Towards an understanding of the controls on δO2/N2 variability in ice core records
Abstract. Processes controlling pore closure are broadly understood yet defining the physical mechanisms controlling associated elemental fractionation remains ambiguous. Previous studies have shown that the pore closure process leads to a decrease in concentration of small-size molecules (e.g., H2, O2, Ar, Ne, He) in the trapped bubbles. Ice core δ(O2/N2) records – the ratio of O2 to N2 molecules in bubbles trapped in ice cores relative to the atmosphere – are therefore depleted owing to this O2 loss and show a clear link with local summer solstice insolation making it a useful dating tool. In this study, we compile δ(O2/N2) records from 14 polar ice cores and show a new link between δ(O2/N2) and local surface temperature and/or accumulation rate, in addition to the influence of the summer solstice insolation. We argue that both local climate-driven and insolation forcings are linked to the modulation of snow physical properties near the surface. Using the Crocus snowpack model, we perform sensitivity tests to identify the response of near-surface snow properties to changes in insolation, accumulation rate, and air temperature. These tests support a mechanisms linked to snow grain size, such that the larger the grain size for a given density, the stronger the pore closure fractionation, and hence, lower δ(O2/N2) values. Our findings suggest that local accumulation rate and temperature should be considered when interpreting δ(O2/N2) as an insolation proxy.
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