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

Gas diffusion through the porous microstructure of sea ice represents a pathway for ocean-atmosphere exchange and for transport of biogenic gases produced within sea ice. We report on the experimental determination of the bulk gas diffusion coefficients, D, for oxygen (O₂) and sulphur hexafluoride (SF₆) through columnar sea ice under constant ice thickness conditions for ice surface temperatures 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 for solubility partitioning between gas-filled and liquid-filled pore spaces. On average, was 1.3 × 10^-4 cm² s^-1 (±40%) and was 3.9 × 10^-5 cm² s^-1 (±41%). The preferential partitioning of SF₆ to the gas phase, which is the dominant diffusion pathway produced the greater rate of SF₆ diffusion. Comparing these estimates of D with an existing estimate of the air-sea gas transfer through leads indicates that ventilation of the mixed layer by diffusion through sea ice may be negligible, compared to air-sea gas exchange through fractures in the ice pack, even when the fraction of open water is less than 1%.