Ice with everything - Antarctica: Global Science from a Frozen Continent - page 81

Why drill in the ice?

Polar ice sheets contain unique archives of past climate and environmental
changes. They provide information on local, regional and global climate, through
the physical and chemical composition of the water and air preserved inside the ice
matrix. Our knowledge of Antarctic climate based on instrumental records is
frustratingly short, as monitoring instruments started being deployed only 50 years
ago, during the 1957 58 International Geophysical Year. Obtaining past climate

records is essential to characterise better the range of natural climate variability in
Antarctica, from the inter-annual timescale to the past million years. Ice cores
provide estimates of local climate parameters, such as the annual accumulation
rate, a variable that is essential to characterise the mass balance of an ice sheet. In
some low elevation coastal sites, summer melt occurs from time to time and this
can be detected in the structure of the ice cores. In inland sites, where most ice core
drilling operations have been conducted, temperatures are always too cold to allow
surface melting. However, the initial snowfall still undergoes post-deposition
effects due to wind erosion and sublimation/recondensation of water vapour in the
porous snow surface.
The isotopic composition of the water trapped in snow and ice is the
most valuable local climate indicator found in ice cores. Different stable
forms of the water molecule are present on Earth: while most of the H 2 O molecules
are formed by two atoms of oxygen with 16 neutrons ( 16 O) and one atom of
hydrogen, a small fraction of H 2 O molecules are formed with 18 neutrons ( 18 O)
(about 2000 parts per million of ocean water) or with deuterium 2 D (about
300 parts per million of ocean water). These different water molecules have the
same chemical properties, but differ in their mass (due to the different numbers
of neutrons) and also in their diffusivity (due to the different structures of the water
molecules). In the atmospheric water cycle, water is evaporated at the ocean surface,
cooled and therefore condensed in the atmosphere to form clouds, and

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finally
precipitated above Antarctica to form snow. During each phase change, isotopic
fractionation occurs. The moisture formed at evaporation is slightly impoverished
in heavy molecules compared to the initial ocean water. Condensation also
preferentially removes heavy molecules.
As a result, the transport of water vapour towards Antarctica is associated with a
progressive distillation of water masses and a progressive loss of heavy water
molecules. As early as the 1960s, a linear relationship was discovered linking the
ratio of light to heavy water molecules of Antarctic snowfall with local temperature.
About a thousand locations have been so far documented for their modern isotopic
composition (see Figure 3.17 ), con

rming its large-scale validity. This relationship,

called the ' isotopic thermometer ' , is still commonly used to estimate past changes