N₂O production in the eastern South Atlantic: Analysis of N₂O stable isotopic and concentration data

<jats:title>Abstract</jats:title><jats:p>The stable isotopic composition of dissolved nitrous oxide (N<jats:sub>2</jats:sub>O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N<jats:sub>2</jats:sub>O concentration and isotope data from the South Atlantic Ocean, spanning from the western side of the mid‐Atlantic Ridge to the upwelling zone off the southern African coast. In the eastern South Atlantic, shallow N<jats:sub>2</jats:sub>O production by nitrifier denitrification contributed a flux of isotopically depleted N<jats:sub>2</jats:sub>O to the atmosphere. Along the African coast, N<jats:sub>2</jats:sub>O fluxes to the atmosphere of up to 46 µmol/m<jats:sup>2</jats:sup>/d were calculated using satellite‐derived QuikSCAT wind speed data, while fluxes at the offshore stations averaged 0.04 µmol/m<jats:sup>2</jats:sup>/d. Comparison of the isotopic composition of the deeper N<jats:sub>2</jats:sub>O in the South Atlantic (800 m to 1000 m) to measurements made in other regions suggests that water advected from one or more of the major oxygen deficient zones contributed N<jats:sub>2</jats:sub>O to the mesopelagic South Atlantic via the Southern Ocean. This deeper N<jats:sub>2</jats:sub>O was isotopically and isotopomerically enriched (<jats:italic>δ</jats:italic><jats:sup>15</jats:sup>N<jats:sup>bulk</jats:sup>− N<jats:sub>2</jats:sub>O = 8.7 ± 0.1‰,<jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O − N<jats:sub>2</jats:sub>O = 46.5 ± 0.2‰, and Site Preference = 18.7 ± 0.6‰) relative to the shallow N<jats:sub>2</jats:sub>O source, indicating that N<jats:sub>2</jats:sub>O consumption by denitrification influenced its isotopic composition. The N<jats:sub>2</jats:sub>O concentration maximum was observed between 200 m and 400 m and reached 49 nM near the Angolan coast. The depths of the N<jats:sub>2</jats:sub>O concentration maximum coincided with those of sedimentary particle resuspension along the coast. The isotopic composition of this N<jats:sub>2</jats:sub>O (<jats:italic>δ</jats:italic><jats:sup>15</jats:sup>N<jats:sup>bulk</jats:sup>− N<jats:sub>2</jats:sub>O = 5.8 ± 0.1‰,<jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O − N<jats:sub>2</jats:sub>O = 39.7 ± 0.1‰, and Site Preference = 9.8 ± 1.0‰) was consistent with production by diffusion‐limited nitrate (NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) reduction to nitrite (NO<jats:sub>2</jats:sub><jats:sup>−</jats:sup>), followed by NO<jats:sub>2</jats:sub><jats:sup>−</jats:sup>reduction to N<jats:sub>2</jats:sub>O by denitrification and/or nitrifier denitrification, with additional N<jats:sub>2</jats:sub>O production by NH<jats:sub>2</jats:sub>OH decomposition during NH<jats:sub>3</jats:sub>oxidation. The sediment surface, benthic boundary layer, or particles resuspended from the sediments are likely to have provided the physical and chemical conditions necessary to produce this N<jats:sub>2</jats:sub>O.</jats:p>