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. H2, O2, Ar, Ne, He) in ice core bubbles relative to larger-sized molecules like N2.
This size-dependent fractionation, identified using ice core dO2/N2 records, exhibits a clear anti-correlation with local summer solstice insolation, making a valuable ice core dating tool. Mechanisms controlling this relationship are attributed to the physical properties of deep firn. In this study, we compile dO2/N2 records from 15 polar ice cores and show a new additional link between dO2/N2 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 dO2/N2 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 as a local insolation proxy.
Autors: R. Harris Stuart, A. Landais, L. Arnaud, C. Buizert, E. Capron, M. Dumont, Q. Libois, R. Mulvaney, A. Orsi, G. Picard, Frédéric Prié, J. Severinghaus, B. Stenni, and P. Martinerie
Article: The Cryosphere, 18, 8 (2024) https://doi.org/10.5194/tc-18-3741-2024