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, D_SF6 was 1.3 × 10^-4 cm² s^-1 (±40%) and D_O2 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 ofD 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%.