Heterogeneous distributions of greenhouse gas concentration in snow layers of Styx Glacier, East Antarctica

In the top unconsolidated snow layer (firn) of ice sheet, air is mixed by diffusion and convection. Atmospheric air is gradually trapped in closed pores along with depth increase in the firn. However, on small depth scales (e.g., mm or cm), the mean air ages in the bubbles at certain depth intervals can be variable due to difference in history of air trapping. Consequently, gas concentration can be also variable on mm or cm scales given that atmospheric concentration changes with time. Greenhouse gas concentrations in deep ice cores often reveal inhomogeneous distribution on cm scale, which might be due to the variable air trap history associated with snow density variations in the firn. In order to quantitatively check this theory, we analyzed greenhouse gas concentrations in different density layers in the firn. In general, low (high) gas concentrations were observed in the high-density (low-density) layers on cm scale. We interpret that air is trapped earlier in the high density layers than in the low density ones. Alternatively, however, low density layers with high open porosity can be highly contaminated by storage air because part of the open pores become closed during ice storage. To distinguish the two processes, we analyzed the greenhouse gas (CO₂, CH₄ and N₂O) concentrations in the closed bubbles and compared with theoretical values which were calculated from a firn air transport model. Our observations may help us better understand inhomogeneous gas concentration distribution in cm scales that may prevent high-resolution reconstruction of atmospheric concentration changes.