Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 133

Accumulation change
Past temperatures have also been estimated from the amount of accumulation of
snow per year. Basically, as the temperature decreases, the saturation vapor press-
ure of water decreases and, as a result, there is less precipitation. However,
changes in atmospheric circulation also affect the amount of precipitation at any
locality (Jouzel et al., 1997).
3.3.2 Climate variations
Ice cores can provide data on precipitation in the past (Kotlyakov, 1996). The rate
of snow accumulation on large ice sheets depends on the temperature above the
layer of ice-cooled air near the surface. The atmospheric moisture content shows a
dramatic drop when the global temperature decreases. It has been estimated that
under the colder conditions of a major ice age the amount of atmospheric deposi-
tion of snow on the glacier surface would have been 50% lower than during an
interglacial period. This can be discerned by the evident reduction in layer
thicknesses during glacial epochs.
In addition to reduced precipitation, it is believed that glacial epochs were
characterized by stronger oceanic currents and winds, as well as higher dust
content due to (a) sharper temperature gradients between continents and oceans,
(b) expansion of deserts as water is transferred to ice sheets, and (c) the exposure
of continental shelves due to lowering of the sea level. Evidence for this occurs in
ice cores where the concentration of aerosols and dust is considerably higher
during glacial periods.
3.3.3 Trapped gases
As polar snow is transformed to ice, atmospheric air is trapped in bubbles.
Therefore, by extracting the gases contained in ice cores, data can be obtained on
the composition of the atmosphere in the past—specifically, on the concentration
of greenhouse gases. In the absence of melting, the closure of ice pores proceeds at
a slow pace: in central East Antarctica this process may take as long as 4,000
years, during which time some exchange of air between the pores and the free
atmosphere takes place. Consequently, the air extracted from polar ice cores is
younger than the accompanying snow. Present day analytical procedures enable us
to extract some gases from the ice—carbon dioxide (CO 2 ) and methane (CH 4 )—
and measure them with great accuracy.
Down to a depth of perhaps 1,200-1,400m, the ice contains a high
concentration of encapsulated gas bubbles. Up to 10% of the volume of the ice is
compressed air, so that the density of the ice is about 0.83 as compared with a
density of 0.93 for pure ice. The bubbles decrease in size with increasing depth
until they disappear in the range 1,200-1,400m. However, the air is still contained
in the ice as a molecular complex at high pressure and, upon decompression, the
bubbles reappear (Oard, 2005).