Structural studies of Vostok ice cores

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The purpose of the present study is to carry out a joint research of Japan, France and Russia on microphysical properties of Vostok ice cores by using new methods developed by our research group. The main results obtained are summarized as follows :(1) Close-off process and number density profile of clathrate hydrates : We found that number densities of clathrate hydrates show a good correlation with isotope data. This number density profile must be a new indication of climate changes recorded in the ice cores.(2) Growth process of clathrate hydrates : We successfully developed a theoretical model to demonstrate a growth process of clathrate hydrates from air-bubbles.(3) Fractionation of air-molecules : We found very large fractionation of N_2 and O_2 during the transition from air-bubbles to clathrate hydrates. This result can be also understood by taking the growth process of clathrate hydrates into account.(4) Microstructures of ice grains : We found by synchrotron radiation topography that very large grains of ice crystals are composed of many small sub-grains with very small misorientation angles.