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Gas diffusion through columnar laboratory sea ice: Implications for mixed-layer ventilation of CO2 in the seasonal ice zone


The porous microstructure of sea ice represents a pathway for ocean-atmosphere exchange and for transport of biogenic gases produced within sea ice. We use laboratory sea ice experiments to determine the bulk gas diffusion coefficients, D, for oxygen (O2) and sulfur hexafluoride (SF6) through columnar sea ice under constant ice thickness conditions, between -4 and -12 °C. Profiles of SF6 through the ice indicate decreasing gas concentration from the ice-water interface to the ice-air interface, with evidence of exchange between gas-filled and liquid-filled pore spaces. On average, DSF6 was 1.3 x10-4 cm2s-1 (± 40%) and DO2 was 3.9 × 10-5 cm2s-1 (± 41%). The preferential partitioning of SF6 to the gas phase, which is the dominant diffusion pathway produced the greater rate of SF6 diffusion. A comparison of the diffusive gas flux, based on these estimates of D, with a literature estimate of the air-sea gas transfer through leads, indicates that mixed layer ventilation by diffusion through sea ice is not an efficient mechanism, as compared with air-sea gas exchange through openings in the ice pack, even when the fraction of open water is less than 1%. However, this conclusion is predicated on an incomplete understanding of air-sea gas exchange in ice-covered waters.