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Field studies of air bubble and clathrate inclusions in the NGRIP ice core reveal two stages of clathrate formation. In the transition zone translucent primary clathrates with rough surfaces and irregular shapes, formed from individual bubbles, split up and form smaller specimens. Deeper down in the transition zone (~1100 m), clathrate metamorphosis sets in, giving rise to the formation of transparent secondary clathrates, with smooth and regular, often poly-hedral or faceted shapes. Rod-like crystals of great length (~2 mm) often show signs of separation into smaller units that fragment into smaller specimens. These findings imply that the simplified picture of one bubble forming one clathrate has to be modified.

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.

Raman spectroscopic measurements on synthetic air clathrates, prepared at different pressures and temperatures, are presented. The gas fractionation for clathrates formed at -2 and -20 degrees C is determined from the integrated vibron mode intensities of nitrogen and oxygen molecules. An increasing fractionation in favour of oxygen with increasing pressure is found. An asymmetric peak shape for the O-2 vibron mode was observed in air clathrates prepared at 300 bar and -2 degrees C. The asymmetry of the N-2 vibron mode in the same sample is much less pronounced.

We present the results of Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core, which differ markedly from previous measurements on the Dye 3 ice core. The N-2/O-2 ratio we observe is much closer to the atmospheric value. This has new implications for the interpretation of gas distributions in ice sheets. Raman spectroscopic scans to determine the N-2/O-2 ratios on different planes through a clathrate, in which the two axes of the scans are perpendicular to each other, give no indication of fractionation effects on the N-2/O-2 concentrations within a clathrate specimen. The frequency shift of the N-2 and O-2 peaks due to decomposition of a clathrate to an air bubble is shown qualitatively. From their peak integrals there is no indication of different retransformation rates to air bubbles between the oxygen and the nitrogen contents of clathrates. In air bubbles resulting from clathrate decomposition, the N-2/O-2 ratio shows similar values to those observed in clathrates and present atmospheric values. Statistical studies on the size, shape, and number concentration of clathrates are intended to give an estimate of the total amount of gas occluded in the clathrates. We present preliminary results obtained from 27 samples in a depth range between 1100 and 3000 m. The first neutron powder diffraction experiments reveal an overall degree of filling of 96.4% for a clathrate at a pressure of 449 bars and the existence of a type I phase at 1311 bars with an overall degree of filling of 107.5%.

This paper reports pressure dependent high resolution neutron diffraction work on N-2-clathrates, which for the first time provides numbers on the compressibility as well as the location and degree of filling of the guest molecules in the small and large cages. N-2-clathrates crystallize, at least at lower pressures and temperatures near 0 degrees C, in the Stackelberg type II structure. However, during the diffraction experiments we have observed the transient and partial formation of the von Stackelberg type I N-2-clathrate at pressures exceeding several hundred bar. The filling of the small cages in the type II clathrate roughly follows a Langmuir isotherm, In contrast to most previous assumptions there is strong evidence that the large cages are doubly occupied in both type I and type II N-2-clathrates. The observed filling can be fitted reasonably well by a two-constant Langmuir model.

We present the results of Raman spectroscopic scans to determine the N-2/O-2 ratios on different planes through a clathrate in the GReenland fee Core Project (GRIP) deep ice core, in which the two axes of the scans are perpendicular to one another. The spectra of the individual focal planes do not point to fractionation effects of the N-2/O-2 concentrations within a clathrate specimen in the range of the precision of Raman spectroscopy. In a second study, the frequency shift of the N-2 and O-2 peaks due to decomposition of a clathrate to an air bubble is shown qualitatively. From their peak integrals, there is no indication of different retransformation rates to air bubbles between the oxygen and the nitrogen contents of clathrates. The N-2/O-2 ratios we observe in clathrates and on clathrate decomposition at different stages are retained in our measurements of air bubbles resulting from clathrate decomposition.

We have carried out Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core. We present new N-2/O-2 measurements that markedly differ from previous measurements of the Dye-3 ice core: the N-2/O-2 ratio we observe is much closer to atmospheric. This has new implications for the interpretation of gas distributions in ice sheets and the reconstruction of past atmospheric conditions.