Laboratory Estimates of the Pathways for Air-sea Gas Transport Through Sea Ice-covered Waters

Gases in the polar surface ocean can be affected by sea ice in three important ways: sea ice may restrict gas exchange at the air-sea interface causing excesses or deficits in the underlying water column, gases may be transported beneath the mixed layer by brine rejection, and some biologically-active gases may be produced or consumed within sea ice by algae and microbes. We explored the possible pathways by which gas can be transported between the surface ocean, the ice and the atmosphere. Specifically, we used laboratory sea ice experiments to determine the rate of bulk gas diffusion, D, for oxygen (O₂) and sulfur hexafluoride (SF₆) through columnar sea ice under constant ice thickness conditions, for 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 of exchange between gas-filled and liquid-filled pore spaces. On average, DSF₆ was 1.3 x10-4cm2s-1 (± 40 %) and DO2 was 3.9 x 10-5 cm2s-1 (± 41 %).

We also used similar laboratory methods to measure the gas transfer velocity, k, through partially ice-cover water to explore the relationship between ice cover and k. The results indicate that k exceeded that expected from a linear relationship between 100% open water (k100%) and complete ice cover: at 15% open water, k was 25% of k100%. These results indicate that the net flux of gas through the ice pack may not scale linearly with open water area, as circulation processes under the ice and the related turbulence affect the gas exchange rate.

Finally, we compared the diffusive gas flux of CO2, based on these estimates of D, with a literature estimate of the air-sea gas transfer through partially ice-covered water. The comparison indicates that mixed-layer ventilation by diffusion through sea ice is not an efficient pathway for transport when contrasted with air-sea gas exchange through openings in the ice pack, even when the fraction of open water is less than 1 %. However, these results are only representative of transport through stable winter ice, while melting ice in spring may have much greater rates of diffusivity, and the existing estimates of k are predicated on an incomplete understanding of the magnitude of turbulent forcing in partially ice-covered waters.