Effects of molecular diffusion on trapped gas composition in polar ice cores

Enrichment of nitrogen gas has been found from gas analyses of ice cores retrieved from deep parts of Antarctica. Neither climate change nor gas loss through ice cracks explain the enrichment. In order to investigate the mechanism of the gas composition change, we develop a model of gas loss caused by molecular diffusion from clathrate hydrates toward the ice-core surface through ice crystal. We apply the model to interpret the data on the gas composition change in the Dome Fuji ice core during the storage for 3 years at 248 K. The mass transfer coefficients determined using the model are 1.4×10−9 and 4.3×10−9 m·s−1 at 248 K for N2 and O2, respectively. The difference in the coefficient between N2 and O2 causes the change in the O2/N2 ratio of the trapped gas in the ice core during the storage. During the storage period of 1000 days at 248 K, the O2/N2 ratio changes from −9.9‰ to −20.5‰. The effect of the gas loss decreases as the storage temperature decreases. The results have important implications for the accurate reconstructions of the paleo-atmosphere from polar ice cores.