Nitrous oxide in the Bellingshausen Sea and Drake Passage

Concentrations of dissolved and atmospheric nitrous oxide, N2O, were measured in the austral spring of 1992 in the Drake Passage and Bellingshausen Sea as part of the United Kingdom Joint Global Ocean Flux Study expedition to the Southern Ocean. The measured atmospheric mixing ratio was 313±5 parts per billion by volume, in agreement with the hemispherically corrected global mean. In the Drake Passage, surface N2O saturations were generally very close to atmospheric equilibrium, 99.7±3%, although several anomalous points were associated with the presence of frontal and eddy-like features within the Antarctic Polar Frontal Zone and at the Continental Water Boundary. Further to the south, a series of oceanographic stations and two surface transects along the 85°W meridian between 65.28°S and 70.32°S revealed a transition from undersaturated conditions in open water to oversaturated conditions in the marginal ice zone, in the upper mixed layer (75–100 m). These observations reflect upwelling of Circumpolar Deep Water at approximately 70°S, resulting in the accumulation of N2O under the winter sea ice and its subsequent release to the atmosphere following the ice retreat. Sea-air N2O fluxes were estimated from the product of the surface N2O anomaly and the modeled gas transfer coefficients of Liss and Merlivat [1986] and Wanninkhof [1992] to find a maximum rate of +3.1 μmol N2O m−2 d−1. North of the upwelling region, Antarctic Surface Water formed from the mixing of surface waters and ice melt was moderately depleted in N2O with respect to the atmosphere, a minimum 90% of saturation. This sink area was estimated to extend between 65.28°S and 69.57°S with a mean sea-air flux of between −0.6±0.4 and −0.9±0.7 μmol N2O m−2 d−1. The region studied at 85°W (65.28°S to 70.32°S) revealed source and sink areas which were largely determined by the changing physical hydrography, so that overall there was a small net negative flux of between −0.06±0.9 and −0.09±1.4 μmol N2O m−2 d−1.