Air clathrate crystals from the GRIP deep ice core, Greenland: a number-, size- and shape-distribution study

We performed microscopic observations and a statistical study of the number, size and shape distribution of clathrates in the GRIP (Greenland Ice Core Project) deep ice core, using 185 samples from a depth range of 1016-3014m, spanning a period of 6 to >110 ka sp and encompassing the Holocene, Wisconsin and Eemian periods. The number concentration of the clathrates varied considerably with climatic changes. It was possible to detect the rapid climatic oscillations in the last glacial between 13 and 110 ka BP, the Dansgaard-Oeschger cycles, in the number-concentration profile of clathrates. The mean volume of clathrates is less clearly influenced by climatic factors, with a tendency towards greater volumes in warmer periods, but also a growth of clathrates with depth could be detected. This growth rate was calculated to be 3.1 x 10(-12) cm(3) a(-1). The amount of gases captured in the clathrates is estimated to be significantly smaller than the total amount of air determined by gas-concentration measurements. This points to diffusion processes of atmospheric gases within the ice matrix.