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Gases in ice cores are invaluable archives of past environmental changes(e.g., the past atmosphere). However, gas fractionation processes afterbubble closure in the firn are poorly understood, although increasingevidence indicates preferential leakages of smaller molecules (e.g., neon,oxygen, and argon) from the closed bubbles through the ice matrix. Thesefractionation processes are believed to be responsible for the observedmillennial δO2/N2 variations in ice cores, linking icecore chronologies with orbital parameters. In this study, we investigatedhigh-resolution δAr/N2 of the GISP2 (Greenland Ice SheetProject 2), NGRIP (North Greenland Ice Core Project), and Dome Fuji ice coresfor the past few thousand years. We find that δAr/N2 atmultidecadal resolution on the "gas-age scale" in the GISP2 ice core has asignificant negative correlation with accumulation rate and a positivecorrelation with air contents over the past 6000 years, indicating thatchanges in overloading pressure induced δAr/N2 fractionationin the firn. Furthermore, the GISP2 temperature and accumulation rate for thelast 4000 years have nearly equal effects on δAr/N2 withsensitivities of 0.72 ± 0.1 ‰ °C−1 and−0.58 ± 0.09 ‰ (0.01 m ice year−1)−1,respectively. To understand the fractionation processes, we applied apermeation model for two different processes of bubble pressure build-up inthe firn, "pressure sensitive process" (e.g., microbubbles: 0.3–3 % ofair contents) with a greater sensitivity to overloading pressures and"normal bubble process". The model indicates that δAr/N2 inthe bubbles under the pressure sensitive process are negatively correlatedwith the accumulation rate due to changes in overloading pressure. On theother hand, the normal bubbles experience only limited depletion(< 0.5 ‰) in the firn. Colder temperatures in the firn inducemore depletion in δAr/N2 through thicker firn. The pressuresensitive bubbles are so depleted in δAr/N2 at the bubbleclose-off depth that they dominate the total δAr/N2 changes inspite of their smaller air contents. The model also indicates that δAr/N2 of ice cores should have experienced several per mil ofdepletion during the storage 14–18 years after coring. Further understandingof the δAr/N2 fractionation processes in the firn, combinedwith nitrogen and argon isotope data, may lead to a new proxy for the pasttemperature and accumulation rate

Post-bubble close-off fractionation of gases in polar firn and ice cores: effects of accumulation rate on permeation through overloading pressure