methods confirm characteristics of the region found in previous work, i.e. the subpolar gyre is a sink for atmospheric CO2 throughout the year, and exhibits a strong seasonal variability with the highest undersaturations occurring in spring and summer due to biological activity. As an annual average the surface pCO2 is higher than estimates based on available syntheses of surface pCO2. This supports earlier suggestions that the sink of CO 22 in the Atlantic subpolar gyre has decreased over the last decade instead of increasing as previously assumed. The neural network is able to capture a more complex distribution than can be well represented by linear regressions, but both techniques agree relatively well on the average values of pCO2 and derived fluxes. However, when both techniques are used with a subset of the data, the neural network predicts the remaining data to a much better accuracy than the regressions, with a residual standard deviation ranging from 3 to 11 μatm. The subpolar gyre is a net sink of CO2 of 0.13 Gt-C yr-1 using the multiple linear regressions and 0.15 Gt-C yr-1 using the neural network, on average between 1995 and 1997. Both calculations were made with the NCEP monthly wind speeds converted to 10 m height and averaged between 1995 and 1997, and using the gas exchange coefficient of Wanninkhof. Copyright © Blackwell Munksgaard, 2005.

Phytoplankton biomass modifies the penetration of light and impacts the physical properties of the upper ocean. We quantify these impacts and the feedbacks on phytoplankton biomass for the global ocean using an Ocean General Circulation Model coupled to an ocean biogeochemistry model. Phytoplankton biomass amplifies the seasonal cycle of temperature, mixed layer depth and ice cover by roughly 10%. At mid and high latitudes, surface temperature warms by 0.1-1.5°C in spring /summer and cools by 0.1-0.3°C in fall/winter. In the tropics, phytoplankton biomass indirectly cools the ocean surface by 0.3°C due to enhanced upwelling. The mixed layer stratifies by 4-30 m everywhere except at high latitudes. At high latitudes, the sea-ice cover is reduced by up to 6% in summer and increased by 2% in winter, leading to further feedbacks on vertical mixing and heat fluxes. Physical changes drive a positive feedback increasing phytoplankton biomass by 4-12% and further amplifies the initial physical perturbations. Copyright 2005 by the American Geophysical Union.

It has been argued that diapycnal mixing has a strongly stabilizing role in the global thermohaline circulation (THC). Negative feedback between THC transport and low-latitude buoyancy distribution is present in theory based on thermocline scaling, but is absent from Stommel's classical model. Here, it is demonstrated that these two models can be viewed as opposite limits of a single theory. Stommel's model represents unlimited diapycnal mixing, whereas the thermocline scaling represents weak mixing. The latter limit is more applicable to the modern ocean, and previous studies suggest that it is associated with a more stable THC. A new box model, which can operate near either limit, is developed to enable explicit analysis of the transient behaviour. The model is perturbed from equilibrium with an increase in surface freshwater forcing, and initially behaves as if the only feedbacks are those present in Stommel's model. The response is buffered by any upper ocean horizontal mixing, then by propagation of salinity anomalies, each of which are stabilizing mechanisms. However, negative feedback associated with limited diapycnal mixing only prevents thermohaline catastrophe in a modest parameter domain. This is because the time-scale associated with vetical advective-diffusive balance is much longer than the time required for the THC to change mode. The model is then tuned to allow equilibrium THC transport to be independent of the rate of mixing. The equilibrium surface salinity difference controls the classical THC-transport/salinity positive feedback, whereas the equilibrium interior density difference controls the mean-flow negative feedback. When mixing is strong, unrealistic vertical homogenization occurs, causing a convergence in surface and interior meridional gradients. This reduces positive feedback, and increases stability, in the tuned model. Therefore, Stommel's model appears to overestimate, rather than underestimate, THC stability to high-frequency changes in forcing. Copyright © Blackwell Munksgaard, 2005.

Ecosystem processes are important determinants of the biogeochemistry of the ocean, and they can be profoundly affected by changes in climate. Ocean models currently express ecosystem processes through empirically derived parameterizations that tightly link key geochemical tracers to ocean physics. The explicit inclusion of ecosystem processes in models will permit ecological changes to be taken into account, and will allow us to address several important questions, including the causes of observed glacial-interglacial changes in atmospheric trace gases and aerosols, and how the oceanic uptake of CO2 is likely to change in the future. There is an urgent need to assess our mechanistic understanding of the environmental factors that exert control over marine ecosystems, and to represent their natural complexity based on theoretical understanding. We present a prototype design for a Dynamic Green Ocean Model (DGOM) based on the identification of (a) key plankton functional types that need to be simulated explicitly to capture important biogeochemical processes in the ocean; (b) key processes controlling the growth and mortality of these functional types and hence their interactions; and (c) sources of information necessary to parameterize each of these processes within a modeling framework. We also develop a strategy for model evaluation, based on simulation of both past and present mean state and variability, and identify potential sources of validation data for each. Finally, we present a DGOM-based strategy for addressing key questions in ocean biogeochemistry. This paper thus presents ongoing work in ocean biogeochemical modeling, which, it is hoped will motivate international collaborations to improve our understanding of the role of the ocean in the climate system. © 2005 Blackwell Publishing Ltd.

The Faroe Bank Channel is the deepest passage for dense water leaving the Nordic Seas into the North Atlantic. The contribution to this part of the Greenland-Scotland Overflow by intermediate water from the Greenland Sea is investigated by the tracer sulphur hexafluoride (SF6) that was released into the central Greenland Sea in summer 1996. Continuous monitoring has since traced it around the Nordic Seas and into the connecting areas. It was observed for the first time close to the Faroe Islands in early 1999, indicating a transport time from the Greenland Sea of around 2.5 years. This study estimates that approximately 16 kg of SF6 had passed the Faroe Bank Channel by the end of 2002, that is 5% of the total amount released. Both the arrival time and the amount of exported SF6 deduced from the observations are consistent with the results from a numerical ocean model simulating the tracer release and spreading. © 2004 Elsevier Ltd. All rights reserved.

The seasonal evolution of total inorganic carbon and CO2 air-sea fluxes in the Eastern North Atlantic Subtropical Gyre (Azores area) was investigated by means of studying a data set from 10 cruises covering a seasonal cycle. Monthly CO2 fugacity was modelled as a function of surface temperature and month for 1998. So, the seasonal cycle of CO2 and its air-sea fluxes were obtained using monthly average surface data in the area. Over the year, the Azores area (2.25·1012 m2) acts as a weak net sink of CO2 (0.38 mmol m-2 day -1). From December to May, the zone is a rather strong sink for CO2 (10.3 mmol m-2 day-1), while between June and November, it behaves as a CO2 source (9.9 mmol m-2 day-1), August presents the highest outgassing (3.88 mmol m -2 day-1). Moreover, a box budget was established to evaluate the relative contribution of the physical and biological processes affecting the seasonal CO2 variability in the mixed layer of the Azores area. The most important contributor to the average mass balance of CO2 was the mixing with the lower layer (7.8 mmol m-2 day-1) and biological activity (-8.9 mmol m-2 day -1). Conversely, air-sea exchange (0.17 mmol m-2 day -1) and advection (1.7 mmol m-2 day-1) contribute with a very small input. There is a strong coupling between biological activity, advection, and mixing in the mixed layer depth. The biological activity is supported by mixing and advection that provide CO 2 and nutrients to the mixed layer, so we combine the three processes in only one term (ΔCAMB) that represents the net biology production in the water column, and re-evaluated the CO2 mass balance to discriminate the importance of the physical and biological contributions. The effect of temperature, wind, and net biological process contribute in 42%, 12%, and 46%, respectively, to the explained variance of total CO2 mass balance in the upper layer. © 2004 Elsevier B.V. All rights reserved.

Global ocean carbon models and available syntheses of the oceanic CO2 flux suggest that the North Atlantic subpolar gyre (50°N-70°N, 80°W-10°W) is a region of increasing uptake of CO2 from the atmosphere, with the oceanic partial pressure of CO2 (pCO2) increasing more slowly than the atmospheric CO2 over time. Our analysis of available CO2 data shows that, on the contrary, seawater pCO2 has increased faster than the atmosphere in recent decades, especially in summer, resulting in a decrease in uptake from the atmosphere. A decrease in the biological productivity of the region may be the underlying cause of this trend. From the observed trend we estimated a significant decrease in the annual carbon uptake in this region. Copyright 2004 by the American Geophysical Union.

The flux of gases between the atmosphere and the oceans can be calculated from the product of the concentration difference across the sea surface and a kinetic term, often called a transfer velocity. The transfer velocity is frequently parameterized in terms of wind speed, although the actual exchange process is also affected by waves, bubbles, wind fetch, and less certainly by surfactants and chemical reactivity. There is currently an uncertainty of about a factor of two in using the wind speed parameterization. In view of the windiness of the Southern Ocean, transfer velocities will often be high, although there are few published in situ measurements of transfer rates made in the region. Data for gas concentration fields in the Southern Ocean are generally sparse compared to other better studied oceanic areas. In this paper we discuss what is known for the region for carbon dioxide, including the oceanic sink for man-made inputs to the atmosphere; dimethyl sulphide, where there appears to be a substantial source, which has the potential for a significant climatic effect due to the low particulate loading in the region; and organo-halogen and alkyl nitrate gases, where marine emissions may play an important role in controlling the oxidation capacity of the Antarctic atmosphere.

The distribution of turbulent diapycnal mixing in the Nordic seas is mapped from observations of internal wave density and velocity fine structure. The uppermost 500-1500 m host two distinct mixing regimes. In the eastern basins, the diapycnal diffusivity (Kρ) straddles 10-5 m2 s-1, whereas in the weakly stratified Greenland and Boreas basins it is raised by an order of magnitude. Below ∼2000 m, low stratification is associated with intense turbulent mixing across the Nordic seas, with diffusivities in the range 3 × 10-4 - 10-2 m2 s-1. These mixing rates agree within uncertainties with three tracer-based diffusivity estimates in the region and are associated with turbulent dissipation rates (ε) that are at most moderately enhanced above typical open ocean values. A minimum in both ε and Kρ is commonly found at ∼1500 m, a depth level that is most efficiently sheltered from shallow and bottom energy sources for the mixing. Available evidence points to wind work on upper ocean inertial motions as a shallow source, with semidiurnal internal tides generated at different levels of the topography contributing to both shallow and deep turbulence. While the closure of the North Atlantic meridional overturning circulation in the Nordic seas appears to be primarily driven by air-sea interaction, turbulent mixing has the potential to play a critical role in shaping the stratification and ventilation of the region via a range of complex interactions with convection. Copyright 2004 by the American Geophysical Union.

We examine the effect on atmospheric CO2 of the occurrence of increased shallow water carbonate deposition and regrowth of the terrestrial biosphere following the last glacial. We find that contrary to recent speculations that changes in terrestrial carbon storage were primarily responsible for the observed ∼20 ppmv late Holocene CO2 rise, a more likely explanation is coral reef buildup and other forms of shallow water carbonate deposition during this time. The importance of a responsive terrestrial carbon reservoir may instead be as a negative feedback restricting the rate of CO2 rise possible in the early stages of the deglacial transition. This separation in time of the primary impacts of regrowth of the terrestrial biosphere and increased shallow water carbonate deposition explains the occurrence of an early Holocene carbonate preservation event observed in deep-sea sediments. We demonstrate that their combined influence is also consistent with available proxy estimates of deep ocean carbonate ion concentration changes over the last 21 kyr. Accounting for the processes that act on the carbonate chemistry of the ocean as a whole then allows us to place strong constraints on the nature of the remaining processes that must be operating at the deglacial transition. By subtracting the net CO2 effect of coral reef buildup and terrestrial biosphere regrowth from recent high-resolution ice core data, we highlight two periods, from 17.0 to 13.8 kyr and 12.3 to 11.2 kyr BP characterized by sustained rapid rates of CO2 increase (>12 ppmv kyr-1). Because these periods are coincident with Southern Hemisphere "deglaciation," we argue that changes in the biogeochemical properties of the Southern Ocean surface are the most likely cause.

Dispersion of the tracer sulphur hexafluoride (SF6) during the Southern Ocean Iron Enrichment Experiment (SOIREE) provided an estimate of vertical exchange at the base of the surface mixed layer (60 m) at 61°ES 140°E. Budget analysis confirmed that the SF6 patch was well constrained by surface mapping, with the decline in total SF6 showing good agreement with that predicted from wind speed parameterizations. Two approaches were used to calculate the mean effective vertical diffusivity Kz from the diapycnal transfer of SF6, with complementary error function and second-moment fits to the SF6 depth profiles indicating that Kz was less than 0.3 × 10-4 m2s-1. This result was examined using a three-dimensional diffusion model that incorporated lateral dispersion and air-sea exchange losses, which confirmed that vertical shear and subpycnocline dispersion did not influence the Kz estimate. Current shear at the base of the mixed layer was generated by wind-driven inertial oscillation, with a decrease in wind speed and increasing stratification in the latter half of the experiment reducing diapycnal transfer of SF6. A compilation was used to examine the potential of both N (Brunt-Väisäla frequency) and Ri (gradient Richardson number) on the basis of parameterizations of Kz in the seasonal pycnocline. Application of Kz to nutrient gradients in the seasonal pycnocline suggests that vertical diffusion represents a significant pathway for silicic acid supply in late summer. Furthermore, use of the mean effective Kz (0.11 ± 0.2 × 10-4 m2s-1) indicates that vertical diffusion supplies a large proportion of the iron required for new production in this region.

Enhanced aeolian supply of iron to the biota of the Southern Ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric CO2. Declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in CO2, but the reasons behind the dust decline itself are as yet unknown. Here we show that the dust record from the Vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of Patagonian sources of acolian material and the efficiency with which it is transported through the atmosphere. We suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that, in turn, climate is responsive to the dust due to iron fertilization. This positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states, characterized by distinct CO2 concentrations. Recognition of this "glacial subcycle" may help to account for the timing and sequence of events at the terminations.

The Greenland Sea is one of a few sites in the world ocean where convection to great depths occurs-a process that forms some of the densest waters in the ocean. But the role of deep convective eddies, which result from surface cooling and mixing across density surfaces followed by geostrophic adjustment, has not been fully taken into account in the description of the initiation and growth of convection. Here we present tracer, float and hydrographic observations of long-lived ( approximately 1 year) and compact ( approximately 5 km core diameter) vortices that reach down to depths of 2 km. The eddies form in winter, near the rim of the Greenland Sea central gyre, and rotate clockwise with periods of a few days. The cores of the observed eddies are constituted from a mixture of modified Atlantic water that is warm and salty with polar water that is cold and fresh. We infer that these submesoscale coherent eddies contribute substantially to the input of Atlantic and polar waters to depths greater than 500 m in the central Greenland Sea.

It has been suggested that much of the observed glacial-interglacial variability in the atmospheric mixing ratio of CO2 (xCO2) could potentially be driven by a perturbation of the marine Si cycle. To date, only relatively simple steady-state analysis has been made of this hypothesis. In this study, we develop a description of the ocean carbon cycle, incorporating novel descriptions for the recycling of Si, both within the water column and in deep-sea sediments. A high degree of computational efficiency enables model integrations over multiple glacial-interglacial cycles, driven by a time-varying input of dissolved Si to the ocean. Due to the long time constant (∼23 ka) of atmospheric xCO2 response to perturbation in Si supply and the highly nonlinear nature of opal preservation in deep-sea sediments, we find that reduction in the deposition rate of aeolian silicates at the surface ocean can explain little (

Biological productivity in a number of ocean regions appears to be at least partly limited by the availability of iron. Any reduction in the present-day aeolian iron supply to the open ocean is therefore likely to result in further limitation of productivity. The stabilization of soils for the purpose of carbon sequestration could give rise to such an effect. With the aid of a global carbon cycle model, we show that the effectiveness of carbon removal from the atmosphere by sequestration on land will be diminished as a result of a reduction of up to 9% in the rate of anthropogenic CO2 uptake by the ocean. This interconnectedness, both within the 'natural' system and in relation to human activities, highlights the importance of analyzing global change within an integrated 'Earth system' framework.

Biological productivity in a number of ocean regions appears to be at least partly limited by the availability of iron. Any reduction in the present-day aeolian iron supply to the open ocean is therefore likely to result in further limitation of productivity. The stabilization of soils for the purpose of carbon sequestration could give rise to such an effect. With the aid of a global carbon cycle model, we show that the effectiveness of carbon removal from the atmosphere by sequestration on land will be diminished as a result of a reduction of up to 9% in the rate of anthropogenic CO2 uptake by the ocean. This interconnectedness, both within the 'natural' system and in relation to human activities, highlights the importance of analyzing global change within an integrated 'Earth system' framework.

Biological and biogeochemical change in the surface mixed layer of an anticyclonic eddy at 60°N in the North Atlantic were monitored within a Lagrangian time-series study using the tracer sulphur hexafluoride (SF 6). Four ARGOS buoys initially released at the patch centre remained closely associated with the SF 6 patch over a 10-day period, with the near-circular eddy streamlines contributing to the stability and coherence of the patch. Progressive deepening of the surface mixed layer was temporarily interrupted by a storm, which increased mixed-layer nitrate and accelerated the transfer of SF 6 to the atmosphere. Diapycnal exchange of SF 6 was relatively rapid due to the shallow pycnocline gradient, and a vertical eddy diffusivity (K z) of 1.95 cm 2s -1 at the base of the mixed layer was estimated from vertical SF 6 profiles at the patch centre. Application of K z to the nutrient gradients inferred vertical nitrate and phosphate fluxes of 1.8 and 1.25 mmol m -2d -1, respectively, for the pre-storm period, which accounted for 33% and 20% of the reported in vivo uptake rates. Integration of the vertical nitrate flux and decline in surface layer nitrate suggest a total loss of 0.54 mmol N m -3d -1 during the 5-day pre-storm period, of which in vivo nitrate consumption only accounted for 49%. Vertical transport of ammonium regenerated in the pycnocline accounted for up to 25% of in vivo phytoplankton uptake. The results suggest that the contribution of vertical turbulence to the mixed-layer nutrient pool was less important than that recorded in other regions of the open ocean, inferring that advective processes are more significant in an eddy. This study also emphasises the potential of SF 6 for oceanic Lagrangian time series studies, particularly in dynamic regions, and in constraining estimates of new production. © 2001 Elsevier Science Ltd.

The spreading and mixing of deep and bottom waters from the Weddell Sea (originating with potential temperature

Sulphur hexafluoride (SF6) has potential as a transient tracer of recently ventilated water masses, as its atmospheric burden continues to increase. Northern Arabian Sea hydrography was examined using measurements of atmospheric and dissolved SF6, CFC-11, CFC-12 and CFC-113. Persian Gulf Water (PGW) was characterised by its SF6 signal, and the time elapsed since its formation was evaluated by two approaches. Four ventilation age estimates were derived from SF6/CFC-11, SF6/CFC-12, CFC-113/CFC-11 and CFC-113/CFC-12, and their agreement at the oceanic stations confirms the validity of SF6 as a transient tracer. A second approach, of correcting SF6 partial pressure for PGW dilution by an optimal mixing model and referencing to the atmospheric SF6 chronology, provided a relative tracer age. This indicated a PGW flow of 0.016 (+/- 0.003) m/s across the northern Arabian Sea, with an associated oxygen consumption of 10.1 μmol/l p.a. that exceeds tracer-derived estimates but confirms rates derived from export flux. Recent observations suggest that the impact of PGW upon the oxic and nutrient status of the northern Arabian Sea is reduced by rapid mixing and dilution within the GOM (Morrison et al. 1997; Banse et al. 1997). Constraining estimates of PGW flow rate and oxygen demand is therefore critical in establishing the factors that maintain the ODZ. Sulphur hexafluoride (SF6) is an inert man-made volatile produced since the early 1970's for its dielectric properties. The documentation of an annual atmospheric increase of 5.5-7% (Maiss et al. 1996; Geller et al. 1997) and the establishment of methodologies for reproducible measurement of background SF6 in the ocean now suggest a viable alternative application for SF6 as a transient tracer of ocean dynamics (Law et al. 1994). This potential was first examined within a study aimed at determining the chronology and velocity of Persian Gulf Water (PGW) in the northern Arabian Sea. Transient tracers were measured at 4 station in the Gulf of Oman (GOM1-GOM6) and 7 stations (A1-17) on a south-easterly transect from the Omanim coastline (19°N 59°E) to the southern Arabian Sea (8°N 67°E) during the UK JGOFS ARABESQUE cruises (Fig. 1). Dissolved and atmospheric measurements were obtained for SF6 during September 1994 and for CFCs during November-December 1994. Dissolved SF6 was analysed in 650 ml samples with a precision of 0.012 fmol/kg SF6 and reproducibility of 1.2% for surface waters (Law et al. 1994), with standards cross-calibrated with atmospheric standards (Max-Planck Institute). Precisions of 0.022, 0.02 and 0.006 pmol/kg and reproducibilities of 2.7%, 1.7% and 4.8% were obtained for CFC-11, CFC-12 and CFC-113 analysis, respectively (Haine et al. 1995), with standards cross-calibrated with the SIO-93 scale. Atmospheric ratios were obtained from recent CFC and SF6 chronologies for the northern hemisphere (Walker et al. 2000; Geller et al. 1997) and surface saturatential of SF6 as a transient tracer. The stabilisation of atmospheric concentrations has introduced ambiguity into the interpretation of CFCs as tracers of recently-ventilated waters. SF6 is not yet subject to production controls and so represents a valuable transient tracer for future studies of ocean mixing and circulation.

A study of the year-to-year variation in net CO2 uptake by the ocean helps in assessing the mechanisms of global climate change.

The effect of iron supply on phytoplankton growth and the marine carbon cycle was tested in situ at 61°S 141°E in the Southern Ocean Iron Release Experiment (SOIREE). On 9 February 1999 iron and the tracer sulphur hexafluoride (SF6) were added to the mixed layer with additional iron infusions after 3, 5 and 7 days. A small decrease of the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) by iron-induced algal growth was observed 4-5 days after the first infusion. From then onwards fCO2 and DIC steadily decreased, and the iron-enriched waters became a sink for atmospheric CO2. The region with surface-water fCO2 drawdown closely matched the shape of the patch, as indicated by SF6. Surface-water fCO2 and DIC drawdown were relatively constant across the patch, whereas SF6 decreased from the patch centre outwards. This pointed to uniform algal carbon uptake, not limited by iron, in the patch. After 13 days surface-water fCO2 and DIC in the patch centre had decreased by 32-38 μatm and 15-18 μmolkg-1, respectively. Surface-water fCO2 outside the patch had increased by 8 μatm, partly as a result of surface-water warming. The iron-induced fCO2 change exceeded seasonal fCO2 variability in this region by a factor of two. From the surface-water fCO2 distribution we estimate a net DIC drawdown of 1353 t of carbon (± 14%) (1 t = 106 g) across the patch after 12 days, assuming uniform drawdown in the upper 50 m. Correction for vertical diffusion and air-sea exchange results in a gross DIC drawdown of 1408 t of carbon. The decrease of fCO2 and DIC, integrated over the mixed layer, was remarkably similar in size after 13 days of SOIREE as changes observed after 6-9 days during IronEx II, if we consider the 4-5 days lag in algal carbon uptake at the Southern Ocean site. SOIREE has demonstrated in situ the occurrence of algal iron limitation and of iron-induced carbon uptake in these Southern Ocean waters. The subsequent fate of the fixed inorganic carbon can only be speculated upon. © 2001 Elsevier Science Ltd.

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis'. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial approximately 40 p.p.m. of glacial-interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known.

Measurements of air-sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993. A conservative tracer, spores of the bacterium Bacillus globigii var. Niger, was used for the first time in an in situ air-sea gas exchange experiment. This nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period. The first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings suggest that results from all dual tracer experiments are mutually consistent. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas ex- change relationship that shows a dependence on wind speed intermediate between those ofLiss and Merlivat [1986] and Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb-derived radiocarbon. The degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates.

We review recent results of experiments using sulfur hexafluoride tracer releases to investigate ocean mixing, gas exchange, and response to iron fertilization. A release method has been devised that allows large-scale mixing experiments to be initiated using ∼100 kg or more of tracer accurately targeted on a given density surface. Combined with the very low detection limit of SF6, this means that these experiments can last up to several years and cover thousands of kilometers. The experiments have revealed that in the open ocean pycnocline, rates of mixing are low (0.1-0.2 cm2 s-1 at 300 m in the subtropical North Atlantic, for example). An approximately inverse relationship between diapycnal mixing and buoyancy frequency is suggested by most (but not all) tracer investigations to date, though this may not hold in the abyssal ocean far from any boundary. Much smaller scale releases into surface waters have been used to obtain measurements of gas exchange and to enable biogeochemical studies such as iron fertilization experiments on well-defined volumes of water. These experiments take advantage of the fact that in surface water, continuous rapid analysis of sulfur hexafluoride enables the tracer to be tracked and mapped by the observing ship. Recent gas exchange studies using the "dual-tracer" method are summarized; we suggest that they may be reconciled to values based on global 14C exchange by appealing to the effect of organic films observed in coastal waters by Frew [1997]. The tracer technique makes possible biogeochemical "patch" studies such as the Ironex I and II experiments in the equatorial Pacific. The use of the tracer measurements to guide and normalize measurements of the effect of added iron is illustrated. Copyright 2000 by the American Geophysical Union.

In March-April 1995, as part of the World Ocean Circulation Experiment section A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shelf (72.5° S, 16.5° W) to South Georgia (55° S, 34.5° W). Chlorofluorocarbon (CFC-11, CFC-12, and CFC-113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Water (defined as waters with potential temperatures less than 0°C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potential temperatures less than -0.7°C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing northern Weddell Gyre than in the westward flowing southern limb. Our CFC measurements suggest that distinct sources of Weddell Sea Bottom Water exist in the western Weddell Sea, in agreement with previous descriptions based on potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0°C and -0.7°C, confirm the long-recognized sources for this water mass in the western and southwestern Weddell Sea. In the southern Weddell Gyre at about 20° W and along the Antarctic continental slope, Weddell Sea Deep Water with potential temperatures around -0.45° C shows a deep CFC maximum about 1000 m above the seafloor. CFC concentrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30° W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea. Instead, these tracers suggest that an important portion of the Weddell Sea Deep Water in the southern Weddell Gyre originates outside the western Weddell Sea, probably near the Amery Basin and environs, around 75° E. These features of the circulation and renewal of the deep Weddell Gyre should be carefully considered in simulations dealing with fluxes, pathways, and formation rates of Antarctic Bottom Water. Copyright 2000 by the American Geophysical Union.

Seasonal CO2 fluxes are estimated from quarterly maps of ΔpCO2 (difference between the oceanic and atmospheric partial pressure of CO2) and associated error maps. ΔpCO2 maps were interpolated from pCO2 measurements in the North Atlantic and the North Pacific Oceans using an objective mapping technique. Negative values correspond to an uptake of CO2 by the ocean. The CO2 flux for the North Atlantic Ocean, between 10°N and 80°N, ranges from -0.69 GtC/yr, for the first quarter (January-March), to -0.19 GtC/yr for the third quarter (July-September) using the gas exchange coefficient of Tans et al. [1990], satellite wind speeds, and a correction for the skin effect. On annual average, the North Atlantic ocean (north of 10°N) is a sink of CO2 ranging from -0.23 ± 0.08 GtC/yr (gas exchange coefficient of Liss and Merlivat [1986] with Esbensen and Kushnir [1981] wind field) to -0.48 ±0.17 GtC/yr (gas exchange coefficient of Tans et al. with satellite wind field). The CO2 flux for the North Pacific, between 15°N and 65°N, ranges from -0.66 GtC/yr from April to June to zero from July to September. For the Atlantic, the errors are generally small, that is, less than 0.19 GtC/yr, but for the Pacific considerably larger uncertainties are generated due to the less extensive data coverage. The northern hemisphere ocean (north of 10°N) is a net sink of CO2 to the atmosphere which is stronger in spring (April-June), due to the biological activity, with an estimate of-1.23 ± 0.40 GtC/yr averaged over this period. The annual mean northern hemisphere ocean flux is -0.86 ± 0.61 GtC/yr.

This paper is included in the Special Publication entitled 'James Hutton - present and future', edited by G.Y. Craig and J.H. Hull. Observation suggests that the Earth's surface environment is maintained by processes in which non-living and living causes are linked inextricably. Once established on Earth, life rapidly became a dominant influence on the evolution of the planetary environment. But life was also shaped by that evolution, constrained and directed by the physical and chemical processes that moulded the planet's surface. Life and the planetary environment form a closely coupled entity, a view of the Earth as a complex system which is prefigured in the writings of James Hutton. Hutton compared the workings of the Earth's surface to the body of an animal, being both wasted and repaired continually. In modern times, James Lovelock has argued that it is a property of this system that it acts to maintain the planet in a habitable conditon. Alternatively, perhaps it is pure chance that the planet has always remained hospitable for life - it could just as easily have followed an infinity of different evolutionary paths, many and perhaps most of which would lead rapidly to global extinction. Consideration of the fates of our near- neighbour planets, Mars and Venus, and the dangerous nature of the inner solar system, leads to the conclusion that there is indeed a substantial element of luck involved in the Earth's biosphere having survived as long as it has. The fact that our own existence is dependent on it having survived makes it nearly impossible to accurately assess a priori the probability of survival. Abstract models such as 'Daisyworld' can capture some of the complex behaviour of the Earth-life system. This may include periods of stasis and sudden changes to new states, the stasis being an example of regulatory behaviour where the system is dominated by negative feedback, and the sudden changes being essentially the opposite - brief but traumatic periods where the dominant feedbacks are positive. GEOCARB, a biogeochemical model for the Phanerozoic which links changes in the long-term carbon cycle to planetary temperature, shows examples of both regulatory and destabilizing behaviour in a less abstract system, and suggests that such responses have indeed characterized Earth history. I conclude that the properties of the Earth-life system are complex and not easily predictable. The longevity of Hutton's animal (nearly 4 Ga) is no guide to its future life expectancy, and even if the system as a whole lasts many aeons into the future, any given species (such as humans) is most unlikely to.

Milankovitch theory seeks to explain the Quaternary glaciations via changes in seasonal insolation caused by periodic changes in the Earth's obliquity, orbital precession, and eccentricity. However, recent high-resolution spectral analysis of δ18O proxy climate records have cast doubt on the theory [Muller and MacDonald, 1997a, b]. The spectral signature of the '100 kyr' component, which dominates the climate record over the past 0.6-0.8 Myr does not match the frequencies of the eccentricity variation. Muller and MacDonald [1997b, c] have therefore argued that a more likely pacemaker for the climate cycles is the variation in inclination of the Earth's orbit relative to the invariant plane of the solar system. Here we show that the spectral signature of δ18O records are entirely consistent with Milankovitch mechanisms in which deglaciations are triggered every fourth or fifth precessional cycle. Such mechanisms may involve the buildup of 'excess' ice due to low summertime insolation at the previous precessional 'high'.

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior. Deep-water formation varies substantially on annual and decadal timescales, and responds to regional climate signals such as the North Atlantic Oscillation; its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change. Here we report direct measurements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. In summer, we found rapid turbulent vertical mixing of about 1.1 cm2s-1. In the following late winter, part of the water column was mixed more vigorously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing outside the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically transported in convective plumes, which reached from the surface to, at their deepest, 1,200-1,400 m. Convection was limited to a very restricted area, however, and smaller volumes of water were transported to depth than previously estimated. Our results imply that it may be the rapid year-round turbulent mixing, rather than convection, that dominates vertical mixing in the region as a whole.

Iron occurs at very low concentrations in seawater and seems to be a limiting factor for primary production in the equatorial Pacific and the Southern Ocean. The global distribution of iron is still not well understood because of a lack of data and the complex chemistry of iron. We develop a 10-box model to study the oceanic distribution of iron and its effect on atmospheric CO2 concentration. Subject to our assumptions, we find that a lack of interocean fractionation of deep sea iron concentrations, as suggested by Johnson et al. [1997a], is not readily explained by a balance of eolian deposition, scavenging, and regeneration. Incorporation of organic complexation in the model, as suggested by Johnson et al. to reduce the scavenging rate of iron when concentrations fall below some ligand-stabilized concentration, is one solution to this difficulty. Alternatively, the deep-sea concentration may be more variable than the current, rather sparse data coverage suggests. In the model, deep-sea iron concentrations are responsive to the atmospheric source, even if we adopt stabilization of concentrations by a ligand as modeled by Johnson et al. [1997a]. In the Southern Ocean, where the model suggests iron supply has an important limiting effect on the biota, more than 99% of the iron supply to the surface in the present day comes from upwelling and not from the local atmospheric flux. In the context of glacial-interglacial changes to atmospheric CO2 the model suggests that increasing atmospheric iron to the entire global ocean by a factor of 2, leads to decreases in atmospheric CO2 of 10-30 ppm, depending on assumptions. However, in our model, CO2 concentrations are almost unaffected by changes in Southern Ocean atmospheric fluxes alone, unless these are unrealistically large (>100 times present day). The effect on atmospheric carbon dioxide is slightly stronger if accompanied by increased stratification of the Southern Ocean. The model suggests that eolian 'iron fertilization' of the ocean could have importantly influenced glacial atmospheric CO2 concentrations but that other processes must also be at work to account for the full magnitude of the glacial-interglacial change.

There have been several recent advances in our understanding of the geochemistry of iron and its effect on the marine biota. In this contribution, we highlight two such advances, namely results of the Ironex experiments in the equatorial Pacific and the recent publication of the first global data set for iron concentrations in the oceans. These have profound consequences for our understanding of the factors that set the pre-anthropogenic concentration of carbon dioxide in the atmosphere, and how these may have changed between glacial and interglacial time. Some of these consequences we are able to quantify and explore, but others open new questions for which we have as yet no answers.

We review aspects of the influence of the marine biota on climate, focusing particularly on their role in mediating surface temperatures via their influence on atmospheric carbon dioxide (CO2) and dimethyl sulphide (DMS) concentrations. Variation in natural CO2 concentrations occurring over 103-105 years are set by oceanic processes, and in particular by conditions in the Southern Ocean, so it is to this region that we must look to understand the glacial-interglacial changes in CO2 concentrations. It seems likely that marine productivity in the Southern Ocean is limited by a combination of restricted iron supply to the region and insufficient light. Plankton-produced DMS is thought to influence climate by changing the numbers of cloud condensation nuclei available in remote regions; the efficiency of this mechanism is still unknown, but calculations suggest it may be a powerful influence on climate. It has a much shorter time-scale than the CO2 effect, and as a consequence may well be a player on the 'global change' time-scale. The direction of both the CO2 and the DMS mechanisms is such that more marine productivity would lead to lower global temperatures, and we speculate that the overall effect of the marine biota today is to cool the planet by ca. 6 C as a result of these two mechanisms, with one-third of this figure being due to CO2 effects and two-thirds due to DMS. While the marine biota influence climate, climate also influences the marine biota, chiefly via changing atmospheric circulation. This in turn alters ocean circulation patterns, responsible for mixing up subsurface nutrients, and also influences the transport of nutrients, such as iron, in atmospheric dust. A more vigorous atmospheric circulation would be expected to increase the productivity of the marine biota on both counts. Thus during glacial time, the colder and drier climate might be expected to stimulate greater marine productivity than occurs today. Since more production leads to greater cooling by reduction in CO2 and increase in DMS, the marine biota-climate system appears to have been in positive feedback in the glacial-interglacial transition, with the changes in the climate system being reinforced by changes in the marine biota. In the context of anthropogenic change, we cannot at present say what sign the feedback on climate will have, because we have no clear idea whether circulation will become more or less vigorous in the future.

A patch of sulfur hexafluoride was released in May 1992 in the eastern North Atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces. The diapycnal eddy diffusivity K estimated for the first 6 months was 0.12 +/- 0.02 cm(2)/s, while for the subsequent 24 months it was 0.17 +/- 0.02 cm(2)/s. The vertical tracer distribution remained very close to Gaussian for the full 30 months, as the root mean square (rms) dispersion grew from 5 to 50 m. Lateral dispersion was measured on several scales. The growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0.07 m(2)/s at scales of 0.1 to 1 km, larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing. Teasing of the overall patch, initially about 25 km across, into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 +/- 0.5 x 10(-7) s(-1). The rms width of these streaks, estimated as 3 km and maintained in the face of the streak growth, indicates an isopycnal diffusivity of 2 m(2)/s at scales of 1 to 10 km, much greater than expected from internal wave shear dispersion. The patch was painted in, albeit streakily, by 12 months, confirming expectations from analytical and numerical models. Homogenization of the patch continued during the subsequent 18 months, while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m(2)/s, acting at scales of 30 to 300 km.

An 8 x 8 km tracer-enriched patch was successfully formed in an open-ocean mixed layer in the equatorial Pacific during the first uncontained test of the iron hypothesis. To minimize the effects of horizontal advection, the patch formation and subsequent rapid underway sampling of the patch properties were performed in a Lagrangian reference frame centered on a navigated, drogued buoy that tracked the mixed layer movements on O(1 d) timescales. Daily maps of the evolving patch shape, and corrections for the buoy drift relative to the patch center, were based on objectively analyzed surface concentration maps of an inert tracer, SF6, which had been injected into the ocean surface with trace quantities of iron during the patch formation. The Lagrangian reference frame significantly reduced large scale circular and lateral advection errors in maps of the surface patch shape. A strong pycnocline at approximately 35 m depth and very constant 6 m s-1 wind forcing greatly limited turbulent diffusion below the mixed layer. Rapid small scale mixing over the first 24 h was followed by a four day period of slow spreading, primarily in the along-wind direction. Estimates of along wind horizontal diffusivity using a Fickian model were 600 ± 100 m2 s-1, with a mean cross wind value of 200 ± 30 m2 s-1 over a 4 d period. On the fifth day an intruding low salinity surface front effectively capped the patch between a 10-20 m thick fresh surface layer and the pycnocline. This experiment demonstrated that an O(100 km2) open ocean mixed layer tracer tagged patch could be formed, and its evolution mapped the presence of advection over 5-10 d periods.

The first open-ocean experiment to test the iron hypothesis in the equatorial Pacific was undertaken using the tracer gas sulphur hexafluoride (SF6) to locate and track the fertilised surface water. Continuous surface measurements showed that the SF6 patch spread rapidly in the first 24 h, from an initial release area of ~64 km2 to a total area of 214 km2, and remained relatively constant in size for the following three-day period. SF6 data was mapped in a Lagrangian frame of reference by the use of a drogued GPS buoy released at the centre of the patch. The SF6 patch remained coherent and exhibited a slow, anti-cyclonic oscillation during the first four days. The buoy was transported downwind of the patch in a northwesterly direction within two days, which has implications for the future use of buoys in surface-water advection studies. Following subduction below a low-salinity front 3-4 days after release, the patch centre was relocated by its SF6 signal at a depth of 25-30 m to the east of the residual surface patch the latter spread rapidly to the southwest during the remainder of the experiment, whilst the subducted patch remained relatively stationary. Density-corrected SF6 profiles were used to calculate a mean vertical eddy diffusivity of 0.25 cm2/s across the thermocline following the subduction event. A vertical flux of nitrate of 2.5 mmol/m-2 d-1 into the mixed layer was estimated, which implied an f-ratio value of 0.4 on comparison with productivity data. The results demonstrate that SF6 is a successful tracer of water masses, and emphasise the potential of this technique for the in situ measurement and manipulation of open-ocean processes.

Observations of chlorofluorocarbons (CFCs), carbon tetrachloride, temperature, and salinity from five sections following the outflow path of Antarctic Bottom Water (AABW) into the southwest Indian Ocean are reported. The transient tracer data clearly show the plume of recently ventilated water whose hydrographic properties are progressively altered by mixing with the overlying waters. We use the CFC measurements to estimate the mean speeds (or transit times) and mixing rates (or dilutions) of the abyssal flow at each section using simple kinematic circulation models. Given our assumptions, the CFC ventilation age equals the transit time. The results suggest a transit time of 23 +/- 5 years (outflow speed of 1.2 +/- 0.3 cm s(-1)) to the Crozet-Kerguelen Gap with a dilution of 8-15 from the surface waters of the Weddell Sea. The estimated horizontal diffusivity is 30-70 m(2) s(-1), and the vertical diffusivity is 3-7 x 10(-4) m(2) s(-1). Combined with the estimate of R. R. Dickson (unpublished data, 1998) for the AABW transport at this point, we conclude that a volume flux of 0.8-1.6 Sv (10(6) m(3) s(-1)) is leaving the continental shelves of the Weddell Sea to eventually enter the abyssal Indian Ocean past Crozet Island.

Atmospheric and oceanic partial pressures of CO2 (pCO2) have been recorded automatically along two Atlantic meridional transects in 1995. The tropical Atlantic ocean (20°S-20°N) is generally a source of CO2 for the atmosphere, but in the region of the North Equatorial Countercurrent an undersaturation of CO2 has been observed. Undersaturations previously reported in the literature are explained by the decrease of salinity due to the high precipitations associated with the Intertropical Convergence Zone. In June 1995, strong CO2 Undersaturations (ΔpCO2 = -70 μatm) were observed near 8°N, which suggests, in addition of the salinity effect, an uptake of CO2 due to biological activity. This undersaturation, although weaker than in spring, also appeared at other periods of the year 1995.

Underway measurements of the partial pressure of carbon dioxide (P(CO2)) in the ocean surface and overlying atmosphere have been made using a fully automated instrument installed on a merchant ship. This ship travelled between the U.K. and the Caribbean with an unattended P(CO2) instrument operating between June 1994 and August 1995. Each voyage was five weeks long with slightly different, repeated routes for the outbound and return transects. Seasonal variability in the ocean surface saturation state of carbon dioxide has been quantified from the two resultant P(CO2) time series. The large scale seasonal changes in a broad region of the mid-Atlantic are consistent with the thermodynamic change in P(CO2) due to seasonal temperature fluctuation. However, to the north and east of the region studied the P(CO2) variation was in antiphase to the temperature signal, due to deep winter mixing and spring-summer biological activity. Large variations in oceanic P(CO2) were also observed on relatively small spatial scales, particularly in coastal and shelf waters, where the effects of phytoplankton activity are more obvious than in the open ocean. Our data return demonstrates the feasibility of unattended data acquisition, a cost effective method of substantially expanding global oceanographic and biogeochemical databases. The design, installation and technical details of the P(CO2) instrument are also described in this paper.

THE equatorial Pacific Ocean is a 'high-nitrate, low-chlorophyll' region where nitrate and phosphate are abundant all year round, These nutrients cannot therefore be limiting to phytoplankton production. It has been suggested that the bioavailability of iron-a micronutrient-may be preventing full biological utilization of the major nutrients(1-3). The results of a previous in situ iron fertilization experiment in this region provided support for this hypothesis(4), but the observed biological response resulted in only a small decrease in surface-water CO2 fugacity(5). Here we report a much larger, biologically induced uptake of surface-water CO2 that occurred during a second study(6). The fugacity of CO2 in the centre of the (iron-fertilized) patch of surface ocean fell from a background value near 510 mu atm to approximately 420 mu atm, corresponding to a transient 60% decrease in the natural ocean-to-atmosphere CO2 flux. We conclude that iron supply to this ocean region can strongly modulate the local shortterm source of CO2 to the atmosphere, but has little long-term influence on atmospheric CO2 partial pressure. However, if such a modulation also occurs in the Southern Ocean, then iron bioavailability at high southern latitudes could have a significant effect on atmospheric CO2 partial pressure(7-11), for example over glacial-interglacial periods.

Antarctic bottom waters have long been known to be a mixture of Circumpolar Deep Water and Shelf Water. Recent observations show that in the Antarctic bottom waters of the Scotia Sea and northern Weddell Sea, the ratios of carbon tetrachloride (CCl4) to chlorofluorocarbons (CFC-11, 12) are inconsistent with the ratios observed in the surface layer of the Weddell Sea. This is the result of a deficit of CCl4 in the bottom waters, and renders the compound unsuitable for use as a transient tracer from which apparent ages can be derived directly. The summer near-surface temperature minimum of Antarctic Surface Water exhibits a similar inconsistency, demonstrating that CCl4 can be removed from cold waters with high oxygen levels, probably through a biological process. It is inferred that Shelf Water features a similar CCl4 deficit which is transferred to the new Antarctic bottom waters upon formation, accounting for the observed deep CCl4 deficit. Copyright 1996 by the American Geophysical Union.

Measurements of surface water dissolved carbon dioxide are presented from two cruises in the Southern Ocean from 88°W (Bellingshausen Sea) to 80°E (Princess Elizabeth Trough) with a number of observations close to the ice edge. The data, collected from early to late Austral summer 1992/1993, indicate that the Southern Ocean in these regions was acting as a sink for atmospheric carbon dioxide at this time. No correlation between carbon dioxide levels and the standing stock of phytoplankton or sea-surface temperature was observed, except in isolated regions of high chlorophyll concentration and across small source regions associated with fronts, respectively. Assuming that the observations can be generalised to the region encircled by the cruise tracks (approximately 20% of the Southern Ocean south of 50°S by area), we calculate C02 uptake for this region during the four months that the cruises took place. Using transfer velocities based on observed winds, we find an uptake of 0.07 Gt C using the Liss-Merlivat (The role of air-sea exchange in geochemical cycling; Reidal Publ. the Netherlands, 1986) parameterisation of transfer velocity, or 0.10 Gt C using that due to Wanninkhof (Journal of Geophysical Research, 97, 7373-7382, 1992) or Tans et al. (Science, 247, 1431-1438, 1990), including an additional flux from the skin effect. No winter-time data are available to assess the sign of the flux annually (sink or source); however, the size of the sink observed during the summer suggests that, if representative of the whole of the Southern Ocean, there is a drawdown of between 0.35 and 0.50 Gt C over 4 months. The observation of a sink is consistent with the most recent estimates of the regional budgets derived from atmospheric isotope data. Most data sets from earlier years show the region as neutral with respect to atmospheric CO2, so it appears possible that the widespread sink we observed, at the very least between 88°W and 80°E, is a recent phenomenon not present in some previous surveys. © 1995.

An automated gas chromatographic technique to measure the concentrations of chlorofluorocarbon 113 (CFC-113:CCl2FCClF2) dissolved in seawater has been developed. The method also quantifies chlorofluorocarbons II and 12 (CFC-11:CCl3F and CFC-12:CCl2F2). Seawater collected from Niskin bottles in ground-glass syringes is stripped by a gas stream and concentrated on a cryogenic trap in the manner of Bullister and Weiss (1988) and Gammon et al. (1982). By isolating and heating the trap, the chlorofluorocarbon compounds are reliberated and injected onto a high-resolution capillary gas chromatographic column, followed by electron-capture detection. The analysis time for each sample is less than 15 min. Surface seawater precisions are 2.9%, 2.4%, and 1.2% for CFC-113, CFC-11, and CFC-12, with detection limits of 0.003-0.004, 0.02, and 0.03-0.05 pmol L(-1), respectively. Although these statistics do not compare favorably with other CFC-11 and CFC-12 techniques (precision similar to 1%, detection limit similar to 0.005 pmol L(-1) (Bullister and Weiss, 1988)), the dynamic ranges of the CFC-113:CFC-11 and CFC-113:CFC-12 ''ventilation ages'' are 20:1, better than that of the best CFC-11:CFC-12 age, albeit with inferior precisions. Estimates of the solubility ratios of CFC-113:CFC-11 and CFC-113:CFC-12 are 0.303 and 1.22, disagreeing with the work of Wisegarver and Gammon (1988), whose CFC-113 results are believed to be boosted by coelution with methyl bromide. The optimum tracer ventilation age resolution is +/-0.9 years for both CFC-113:CFC-11 and CFC-113:CFC-12 from a cast considered in the northeastern Atlantic. A plot of CFC-113:CFC-12 ventilation age is presented on an outcropping isopycnal. A strong correlation with pressure and dissolved oxygen concentration is noted and an oxygen utilization rate between 4.2 and 5.5 +/- 0.4 mu mol L(-1) is implied, depending on the choice of CFC-113 atmospheric history.

Knowledge of the uptake of atmospheric carbon dioxide by the North Atlantic is important in understanding the global carbon budget. Obtaining an accurate value for the ocean sinks in the northern hemisphere temperate zone, in particular, would make it possible to be more quantitative about the enigmatic land sink in these latitudes. The global ocean sink is dominated by the North Atlantic, despite its small area in comparison with the North Pacific. of the possible methods for calculating the uptake, potentially the most informative is that based on the gas exchange equation because information about the seasonal and spatial trends can be obtained. However, there still remain serious questions about which form of the gas exchange coefficient is appropriate to carbon dioxide. Here we summarize current knowledge of the gas exchange coefficient, including recent new evidence which supports lower gas exchange rates and which, therefore, generally supports the Liss-Merlivat prediction for less soluble gases. In view of the uncertainties still surrounding the direct calculation, we investigate methods for calculating the basin-wide uptake by exploiting what is known about the general circulation of the North Atlantic. The uptake can be estimated by dividing it into pre-industrial steady state and anthropogenic contributions. We make a new estimate of the pre-industrial flux based on the heat budget of the North Atlantic, and also consider two earlier calculations of the same quantity. Taking a best value, adding the better-known anthropogenic flux and making small corrections, we find a value of 0.7 +/- 0.15 Gt C a(-1) for the uptake of the North Atlantic, north of 15 degrees N, in the mid 1980s. Though larger than the gas exchange based estimates of Tans et al. (1990), this value is not enough to obviate the need which they deduced for a sizeable 'land-based' northern hemisphere sink for CO2.

A sediment trap experiment was carried out in the Bay of Calvi (Corsica, France) over a Posidonia oceanica seagrass bed, at a depth of 36 m. The traps collected 44 samples over a complete annual cycle and allowed a rough partition between 'sinking' and 'resuspended' matter. The measured annual flux of particulate matter amounted to 1.3 kg m(-2), of which about 70% arose from sediment resuspension, this process being particularly enhanced during northerly gales. Resuspended material consisted mainly of seagrass-derived detritus, as evidenced by SEM photomicrographs. Chemical analyses of trapped material showed seasonal variations in inorganic carbon content, organic C/N ratio and delta(13)C. Benthic signature (C-inorg, C-org- and C-13-enriched Posidonia-derived matter) was emphasized when resuspension occurred, while planktonic features (N- and C-12-enriched matter) prevailed during bloom conditions.

Understanding the: role the oceans play in sequestering anthropogenic CO2 is crucial to understanding global climate change. Correct parameterization of air-sea flux of CO2 is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO2 solubility (Robertson and Watson, 1992) We re-evaluate the effect of the cool thermal skin anti present a high-resolution seasonal estimate of its effect on the air-sea, flux of CO2. We. compare air-sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO2 (Tans et al. 1990), we estimate that excluding the southern ocean, the surface. skin correction increases the air-sea flux of carbon by 0.48 Ct yr(-1). This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Ct yr(-1) Conservatively, adding a. 10% contribution fi om the southern ocean, we estimate a mean global increase. in CO2 flux clue to the skin effect of 0.39 Ct C yr(-1) This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO2 flux anomaly of only 0.17 Gt C yr(-1) which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large-settle flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (-0.75 degrees C) are found during the northern hemisphere winter in the regions of the. Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient during the summer months.

A fully-automated analysis system for the tracer gas sulphur hexafluoride (SF6) is described. The system could be readily adapted to analyse seawater samples of varying volume and a concentration range exceeding 7 orders of magnitude. Automation facilitated a high precision despite continual analysis for periods of 30 days, and the performance of the instrument over a two year period of intensive use is discussed. Background SF6 profiles from different oceans are compared and the surface concentrations utilised to obtain atmospheric concentrations. These are incorporated with other published data to derive a history for atmospheric SF6 concentration. The data suggest an initial exponential rise in atmospheric SF6 to 1979, followed by a linear increase to late 1993 at a rate of 5.5% p.a. With the sensitive analytical system and documented atmospheric history described in this paper, SF6 has potential application as a transient tracer of recently ventilated waters, as demonstrated by comparison with the analytical parameters of the chlorofluorocarbons. © 1994.

IT has long been hypothesized that iron concentrations limit phyto-plankton productivity in some parts of the ocean1-3. As a result, iron may have played a role in modulating atmospheric CO2 levels between glacial and interglacial times4, and it has been proposed5 that large-scale deposition of iron in the ocean might be an effective way to combat the rise of anthropogenic CO2 in the atmosphere. As part of an experiment in the equatorial Pacific Ocean6, we observed the effect on dissolved CO2 of enriching a small (8x8 km) patch of water with iron. We saw significant depression of surface fugacities of CO2 within 48 hours of the iron release, which did not change systematically after that time. But the effect was only a small fraction (∼10%) of the CO2 drawdown that would have occurred had the enrichment resulted in the complete utilization of all the available nitrate and phosphate. Thus artificial fertilization of this ocean region did not cause a very large change in the surface CO2 concentration, in contrast to the effect observed in incubation experiments3, where addition of similar concentrations of iron usually results in complete depletion of nutrients. Although our experiment does not necessarily mimic all circum-stances under which iron deposition might occur naturally, our results do not support the idea that iron fertilization would signifi-cantly affect atmospheric CO2 concentrations. © 2002 Nature Publishing Group.

Measurements of the carbonate system in the surface waters of the northeast Atlantic during summer 1991, following the main growth phase of a bloom of the coccolithophore Emiliania huxleyi are presented. We examine the perturbation of the carbonate system and assess the effect of calcification on the air-sea gradient of dissolved carbon dioxide in the surface ocean. An estimate of 1:1 organic to inorganic carbon uptake is calculated using the measurements of the surface carbonate parameters which is consistent with other estimates for E. huxleyi populations using radio-tracer methods. Using the changing ratio of dissolved carbon dioxide to nitrate concentration we demonstrate a relative increase in dissolved carbon dioxide due to calcification with evidence of this increase supported by estimates of the buffer factor and C:N assimilation ratios. Within the E. huxleyi bloom the effect of calcification on alkalinity appears to have reduced the air-sea gradient by ∼ 15 μatms (corrected to a constant temperature) using measurements from a 440 km section along the 20°W meridian. This reduction could prove to be significant in terms of the overall drawdown of carbon during the spring-summer season in this area. © 1994.

A method for rapid determination of the partial pressure of carbon dioxide at the sea-surface is described. The method was employed along with a pulsed oxygen electrode to monitor daily changes in surface pCO2 and O2 close to a drifting buoy deployed at approximately 59°N, 20°W. During a 4 day period a gradual rise in oxygen saturation and corresponding fall in pCO2 was observed in the surface layer. Corrections are made for gas exchange of O2 using wind speed data, the correction being an important fraction of the supersaturation observed in the water. Estimates of net community production and photosynthetic quotients are derived, giving a range of PQs from 0.9 to 1.5. Though variations in the local hydrography reduce the accuracy of these estimates, the potential of this approach to estimate productivity appears promising. © 1992.

THE distributions of heat, salt and trace substances in the ocean thermocline depend on mixing along and across surfaces of equal density (isopycnal and diapycnal mixing, respectively). Measurements of the invasion of anthropogenic tracers, such as bomb tritium and 3He (see, for example, refs 1 and 2), have indicated that isopycnal processes dominate diapycnal mixing, and turbulence measurements have suggested that diapycnal mixing is small3,4, but it has not been possible to measure accurately the diapycnal diffusivity. Here we report such a measurement, obtained from the vertical dispersal of a patch of the inert compound SF 6 released in the open ocean. The diapycnal diffusivity, averaged over hundreds of kilometres and five months, was 0.11 ± 0.02 cm 2 s-1, confirming previous estimates1-4. Such a low diffusivity can support only a rather small diapycnal flux of nitrate into the euphotic zone; it justifies the neglect of diapycnal mixing in dynamic models of the thermocline25-27, and implies that heat, salt and tracers must penetrate the thermocline mostly by transport along, rather than across, density surfaces. © 1993 Nature Publishing Group.

The most widely used approach for calculating the flux of gases across the sea surface is from the product of the concentration difference across the interface and a kinetic parameter, often called the transfer velocity. During the NERC North Sea Community Research Project (CRP) a considerable effort was made to improve our knowledge of both of these terms. Concentration measurements were made on nine survey cruises (February to October 1989) for dimethyl sulphide (DMS) (and its precursor dimethylsulphonio-proprionate DMSP, both dissolved and particulate), as well as for a variety of natural and man-made low molecular mass halocarbons. To better define the relationship between transfer velocity and wind speed a novel double tracer technique was used on two of the process cruises in the North Sea CRP. The tracers added to the water were SF6 and He-3 and from the measured change in their concentration ratio over time, four estimates of the transfer velocity were made, one at a rather high wind speed (ca. 17 m s-1). The results are in general agreement with the relationship of Liss & Merlivat (1986) based on laboratory and lake studies and theoretical considerations, and constitute their first real test at sea. Combining the above results for the transfer velocity with the detailed concentration fields measured in the CRP has enabled us to calculate fluxes across the sea surface for the measured gases with a much finer time and space resolution than was possible hitherto. Some implications of the calculated fluxes for atmospheric chemistry in Europe are discussed.

The characteristic features of pedogenesis in deserts result largely from aridity but the main classification systems incorporate very few diagnostic criteria for identification of desert soils. The main soil classification systems are described and there is a section on soil mapping in deserts. Parent materials are rarely sand dunes, but mostly extensively gravelly or sandy plains, and mountainous and rocky terrain. Weathering processes include unloading, wetting and drying and salt weathering. Clay pedogenic horizons and saline sodic soils are particularly characteristic of desert areas, as are soil crusts. Diagenetic features are described as well as the relationship between desert soils and geomorphology and the occurrence of paleosols. -K.Clayton

The overall background to the U.K. BOFS (Biogeochemical Ocean Flux Study) Project, designed to investigate oceanic carbon flux processes throughout the water column, is briefly described together with the strategy for the 1990 BOFS Spring Bloom Experiment. The Experiment involved two ships and was carried out in the northeast Atlantic between 46-50°N, 14-22°W in the period 18 April - 25 June 1990 with the objective of monitoring and quantifying the major carbon flux changes associated with the succession of the spring bloom. Sampling was carried out over a 7 week period adjacent to a Lagrangian buoy drogued at 30m. The spatial fields of the major variables were characterized from box grid surveys around the position of the marker drogue at the beginning and end of the time series observations with the time series hydrographical changes being related to features observed in the spatial surveys. The hydrographical and core biological observations made in the Experiment are described and interpreted. The reference drogue was deployed within an anticyclonic eddy in which initially there was little evidence of seasonal thermocline or phytoplankton develooment. The majority of an array of 30m drogues placed around the reference drogue drifted between 75-150km north and east of their origin, probably exiting from the original eddy system after the first 6 days of deployment. The reference drogue moved anticyclonically around the eddy centre for the first 13 days before exiting from the eddy system and becoming entrained in a discontinuity zone located between discrete warmer and cooler water bodies defined between 50-200m. During this latter period, which continued through to the end of the Experiment, the drogue tracked SE overall and alternately grazed the margins of the two water bodies with greater drift speeds being associated with the influence of the cooler water. Phytoplankton development proceeded slowly over the period that the drogue remained in the original eddy and paralleled the gradual development of the seasonal thermocline. A marked increase in phytoplankton occurred concurrently with the exit of the buoy from the eddy system into the boundary region of the cooler water where increased stratification prevailed. The phytoplankton increase persisted for only 6 days and declined sharply, primarily owing to advective influences, as the buoy moved away from the cool water influence with chlorophyll values remaining low for the remainder of the Experiment. Mesoscale influences were observed to have a major influence on the development sequence of the spring bloom in this area of the northeast Atlantic. © 1992.

By representing growth, decay and vertical mixing during the spring phytoplankton bloom as a series of rate constants, a simple model is constructed that predicts phytoplankton abundance, carbon dioxide concentration and oxygen saturation as continuous functions of latitude and time. The predictions are compared with surface distributions of the partial pressure of carbon dioxide (pCO2), total dissolved inorganic carbon (TCO2), chlorophyll and oxygen saturation mapped during May 1989 on a series of surveys between 47 and 60°N in the eastern North Atlantic near 20°W. Altoough the observations were strongly variable on spatial scales of less than 100 km, the systematic changes revealed in the transects are quantitatively described by the theoretical expressions. Total vertical fluxes of carbon can be calculated from the model, and these can be integrated temporally and spatially. During the course of the spring bloom approximately 5 g m-2 of carbon entered the ocean surface from the atmosphere in the northeast Atlantic and the potential net loss of carbon to the deep ocean was about 16 g m-2. © 1992.

An understanding of the natural sources and sinks of atmospheric carbon dioxide is necessary for predictions of future atmospheric loading and its consequences for global climate. Present estimates of emissions and uptake do not balance1, and although some have attributed the imbalance to a terrestrial sink2, the magnitude of the oceanic sink remains unresolved2-4. It is known5-8 that the upper 1 mm or so of the oceans represents a cool 'skin', with a temperature gradient such that the surface is generally cooler than the bulk mixed layer by about 0.3°C as a result of the upward heat flux. Here we discuss the consequences of the 'skin effect' for the global air-sea flux of CO2. We show that it produces an increased oceanic global uptake of about 0.7 Gt C yr-1, when the flux from the atmosphere to the oceans is calculated on the basis of measured surface-ocean partial pressures of CO2. This correction helps to bring into closer agreement the oceanic CO2 uptake calculated by different methods2-4. © 1992 Nature Publishing Group.

Analysis procedures and instrumentation to measure low levels of sulfur hexafluoride in seawater are described. The minimum detectable level for a 500 mL sample is about 3 x 10(-17) mol/L. Concentrations from 1.5 to 300 x 10(-15) mol/L can be measured with approximately 2% precision. The procedure includes a concentration step employing a trap of Porapak-Q(c) in a dry ice/2-propanol bath. The system has been used for tracer experiments in Santa Monica Basin and Santa Cruz Basin, and will be improved for future tracer release experiments in the deep ocean.

THE transfer of dissolved atmospheric gases across the air/sea interface is a strong function of wind speed and state of the sea surface. The form of the dependence on wind velocities is needed to calculate oceanic sources and sinks of climatically significant gases, particularly carbon dioxide1 and dimethylsulphide2,3. Previous measurements at sea have used techniques that reveal this dependence only when averaged over weeks or months4,5, making it impossible to study the effects of high winds, which are generally episodic. Here we report first results from the application of a tracer technique which enables accurate measurements to be made over periods of 24-72 hours, including the first determination of gas exchange in storm conditions. Our results support the wind-speed dependence suggested by Liss and Merlivat6 on the basis of lake and wind-tunnel experiments, and are consistent with most measurements made by the radon-deficit technique. They suggest slower rates for CO2exchange than those obtained by 14C budgeting, and favour recent lower estimates1,7 of the global oceanic sink for anthropogenic CO2. © 1991 Nature Publishing Group.

Two new types of mechanical integrating samplers have been tried in the deep sea, one driven by a constant force spring, the other by a spring-driven hydraulic system. The samplers are triggered simultaneously with the Niskin bottle to which they are attached and fill in about 90 min. They are useful for sampling poorly mixed tracers, and have been used successfully in tracer release experiments at 800 m depth in Santa Monica Basin in 1985, and to depths of 1900 m in Santa Cruz Basin in 1988.

DIRECT calculation of the air-sea flux of CO2 requires detailed knowledge of the partial pressure of carbon dioxide (P CO2) and gas-transfer velocities at the surface of the global ocean1, with the available observations of surface P CO2 suggesting that it varies in a smooth manner with season and position over the major ocean regions2-5. In spring 1989 we mapped surface P CO22, total inorganic carbon (TIC), chlorophyll, temperature and salinity at several locations between 47°N and 60°N in the northeast Atlantic near 20°W, observing large variations in P CO2 on spatial scales of ≪100 km, Correlated with plankton chlorophyll, surface temperature and TIC. The variation of P CO22 with latitude was in the opposite sense to that previously reported for this region3-5. Thus, in this ocean area and season at least, the air-sea flux is strongly modulated by biological activity and variable on short spatial scales. The inhomogeneity observed suggests that estimates of the oceanic sink for fossil fuel inferred from existing data (relatively sparse even in the North Atlantic) will be subject to significant error. © 1991 Nature Publishing Group.

Methods are described for the preparation and release of sulphur hexafluoride (SF6) and 3He to surface sea water for studies of gas exchange and as oceanographic tracers. Known amounts of the tracers (0.28 mol SF6 and 0.05 mol 3He, measured by gas chromatography with thermal conductivity detection) were dissolved in 1000 l of sea water in a gas-tight steel tank and later released at a depth of 10 m without modification of the initial 3He: SF6 ratio, by excluding all air from the tank. Techniques for deploying the tracers as a discrete patch in shallow, tidally active waters (30 ± 2 m, S.E. North Sea) and the labelling of the patch with drifter buoys are also given, Concentrations of SF6 in the patch were mapped from a survey vessel using an analysis system that determined concentrations in surface water by purge and trap followed by gas chromatography with electron-capture detection, in a fully automated, continuous sequence of analyses with a 3-min repeat time. Data were computer-matched with the ship's navigational data and used to construct a near real-time plot of the tracer patch in a reference frame which moved with the water mass. The plot was used to aid further navigation of the ship around the patch. The results from a typical mapping exercise are presented. © 1991.

We have studied cross isopycnal (diapycnal) and lateral mixing and stirring below the sill of Santa Monica Basin by releasing two tracers, sulfur hexafluoride and perfluorodecalin, as close as possible to an isopycnal surface and measuring their subsequent dispersion. The target for the release was a potential temperature surface at about 790 m depth, roughly 100 m above the bottom and 50 m below the sill. Three surveys, performed immediately after, about 7 weeks after, and about 6 months after the release, showed that the time scales for lateral stirring and mixing in the basin were between 2 and 5 months. The diapycnal diffusivity for the whole period was found to be 0.29 +/- 0.06cm2/s near the injection surface, where the buoyancy frequency was about 1.1 cph. This estimate may include some mixing in the turbulent boundary layer near the walls of the basin. Our best estimate for the diapycnal diffusivity in the basin interior is 0.25 +/- 0.08 cm2/s.

We describe the methods used to prepare and inject sulfur hexafluoride (SF6) and perfluorodecalin (PFD) for the Santa Monica Basin tracer experiment. The use of two tracers gave quasi-independent estimates of mixing rates in the basin, as well as a check on faults in the injection procedure. PFD was the easier substance to inject, but SF6 proved to be the most useful tracer for two reasons: the SF6 measurements had a higher signal-to-noise ratio, and PFD showed some tendency to adsorb onto the surfaces of the sampling gear, whereas SF6 was completely inert in this respect. This surface affinity suggests that PFD may be transported to some extent by settling particles. The PFD profiles obtained 6 months after injection show some enrichment towards the bottom relative to SF6, which might be due to particulate transport.

THE renewal of the deep water of the world's oceans is accomplished through the input of dense water in both the northern North Atlantic and around Antarctica but direct measurement of the renewal rate has proved elusive. Here we describe the first successful long-term measurement of the transport of the northern inflow component where it passes south along the Continental Slope off East Greenland. There we observe a cold, bottom-intensified, long-slope flow with maximum speeds of around 100 cm s-1, mean speeds of up to 25-30 cm s-1, a dominant fluctuation timescale of a few days period but with little seasonal or interannual variation in speed or direction. We estimate the transport at densities (σΘ)≥Combining double low line27.80 to be stable at 10.7x 106 m3 s-1 and propose a circulation scheme for waters of this density throughout the northern gyre. © 1990 Nature Publishing Group.

The experimentally-determined relationships between air-water gas transfer velocity and windspeed are presented for two small, rapidly wind mixed lakes in upland SW England. The results significantly extend the existing field database and show a strong dependence of gas transfer velocity, k, normalised to CO2 at 20°C, on windspeed in the range ~2-13 m s-1, corrected to a height of 10 m. The data are fitted with two least-squares straight lines which intersect at a windspeed of 9.5±3 m s-1(at z=10 m), beyond which significant steepening of the k versus windspeed relationship implies a transition from the "rough surface' to "breaking wave' regime. The observed relationships between k and windspeed are not unique. -from Authors

Aerosol concentrations of methanesulphonic acid (MSA), dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2) and major anions have been measured from landbased stations (principally Plymouth, Devon, U.K.) and various shipboard stations in the North Sea and North Atlantic Ocean. Aerosol samples collected between July 1985 and July 1987 are analyzed both in terms of their back trajectories, and variation with time. These analyses suggest that NO3 and NSSS are anthropogenic in origin while DMSO2 appears to have a maritime source. MSA concentrations are highest in air masses with both oceanic and anthropogenic influences. DMSO, MSA, and DMSO2 all show seasonal cycles, with concentrations similar to previous published results. MSA was found to be concentrated on sub-μm particles. Rainfall MSA concentrations were measured over 2 years at Norwich and were found to exhibit a similar seasonality to the aerosol MSA concentrations. © 1990.

Considers the design criteria for installations to control blowing sand (promotion of drifting, barriers and fences, vegetation belts, enhancement of sand transport to scour away problem sand, surface treatments to reduce supply of sand and deflection of moving sand). Explores the morphology and behaviour of desert dunes and the control of mobile dunes (ie whole dune movement as opposed to drifting sand). -C.J.Barrow

The main types of desert crust - silcrete, calcrete, gypsum and halite - are often the products of mineral precipitation from evaporating water. Desert crusts have an important geomorphic influence in many of the earth's arid regions. Ancient crusts can be extremely useful palaeoenvironmental indicators. -from Author

Bagnold's insight into the physical principles covering sand movement remains fundamental. The topic is examined under the headings of wind dynamics and sand movement, including the computation and measurement of sand transport rates. The second half of the chapter discusses the origin and nature of aeolian sediments, including an outline of specific gravity, grain size and sorting, and grain shape. -K.Clayton

Desert gypsum crusts are of three main types: shallow-water evaporites, which are characterized by size-graded beds; subsurface crusts, which are either macrocrystalline groundwater evaporites, or mesocrystalline illuvial accretions; and surface crusts-excluding the bedded type-which are exhumed illuvial crusts. The exposure and degradation of illuvial, pedogenic crusts are characterized by distinct diagenetic features which can provide detailed information about the geomorphic history and palaeoenvironments of many arid regions. -from Author

A plan for a series of tracer release experiments to measure diapycnal mixing in the open ocean during the next two decades is presented. These experiments can be performed on scales of 500km × 12 months using a few hundred kg of fluorinated tracers such as sulfur hexafluoride. Various hypotheses for the processes controlling diapycnal mixing, and formulas based on these hypotheses, are briefly reviewed. The physical measurements that must be included in the tracer release experiments to test these hypotheses are discussed. © 1988, Elsevier B.V.

Aerosol concentrations of methanesulphonic acid (MSA), dimethyl sulphoxide (DMSO) and dimethyl sulphone (DMSO2) have been measured from landbased stations at Plymouth (Devon, U.K.), Galway (EIRE), and from various shipboard stations in the North Sea and the North Atlantic Ocean. MSA, DMSO and DMSO2 all show seasonal cycles with spring/summer maxima and winter minima. The summer concentrations of MSA are approximately an order of magnitude higher than in winter. The general levels of MSA (July 1985 mean = 9.27 × 10-9 mol m-3, December 1986 mean = 1.14 × 10-9 mol m-3) are comparable to those reported from Cape Grim, Tasmania. Modelling indicates that neither MSA nor DMSO2 are present in sufficient quantity to represent major oxidation pathways for dimethyl sulphide (DMS). Rate constant ratios for both the reactions of DMS and DMSO with OH and IO have been estimated. Hydroxyl radical does not appear to be reactive enough for it to be the major sink of atmospheric DMS. It is also shown that the rate constants for the destruction of DMSO (the main reaction product of the DMS/IO system) with either IO or OH are likely to be slow. Thus low tropospheric concentrations of DMSO tend to indicate that it also is not a major product of DMS oxidation. © 1987.

The grain-size characteristics of the sand upon two dunes-a 40 m high longitudinal dune in the central Namib Desert and a 6.0 m high barchan in the Jafurah sand sea of Saudi Arabia-vary with position on the dunes. On the longitudinal dune, median grain size decreases, sorting improves and the grain-size distributions are less skewed and more normalized toward the crest. Though sand at the windward toe is distinct, elsewhere on the dune the changes in grain-size characteristics are gradual. An abrupt change in grain size and sorting near the crest-as described by Bagnold (1941, pp. 226-229)-is not well represented on this dune. Coarse grains remain as a lag on concave slope units and small particles are winnowed from the sand on the steepest windward slopes near the crest. Avalanching down slipfaces at the crest acts only as a supplementary grading mechanism. On the barchan dune median grain size also decreases near the crest, but sorting becomes poorer, though the grain-size distributions are more symmetric and more normalized. The dune profile is a Gaussian curve with a broad convex zone at the apex upon which topset beds had accreted prior to sampling. Grain size increases and sorting improves down the dune's slipface. However, this grading mechanism does not influence sand on the whole dune because variations in wind regime bring about different modes of dune accretion. On both dunes, height and morphology appear to influence significantly the grain-size characteristics. © 1986.

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block‐strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter‐inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium‐rich granitoid rocks weather more slowly than less potassic rocks. Copyright © 1986 John Wiley & Sons, Ltd

53 closed depressions, from 50 m to 400 m in diameter and up to 5 m deep, have been identified. Frequently occurring in pairs or groups, the density of the pans reaches 0.3/km2. All of the depressions are located on remnants of the tilted African Plantation Surface. This is preserved in the region because the Jurassic rhyolites are less susceptible to weathering than are the surrounding basaltic and sedimentary rocks, which have been eroding since the Miocene. The closed depressions develop on the tuffaceous horizons within the acid volcanic sequence. They are probably the result of the local surface lowering, resulting from dissolution of plagioclase weathering products and possibly silica, in which case they are karstic. Though the presence of thick ferricretes suggests that the pans are of considerable age, possibly more than one million years old, there is no evidence of aeolian modification during the late Pleistocene arid phase. -from Author

Domed inselbergs which rise above the erosional plains occur on granitoid migmatite outcrops which are evidently more resistant to weathering and erosion than the surrounding gneisses. Regolith overlying the Basement rocks in the northern half of the Reserve is generally thin and contains unusually large amounts of silt. This reflects the importance of granular disaggregation relative to chemical decomposition during weathering under semi-arid conditions. Calcretes and dolocretes are developed on outcrops of bedrock containing large amounts of Ca and Mg respectively, and in sediments along some drainage channels. -from Authors

At 47 locations in Swaziland coarse, fluvial deposits, termed the Badzala gravels, have been identified at heights of up to 79 m above present river levels. They are generally at heights of between 20 m and 35 m. The pebbles and cobbles making up the deposits are invariably composed of rock types which are highly resistant to weathering; quartzites predominate. On the basis of the heights, the lithologies, the weathering features and the typologies of the associated stone tools, the gravel deposits are ascribed an Early Pleistocene age. A lower, younger gravel, the Shongololo, is probably derived from reworking of the Badzala and therefore does not represent a primary fluvial deposit. The most likely mechanism for the production of the Badzala gravels is tectonic uplift. -from Authors

In the Mdzimba Hills of NW Swaziland huge exposed domes of coarse-grained granite exhibit a variety of micro-relief features which are attributed to the structural, textural and mineralogical fracturing of exfoliated sheets produces the rectilinear patterns. Pans also develop on the flatter, upper surfaces where water collects in hollows, enhancing the weathering of the granite. Karren are confined to exposures of microgranite on the flanks of the domes; their formation appears to be related to accelerated weathering in zones where the rock has more grain boundary cracks. Crazed and honeycombed surfaces are also weathering features; the former are associated with indurated veneers on granite boulders, and the latter with zones of granite containing more readily weathered epidote-bearing xenoliths. -from Authors

On a time scale of several decades, an increase in the atmospheric burden of certain stable trace gases results in a characteristic oceanic depth profile for the concentration of the dissolved gas. If the atmosphere is the only source of the gas to the sea, the time delay inherent in its downward penetration from the surface results in a profile which decreases with depth. By referencing to compounds such as Freon 11 or Freon 12, the atmospheric histories of which are relatively well known, limits can be placed on the increase of a trace gas whose history is unknown. The method may be particularly valuable in distinguishing the contributions of natural and anthropogenic sources of gases such as CCl4 and CF4, which may have both. The method is here applied to estimate the concentration of atmospheric SF6 since 1970. Both exponential and linear fits are investigated, but the best fit is a linear increase, C = 0.34 + 0.084 (Yr-1970), where Yr is the calendar year and C is the concentration in pptv. A preliminary look at two CCl4 profiles suggests that at least 50% of the atmospheric burden is of recent anthropogenic origin. © 1985.

Gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0.10m to 5.0 m thick containing more than 15% by weight gypsum (CaSO4·2H2O) and at least 5.0% by weight more gypsum than the underlying bedrock. The deposits are often, but not invariably, consolidated owing to cementation by gypsum. The crusts are found in many of the world's deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year. Using structural, fabric and textural criteria, three main types of crust may be distinguished:(1) bedded crusts, found either at or beneath the land surface, which are made up of discrete horizontal strata up to 0.10 m thick, each showing a gradation in gypsum crystal size from less than 50 μm at the top to more than 0.50 mm at the base; (2) subsurface crusts, of which there are two forms, one made up of large, lenticular crystals (up to 0.50 m in diameter)—the desert rose crusts—and the other, a mesocrystalline form, with gypsum crystals up to about 1.0 mm in diameter; and (3) surface crusts, which are subdivided into columnar, powdery and cobble forms, all of which are made up of predominantly alabastrine gypsum (crystallites less than 50 μm in diameter). In southern Tunisia and the central Namib Desert, bedded crusts are found around ephemeral lakes and lagoons. They are characterized by size‐graded beds, gypsum contents of 50–80% by weight and comparatively high concentrations of sodium, potassium, magnesium and iron. They are interpreted as shallow‐water evaporites which accumulate when saline pools evaporate to dryness. Desert rose crusts or croûtes de nappe generally contain 50–70% by weight gypsum, and have higher sodium concentrations than the second subsurface form. Texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals. They are interpreted as hydromorphic accretions, which precipitate in host sediments at near‐surface water tables through the evaporation of groundwater. The second form of subsurface crust—the mesocrystalline—often occurs in close association with the various surface forms. Unlike the hydromorphic crusts, they are not restricted to low‐lying terrain. They are characterized by gypsum contents reaching 90% by weight, and have a close chemical and textural similarity to columnar surface crusts. This mesocrystalline form represents an illuvial accumulation; the surface forms—excluding the bedded crusts—are exhumed examples at various stages of solutional degradation. Subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface, results in displacive gypsum accumulation in the soil zone. In southern Tunisia, the gypsum is derived from sand and dust deflated from evaporitic basins; in the central Namib, salts dissolved in fog water are the most likely source. Where other salts are present, differential leaching may form two‐tiered crusts, calcrete—gypsum or gypsum—halite, if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble. Gypsum crust genesis is restricted to arid environments, and if their susceptibility to post‐depositional alteration is acknowledged, they can provide valuable palaeoclimatic indicators. Copyright © 1985, Wiley Blackwell. All rights reserved

In the Eastern Province of Saudi Arabia drift rates reach 30 m3/m width annually and barchan dunes, up to 25 m in height, have an average rate of movement of nearly 15 m per annum. The problems caused by wind blown sand can be tackled in four ways: 1) by enhancing its deposition, using ditches, fences or tree belts; 2) by enhancing its transport, using streamlining techniques, creating a smooth texture over the land surface or by erecting panels to deflect the air flow; 3) by reducing the supply of sand upwind, using surface stabilizing techniques, fences or vegetation; 4) by deflecting the moving sand using fences or tree belts. In Saudi Arabia stabilization of 2.0 m wide strips, laid perpendicular to the direction of dune movement, using oil-based materials, is being widely adopted. -from Author

Rectangular blocks of York Stone and of concrete placed on a sodium chloride sabkha in southern Tunisia for six years suffered very severe breakdown, thereby indicating the power of salt weathering as a process in sabkha environments. Copyright © 1984 John Wiley & Sons, Ltd

These 'islands' are in fact inselbergs which rise precipitiously above the Kenyan countryside. The differential weathering may be due to minor differences in rock composition. The critical constituent appears to be potassic feldspar. -E.Turner

Colluvial deposits, reaching thicknesses of more than 10 m, mantle the pediment slopes and choke the streams and river valleys of an area covering about 20% of Africa south of the Zambezi. The sediments are usually deeply incised by gully systems, termed dongas, which have exposed abundant archaeological remains in the form of stone tools dating from the African Middle Stone Age. While the bedrock types in the different areas exhibiting colluvium vary greatly, the range in present mean annual rainfall is very narrow, being between 600 and 800 mm. The grain size characteristics of 55 samples of colluvium show a broad similarity in the percentages of sand-, silt- and clay-sized material. In the majority of cases the percentage of sand ranges from 45 to 65%, silt from 15 to 25% and clay from 10 to 35%. The 14C ages of palaeosols in the sediments in Swaziland, Natal and Zululand, based upon the dating of calcium carbonate nodules and charcoal from cave sediments containing identical stone tool assemblages, indicate that colluviation took place between about 30,000 and 12,000 years ago. The palaeosols and colluvial sediments of southern Africa are indicative of semi-arid conditions in which the regolith was stripped from poorly vegetated hill slopes and laid down on gently inclined pediment slopes. The age of the deposits supports the contention of several recent workers that the area experienced dry climatic conditions during the Last Glacial Maximum as a result of restricted circulation and reduced sea surface temperatures in the Southern Ocean. © 1984.

Covers physico-chemical considerations, distribution, field occurrence and geomorphic relations, chemistry and mineralogy, micromorphology and genesis. Brief concluding sections cover the palaeoenvironmental significance and occurrence in the stratigraphic record.-K.Clayton School of Geog. Univ. of Oxford, UK.

Includes not only gypsum crusts, but also gypsum soils and aeolian dunes incorporating gypsum. There is a map of the main areas of gypsum crusts: they are restricted to mainly arid regions. Origin, geomorphological relationships, micromorphology, chemistry and mineralogy are described. Concluding sections cover paleoenvironmental significance and occurrence in the stratigraphic record.-K.Clayton

Particle size analyses, chemical determinations and electron microscopy suggest that the colluvia were deposited under very dry conditions, resulting in a reduction of upland vegetation and the formation of colluvial fans. Conversely pedogenesis and river terrace formation are thought to relate to moister conditions. - from Authors

Geomorphological evidence and associated stratified ‘open’ sites have enabled a reconstruction to be made of fluctuations in palaeoclimate and prehistoric activity during the Late Pleistocene over an area of Africa south of the Zambezi exceeding one million square kilometres. Radiocarbon dates from relevant sites in Swaziland suggest that the Middle Stone Age flourished over a large part of the interior of southern Africa between 30,000 and 20,000 years b.p. during which time the south‐eastern part of Africa was experiencing climates considerably more arid than at present. © 1982 Taylor & Francis Group, LLC.

Recent work in Swaziland, south-east Africa, has for the first time elucidated a complex but comprehensible sequence of Quaternary sediments, which have considerable bearing on human activity against a background of climatic change. Stratigraphically distinct riverine and colluvial deposits reflect various fluctuations from more humid to arid conditions. These deposits are integrally linked to various successive phases of the southern African Stone Age. Radiocarbon dates provide a chronological framework for colluvial formations which are both a major geomorphological phenomenon and which appear to be widespread in southern and central Africa.-from Authors

L. Trafton (1980, Icarus 44, 53-61) has pointed out that a substantial methane atmosphere, observed on Pluto by U. Fink, B.A. Smith, D.C. Benner, J.R. Johnson, and H.J. Reitsema (1980, Icarus44, 62-71), appears to be unstable against blowoff. The difficulty is shown to disappear if the actual heat balance and thermal structure are considered, instead of the classic assumption that the upper atmosphere is isothermal. An energy-limited flux (referred to the surface area) of 3.9 × 1010 cm-2 sec-1 is found. The loss of methane ice over the age of the solar system is an acceptable 3 km. © 1982.

This is a review of the Gaia hypothesis which postulates a condition of planetary homeostasis affecting chemical composition and climate. Some criticisms are answered and a new model is introduced for the long term regulation of the mean surface temperature through the biological control of CO2 partial pressure. © 1982.

Geared towards the UK A-level student, the book provides up to date information on the following areas of desert geomorphology. The great variety of desert landscapes, weathering, desert crusts, sand dunes, wind erosion, climatic change and an explanation of desert landform equifinality and uniqueness. -M.Higgins