ABSTRACT. Microscopic observation of air-hydrate crystals was carried out using 34 deep ice-core samples retrieved at Vostok Station, Antarctica. Samples were obtained from depths between 1050 and 2542 m, which correspond to Wisconsin/ Sangamon/Illinoian ice. It was found that the volume and number of air-hydrate varied with the climatic changes. The volume concentration of air-hydrate in the interglacial ice was about 30% larger than that in the glacial ice. In the interglacial ice, the number concentration of air-hydrate was about a half and the mean volume of air-hydrate was nearly three times larger than that in the glacial-a;e ice. The air-hydrate crystals were found to grow in the ice sheet, about 6.7 x 10-1 cm 3 year-I, in compensation for the disappearance of smaller ones. The volume concentration of air-hydrate was related to the total gas content by a geometrical equation with a proportional parameter a . The mean value of a below 1250 m, where no air bubbles were found, was about 0.79. This coincided with an experimentally determined value of the crystalline site occupancy of the air-hydrate in a 1500 m core obtained at Dye 3, Greenland (Hondoh and others, 1990). In the depth profile of calculated a for many samples, a in the interglacial ice was about 30% smaller than that in the glacial-age ice.

An automatic ice fabric analyser has been developed which precisely determines both c- and a-axes of the individual grains in a thin polycrystalline ice specimen by the use of X-ray Laue method (eg Cullity 1956). The method of analysis is computer-aided indexing of the zone-axes in X-ray Laue patterns. Crystal orientations can be automatically determined by the analyser with an error of approximately 0.6_. Time for analysing one specimen containing 100 grains is approximately three hours at present.

Air-hydrate inclusions have been found in deep ice cores from Dye 3, Greenland, which were taken in August 1981 . Although the concentration of the air-hydrate crystals decreased with time, when the core was stored at a temperature of -50°C, they still existed to an appreciable extent in 1985.An ice specimen was cut out from the Dye 3 core at a depth of 1500 m, where the volume fraction of the hydrate crystals was about 10-3 by volume. Its dielectric ?10