Variation in N2/O2 ratio of occluded air in Dome Fuji antarctic ice

Ancient atmospheric gases are trapped in polar ice sheets. The gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate. Significant gas fractionation has been found from measurements of the depth profile of the N-2/O-2 composition ratios in clathrate hydrates and air bubbles of Vostok antarctic ice. To investigate the effect of the ice condition on the fractionation process, we measured the N-2/O-2 ratios in clathrate hydrates and air bubbles from Dome Fuji antarctic ice using Raman spectroscopy. The results showed that the N-2/O-2 ratios in the clathrate hydrates of the Dome Fuji ice are slightly lower than those of the Vostok ice, although the tendency of the variation of the N-2/O-2 ratio with depth is similar. The difference in the N-2/O-2 ratio between the Dome Fuji ice and the Vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate. On the other hand, it is concluded that the difference in the bubble-free ice zone was caused by gas loss from the ice core after coring. The N-2/O-2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of O-2 than of N-2 . Our data suggest that the effect of gas loss in the Dome Fuji ice is relatively small, and so the gas composition in the Dome Fuji ice can be a precise paleoenvironmental indicator.