Variation in N-2/O-2 ratio of occluded air in Dome Fuji antarctic ice

Ancient atmospheric gases are trapped in polar ice sheets. The gas molecule s are stored in air bubbles at shallow depth and are incorporated into clat hrate hydrates below a depth at which the hydrostatic pressure becomes grea ter than the formation pressure of the air clathrate hydrate. Significant g as fractionation has been found from measurements of the depth profile of t he N-2/O-2 composition ratios in clathrate hydrates and air bubbles of Vost ok antarctic ice. To investigate the effect of the ice condition on the fra ctionation process, we measured the N-2/O-2 ratios in clathrate hydrates an d air bubbles from Dome Fuji antarctic ice using Raman spectroscopy. The re sults 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 tend ency of the variation of the N-2/O-2 ratio with depth is similar. The diffe rence 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 th e difference in the bubble-free ice zone was caused by gas loss from the ic e 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.