Dating[edit]
Many different kinds of analysis are performed on ice cores, including visual layer counting, tests for
electrical conductivity and physical properties, and assays for inclusion of gases, particles,
radionuclides, and various molecular
species. For the results of these tests to be useful in the reconstruction of
palaeoenvironments, there has to be a way to determine the relationship between depth and age of the ice. The simplest approach is to count layers of ice that correspond to the original annual layers of snow, but this is not always possible. An alternative is to model the ice accumulation and flow to predict how long it takes a given snowfall to reach a particular depth. Another method is to correlate radionuclides or trace atmospheric gases with other timescales such as periodicities in the earth's
orbital parameters.
[39]
A difficulty in ice core dating is that gases can
diffuse through firn, so the ice at a given depth may be substantially older than the gases trapped in it. As a result, there are two chronologies for a given ice core: one for the ice, and one for the trapped gases. To determine the relationship between the two, models have been developed for the depth at which gases are trapped for a given location, but their predictions have not always proved reliable.
[40][41] At locations with very low snowfall, such as
Vostok, the uncertainty in the difference between ages of ice and gas can be over 1,000 years.
[42]
