Fractionation of O-2/N-2 and Ar/N-2 in the Antarctic ice sheet during bubble formation and bubble-clathrate hydrate transition from precise gas measurements of the Dome Fuji ice core

The variations of delta O-2/N-2 and delta Ar/N-2 in the Dome Fuji ice core were measured from 112 m (bubbly ice) to 2001 m (clathrate hydrate ice). Our method, combined with the low storage temperature of the samples (-50 degrees C), successfully excludes post-coring gas-loss fractionation signals from our data. From the bubbly ice to the middle of the bubble-clathrate transition zone (BCTZ) (112-800m) and below the BCTZ (> 1200 m), the delta O-2/N-2 and delta Ar/N-2 data exhibit orbital-scale variations similar to local summer insolation. The data in the lower BCTZ (800-1200 m) have large scatter, which may be caused by millimeter-scale inhomogeneity of air composition combined with finite sample lengths. The insolation signal originally recorded at the bubble close-off remains through the BCTZ, and the insolation signal may be reconstructed by analyzing long ice samples (more than 50 cm for the Dome Fuji core). In the clathrate hydrate zone, the scatter around the orbital-scale variability decreases with depth, indicating diffusive smoothing of delta O-2/N-2 and delta Ar/N-2. A simple gas diffusion model was used to reproduce the smoothing and thus constrain their permeation coefficients. The relationship between delta Ar/N-2 and delta O-2/N-2 is markedly different for the datasets representing bubble close-off (slope similar to 0.5), bubble-clathrate hydrate transformation (similar to 1), and post-coring gas loss (similar to 0.2), suggesting that the contributions of the mass-independent and mass-dependent fractionation processes are different for those cases. The method and data presented here may be useful for improving the orbital dating of deep ice cores over the multiple glacial cycles and further studying non-insolation-driven signals (e.g., atmospheric composition) of these gases.