EXPOSÉ

From the Vostok Ice core to a Jupiter's Moon

by R. Souchez,

Correspondant de la Classe

Polar ice is a remarkable archive of our environment. This is the consequence of the following features.

First, the oxygen or hydrogen isotopic composition of the ice crystals is dependent on the temperature of their formation. Let us define the 8180 of a snow sample as the relative deviation between the 180/160 ratio of the sample and of seawater. Simi¬ larly, the 8D-D for deuterium -of a snow sample is the relative deviation between the D/H ratio of the sample and of seawater. 5-values of a snow sample, either 8180 or 8D, is function of the isotopic fractionations occurring during water phase changes in the atmosphere. 8-values are lower if the temperature is lower. A seasonal effect is displayed in polar precipitation, the snow having more negative 8-values during winter than during summer. This, incidentally, can be used for dating the ice. A colder climate is thus conducive to more negative 8-values while a warmer period is characterised by less negative 8-values. Since, in polar regions, there is an excellent correlation between surface temperature and 8-values of the snow precipitated, it becomes possible to derive the precise temperature value in a past period from the 8-values of an ice layer corresponding to that period. The isotopic composition of an ice layer is thus the equivalent of a palaeothermometer.

Secondly, during ice formation, air primarily in the pores of the snow cover is progressively entrapped as gas bubbles within the ice. These bubbles are tiny parts of the atmosphere and their composition gives the composition of the atmosphere. The air present in an ice layer, either in the form of gas bubbles or in the form of gas hydrates deeper into the ice sheet, dating from

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