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

Abstract

Dissolved methane (CH4) concentrations were measured in the upper water column (0–200 m) 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 200 m 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 and the South Shetland Islands. In the deeper waters of central 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 interface 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.