Spatial distribution of air molecules within individual clathrate hydrates in polar ice sheets

Abstract

AbstractWe measured the N2/O2 ratios in clathrate hydrate crystals from Vostok Antarctic ice cores using Raman spectroscopy in order to investigate the spatial distribution of air molecules within a crystal. The results showed that the pattern of the spatial distribution of air molecules in clathrate hydrate depends on the crystal. Some clathrate hydrates have inhomogeneous distributions of the N2/O2 ratio within the crystals, while others are practically homogeneous. The spatial distribution of air molecules within an individual clathrate hydrate changes with time due to three processes: (1) the initial selective enclathration caused by the difference between the dissociation pressures of pure N2- and O2–clathrate hydrates, (2) the diffusive mass transfer of air molecules from surrounding air bubbles through the ice matrix, and (3) diffusion of air molecules in the clathrate hydrate crystal. The dissociation pressures and the diffusion rates of air molecules in ice and clathrate hydrate strongly depend on temperature. Therefore, it is concluded that the pattern of the spatial distribution of air molecules in clathrate hydrate is mainly determined by the depth at which they formed and the temperature in the ice sheet.