نبذة مختصرة : International audience ; 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 δ(O 2 /N 2 ) records, exhibits a clear anti-correlation with local summer solstice insolation, making δ(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 δ(O 2 /N 2 ) records from 15 polar ice cores and show a new additional link between δ(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 δ(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 δ(O 2 /N 2 ) as a local insolation proxy.
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