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

Gases in ice cores are invaluable archives of past environmental changes (e.g., the past atmosphere). However, gas fractionation processes after bubble closure in the firn are poorly understood, although increasing evidence indicates preferential leakages of smaller molecules (e.g., neon, oxygen, and argon) from the closed bubbles through the ice matrix. These fractionation processes are believed to be responsible for the observed millennial delta O-2/N-2 variations in ice cores, linking ice core chronologies with orbital parameters. In this study, we investigated high-resolution delta Ar/N-2 of the GISP2 (Greenland Ice Sheet Project 2), NGRIP (North Greenland Ice Core Project), and Dome Fuji ice cores for the past few thousand years. We find that delta Ar/N-2 at multidecadal resolution on the gas-age scale in the GISP2 ice core has a significant negative correlation with accumulation rate and a positive correlation with air contents over the past 6000 years, indicating that changes in overloading pressure induced delta Ar/N-2 fractionation in the firn. Furthermore, the GISP2 temperature and accumulation rate for the last 4000 years have nearly equal effects on delta Ar/N-2 with sensitivities of 0.72 +/- 0.1 parts per thousand degrees C-1 and -0.58 +/- 0.09 parts per thousand(0.01mice year(-1))(-1), respectively. To understand the fractionation processes, we applied a permeation model for two different processes of bubble pressure build-up in the firn, pressure sensitive process (e.g., microbubbles: 0.3-3% of air contents) with a greater sensitivity to overloading pressures and normal bubble process. The model indicates that delta Ar/N-2 in the bubbles under the pressure sensitive process are negatively correlated with the accumulation rate due to changes in overloading pressure. On the other hand, the normal bubbles experience only limited depletion (< 0.5 parts per thousand) in the firn. Colder temperatures in the firn induce more depletion in delta Ar/N-2 through thicker firn. The pressure sensitive bubbles are so depleted in delta Ar/N-2 at the bubble close-off depth that they dominate the total delta Ar/N-2 changes in spite of their smaller air contents. The model also indicates that delta Ar/N-2 of ice cores should have experienced several per mil of depletion during the storage 14-18 years after coring. Further understanding of the delta Ar/N-2 fractionation processes in the firn, combined with nitrogen and argon isotope data, may lead to a new proxy for the past temperature and accumulation rate.