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, Wisco nsin and Eemian periods. The number concentration of the clathrates varied considerably with climatic changes. It was possible to detect the rapid cli matic oscillations in the last glacial between 13 and 110 ka BP, the Dansga ard-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 growt h of clathrates with depth could be detected. This growth rate was calculat ed to be 3.1 x 10(-12) cm(3) a(-1). The amount of gases captured in the cla thrates is estimated to be significantly smaller than the total amount of a ir determined by gas-concentration measurements. This points to diffusion p rocesses of atmospheric gases within the ice matrix.