Dissolved methane distributions, sources, and sinks in the Western Bransfield Strait, Antarctica

B. D. Tilbrook¹ and D. M. Karl²

¹Division of Oceanography, Commonwealth Scientific and Industrial Organization, Hobart, Tasmania, Australia

²School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu

Abstract

Dissolved methane (CH₄) concentrations were measured in the upper water column (0-200m) of western Bransfield Strait and southwestern Drake Passage on four cruises between December 1986 and March 1987 during the Research on Antarctic Coastal Ecosystem Rates (RACER) experiment. Methane concentration profiles were similar on all four cruises and showed distinct geographic variability. In contrast to most oceanic surface waters, Ch₄ in Drake Passage was undersaturated relative to atmospheric equilibrium. The degree of the undersaturation varied from 0% to 15% in a seasonally warmed surface layer to 46% at 200m depth. We suggest that the undersaturations result from the entrainment of CH₄-depleted warm deep water into the surface layer and from seasonal ice cover restricting air-sea gas exchange in winter. South of Drake Passage, a shelf break frontal zone marked a distinct transition to coastal waters that were typically supersaturated with CH₄. The highest CH₄ concentrations were associated with the shelf waters surrounding Bransfield Strait, CH₄ concentrations tended to decrease to near-saturation values. The distribution of CH₄ appears to be largely controlled by mixing, coupled with the addition of CH₄ to waters flowing over the shallow shelves in the region. The calculated fluxes of CH₄ across the air-sea inteface varied from -0.77 to +5.86 µmol m^-2 d^-1, where negative fluxes indicate a net uptake of CH₄ into the surface ocean and positive fluxes indicate a net release of oceanic CH₄ to the atmosphere. The average net flux in the Drake Passage was -0.35 µmol m^-2 d^-1, and the average flux out of coastal waters to the atmosphere was +0.69 µmol m^-2 d^-1. On the basis of our results the net air-sea CH₄ fluxes in the Southern Ocean out to the edge of the seasonal ice zone are small and should not significantly alter current estimates of the oceanic source of CH₄ to the atmosphere.