Abstract
Abstract. We report on methane (CH4) dynamics in landfast sea ice, brine and under-ice seawater at Barrow in 2009. The CH4 concentrations in under-ice water ranged from 25.9 to 116.4 nmol L−1sw, indicating a supersaturation of 700 to 3100% relative to the atmosphere. In comparison, the CH4 concentrations in sea ice ranged from 3.4 to 17.2 nmol L−1ice and the deduced CH4 concentrations in brine from 13.2 to 677.7 nmol L−1brine. We investigated the processes underlying the difference in CH4 concentrations between sea ice, brine and under-ice water and suggest that biological controls on the storage of CH4 in ice were minor in comparison to the physical controls. Two physical processes regulated the storage of CH4 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 CH4 in the ice at the beginning of ice growth. CH4 retention in sea ice was then twice as efficient as that of salt; this also explains the overall higher CH4 concentrations in brine than in the under-ice water. As sea ice thickened, gas bubble formation became less efficient, CH4 was then mainly trapped in the dissolved state. The increase of sea ice permeability during ice melt marked the end of CH4 storage.
Highlights
Methane (CH4) is a well-mixed greenhouse gas
It appears that the mean CH4 concentration and the standing stock increased as sea ice thickened from BRW2 to BRW7, but decreased at BRW10 despite the fact that sea ice was thicker there
The individual profiles of CH4 concentrations in bulk ice (Fig. 3a) for each sampling event further highlight the contrasts between BRW10 and all the previous sampling events (BRW2 to BRW7): all the CH4 concentration profiles in ice from BRW2 to BRW7 can be divided into three main zones
Summary
Methane (CH4) is a well-mixed greenhouse gas. Its concentration in the atmosphere is much lower than that of its oxidation product (CO2) (1.9 vs. 397 ppm respectively) (http: //www.esrl.noaa.gov/gmd/aggi/). Since the CH4 global warming potential is 28 times higher than that of CO2 over a 100-year frame, it accounts for 20 % of the global radiative forcing of well-mixed greenhouse gases (Myhre et al, 2013). Global ocean emission of CH4 is estimated at 19 Tg per year (Kirschke et al, 2013), which is about 3 % of the global tropospheric CH4 input. CH4 supersaturation relative to the atmosphere in estuaries (Borges and Abril, 2011; Upstill-Goddard et al, 2000) and coastal shelves (Kvenvolden et al, 1993; Savvichev et al, 2004; Shakhova et al, 2005, 2010) is larger than that in the open ocean (Bates et al, 1996; Damm et al, 2007, 2008, 2010)
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