[1] Gas exchange through sea ice is a determining factor in the polar ocean budget of climatically-active gases. We use SF 6 and O 2 as conservative gas tracers to observe transport between the water, ice and air during conditions of freezing and partial ice cover in artificial seawater. During ice growth, O 2 and SF 6 , as non-polar solutes, were rejected from the ice into the underlying water at a faster rate than that observed for salt. Measurements of the gas exchange rate, k, through partial ice cover exceeded that expected from linear scaling between 100% open water (k 100% ) and complete ice cover: at 15% open water, k was 25% of k 100% . 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 affect the gas exchange rate.

The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 6 8 Sv (1 Sv 5 106 m3 s-1) across the central Weddell Sea and to intensify to 54 6 15 Sv further offshore. This circulation injects 36 6 13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 6 23 mSv of freshwater, including 13 6 1 mSv as sea ice to the midlatitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13 6 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by midgyre upwelling (at a rate of 2 6 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 6 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6 6 2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.

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%.