On the relationship between δO2∕N2 variability and ice sheet surface conditions in Antarctica

While the processes controlling pore closure are broadly understood, the physical mechanisms driving the associated elemental fractionation remains ambiguous. Previous studies have shown that the pore closure process leads to a depletion in small-sized molecules (e.g. H-2, O-2, Ar, Ne, He) in ice core bubbles relative to larger-sized molecules like N-2. This size-dependent fractionation, identified using ice core delta(O-2/N-2) records, exhibits a clear anti-correlation with local summer solstice insolation, making delta(O-2/N-2) a valuable ice core dating tool. Mechanisms controlling this relationship are attributed to the physical properties of deep firn. In this study, we compile delta(O-2/N-2) records from 15 polar ice cores and show a new additional link between delta(O-2/N-2) and local surface temperature and/or accumulation rate. Using the Crocus snowpack model, we perform sensitivity tests to identify the response of near-surface snow properties to changes in insolation intensity, accumulation rate, and air temperature. These tests support a mechanism linked to firn grain size, such that the larger the grain size for a given density, the stronger the pore closure fractionation and, hence, the lower the delta(O-2/N-2) values archived in the ice. Based on both snowpack model outputs and data compilation, our findings suggest that local accumulation rate and temperature should be considered when interpreting delta(O-2/N-2) as a local insolation proxy.