Fractionation of gases in polar ice during bubble close-off: New constraints from firn air Ne, Kr and Xe observations

Abstract

Gas ratios in air withdrawn from polar firn (snowpack) show systematic enrichments of Ne/N 2, O 2/N 2 and Ar/N 2, in the firn–ice transition region where bubbles are closing off. Air from the bubbles in polar ice is correspondingly depleted in these ratios, after accounting for gravitational effects. Gas in the bubbles becomes fractionated during the process of bubble close-off and fractionation may continue as ice cores are stored prior to analysis. We present results from firn air studies at South Pole and Siple Dome, Antarctica, which add Ne, Kr and Xe measurements to the suite of observations. Ne, O 2 and Ar appear to be preferentially excluded from the shrinking and occluding bubbles, and these gases therefore accumulate in the residual firn air, creating a progressive enrichment with time (and depth) in firn air. Early sealing of gases by thin horizontal impermeable layers into a non-diffusive zone or “lock-in zone” greatly enhances this enrichment. A simple model of the bubble close-off fractionation and lock-in zone enrichment fits the data adequately. The model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles. The effect appears to be size-dependent, because Ne, O 2 and Ar have smaller effective molecular diameters than N 2, and fractionation increases strongly with decreasing size. Ne is fractionated 34 ± 2 times more than O 2 in South Pole firn air and reaches an enrichment of 90‰ in the deepest sample. The large atoms Kr and Xe do not appear to be fractionated by this process, despite the large size difference between the two gases, suggesting a threshold atomic diameter of ∼ 3.6 Å above which the probability becomes very small that the gas will escape from the bubble. These findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature, chronology and past atmospheric composition.