Coring

Close up of core head being carefully removed from moss bank, Elephant Island, South Shetland Islands, 2012.  The moss bank freezes solid during the winter, with the top 30 cm thawing in the austral summer – thus we needed to use a modified ice corer to cut into the frozen peat, which is a solid as rock.

Core hole

Looking down a frozen core hole.

Slicing a core in half

Slicing a core in half, back in the lab in Cambridge.

Sliced core section

Sliced core section showing the layers of moss growth wonderfully preserved by freezing.

Sliced core section

Monolith from the top section of a core showing the slither of annual green growth.

Testate amoebae

The testate amoeba Nebela carinata (not currently found in our sites)

Testate amoebae

The testate amoeba Arcella artocrea (not currently found in our sites)

How?   

Here we provide some more information on the methods we’re using to reconstruct past climate change.  Whilst the glamorous side of a project like this one is always the fieldwork trip to obtain the cores we will work on, the real work is performed over the following years in laboratories back in the UK.  In explaining about the different methods we are using, we hope to also provide an insight into the daily life of a research scientist, which is often repetitive and not at all glamorous!

Coring – life in the field

Because the moss banks we study grow up very slowly over time, if we now stand on their surface and take a core down through those accumulated layers, we are collecting a record of the bank’s growth over time.  Some moss banks in the Antarctic Peninsula have been growing for many thousands of years.  Certain aspects of their growth and the ecosystem they support (most simply growth rates, but also isotope ratios and types of amoebae – see more below) change as climate (for example temperature or the amount of rainfall) changes.  Therefore if we take samples down through our core and analyse them, we are effectively creating a record of how certain aspects of climate have changed in the past.  There’s a lot of detailed science that goes in to making sure that we understand the link between climate and the ‘proxy’ method we are using.

In some parts of the world, taking a core is a relatively simple task and can be achieved by manpower alone.  However, apart from a thin zone near the surface, the moss banks we are studying are frozen solid.  This means we need to use a special permafrost corer which is run by a small engine attached to the top.  What comes out the end is a quite beautiful (depending on your perspective!) frozen core of moss, in which you can see many different layers that show how the moss has built up over time.  If all this equipment needs to be carried to the top of a hill to take a core, it can be a lot of hard work!

Slicing – life in the lab

Most of the hard work on a project such as this one is done back in the lab once the cores arrive back in the UK.  Firstly, the cylindrical cores must be sliced in half length ways.  One of these halves is then preserved, intact and frozen, as part of a scientific archive.  The second half is then precisely sliced in to half-moon shaped sections all ten millimetres in width, which represents about ten years of growth.  Once again, the layering in the cores can be clearly seen when they are sliced open.  Each slice then undergoes several different types of analyses and we then combine all the different data gathered about each slice, or time point, to help us understand what the environmental conditions were like when the moss was growing.  This technique of combining multiple analytical methods is known as a “multi-proxy approach”.  The two main methods we’re using are called isotope analysis and testate amoebae analysis.

Isotope analysis

Isotopes are rare, alternative forms of elements that have essentially the same chemical characteristics but different masses.  Chemical characteristics are largely determined by the number of positively charged protons and negatively charged electrons that comprise an atom and this differs between all elements.  

For example, hydrogen has one proton and one electron, helium has two protons and two electrons.  However, the mass of an atom is determined by the number of protons AND the number of neutrons in the nucleus and this affects the physical characteristics of an atom – for example how quickly it will move (diffuse).

Some isotopes decay over time.  For example, carbon atoms with six protons and eight neutrons (known as carbon-14) break down into nitrogen atoms (with seven protons and seven neutrons) and the rate of this decay can be used to estimate the age of biological material up to about 60,000 years old in a technique known as radiocarbon dating.  These radioactive isotopes tend to be very rare; carbon-14 makes up less than 0.001% of all carbon atoms.

However, there are other isotopes that do not change in abundance over time.  Carbon-13 (six protons, seven neutrons) is a stable isotope of carbon, which makes up 1.1% of all carbon atoms, whilst the remaining 98.89% of carbon atoms (carbon-12) have six protons and six neutrons.

In our analysis we exploit the fact that carbon-12 and carbon-13 atoms behave slightly differently and measure the proportion of each type of carbon in the moss organic matter. This can give us an indication of the environmental conditions at the time that the CO2 was fixed, through the process of photosynthesis, by the moss, as in wetter conditions there tends to be a higher proportion of heavy CO2 fixed and preserved in the organic matter.

Testate amoebae analysis

Testate amoebae are microscopic, single-celled organisms that live in the tiny water films around moss leaves.  Testate means ‘shelled’ and the amoebae are called this because they grow a shell around themselves.  Whilst the amoebae die and decay over time, these shells are preserved.  If we take a moss sample from our core and sieve it down to remove the larger remains, we can put the residue on a microscope slide and hunt for the amoeba shells.  With even the largest amoebae put end to end, you could fit five into a millimetre.  The smallest amoeba are only 0.02 of a millimetre long, so we have to use very powerful microscopes to find them.

There are many different species of amoeba and each one has a particular habitat preference.  For example, some prefer to live in wetter conditions and others in drier conditions.  As climate changes over time, it effects the living conditions in the moss bank and the species of amoebae present change in response. 

By studying live amoebae that live near the surface, we can understand their habitat preferences quite precisely.  In each sample that we take from our core, we find a certain number of amoeba shells and calculate the proportions of different species in the sample.  We can then apply the modern habitat preferences to these results and arrive at a figure that represents the average conditions of that sample at that given point in time.  By analysing hundreds of samples in each core, we can build up a long term record of change. 

Conditions at our sites are at the extremes of the range of conditions in which testate amoebae live and unfortunately for us, that means there are often relatively few shells to be found in our samples.  As a result, each sample can take up to several hours to analyse.  When you consider that we will analyse up to one thousand samples over the course of the project, that represents a lot of microscope time!