| Summary: | We report on methane (CH 4 ) dynamics in landfast sea ice, brine and under-ice seawater at Barrow in 2009. The CH 4 concentrations in under-ice water ranged from 25.9 to 116.4 nmol L -1 sw , indicating a supersaturation of 700 to 3100% relative to the atmosphere. In comparison, the CH 4 concentrations in sea ice ranged from 3.4 to 17.2 nmol L -1 ice and the deduced CH 4 concentrations in brine from 13.2 to 677.7 nmol L -1 brine . We investigated the processes underlying the difference in CH 4 concentrations between sea ice, brine and under-ice water and suggest that biological controls on the storage of CH 4 in ice were minor in comparison to the physical controls. Two physical processes regulated the storage of CH 4 in our landfast ice samples: bubble formation within the ice and sea ice permeability. Gas bubble formation due to brine concentration and solubility decrease favoured the accumulation of CH 4 in the ice at the beginning of ice growth. CH 4 retention in sea ice was then twice as efficient as that of salt; this also explains the overall higher CH 4 concentrations in brine than in the under-ice water. As sea ice thickened, gas bubble formation became less efficient, CH 4 was then mainly trapped in the dissolved state. The increase of sea ice permeability during ice melt marked the end of CH 4 storage.
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