Nitrous oxide in the Somali Basin: the role of upwelling

Nitrous oxide (N20) concentrations in surface water, resulting emissions into the atmosphere, and the distribution in the water column were measured in the Somali upwelling region and the Gulf of Aden during July–August 1992. The N20 concentration in the water column exhibited two maxima: a sharp subsurface maximum around 100–200 m, and a broad deep-water maximum around 600–1000 m. Both maxima intensified northward, reaching concentrations up to 81 nM (1030% saturation) and 76 nM (840% saturation), respectively. Deep-water N20 was correlated negatively with 02 and positively with nitrate, indicating microbial N20 production by with nitrification. At O2 and positively with nitrate, indicating microbial N2O production by consumption increased significantly, suggesting an increasing N20 yield in the nitrification process. Oxygen concentrations, however, never dropped low enough to allow N20 cycling by denitrification. e water N20 saturation was strongly negatively correlated with temperature, indicating that monsoon-driven upwelling of cold N2O-rich water controlled the saturation degree of N20 in surface waters. The maximum N2O saturation was 330% in freshly upwelled waters. The strong monsoon wind driving the upwelling concomitantly induced strong vertical mixing and effective air-sea gas exchange. As a consequence of the simultaneous occurrence of strong N20 saturations and high gas transfer velocities, N20 emissions into the atmosphere reached maximum values of 260–500 μmol m−2 day−1, which is nearly three orders of magnitude above the global mean oceanic N20 flux. Between 21 June and 25 August 1992, the N20 emission from the region of most intense upwelling off Somalia was estimated at (0.6–1.1) × 109 mol [(26–48) × 109 g]. Although this flux equals 0.4–0.8% of the estimated annual marine N20 emission, it was emitted during a timespan of 2 months from an area of less than 0.011 % of the World Ocean.