N_2O production in the eastern South Atlantic: Analysis of N_2O stable isotopic and concentration data

摘要

The stable isotopic composition of dissolved nitrous oxide (N_2O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N_2O 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_2O production by nitrifier denitrification contributed a flux of isotopically depleted N_2O to the atmosphere. Along the African coast, N_2O fluxes to the atmosphere of up to 46 μmol/m~2/d were calculated using satellite-derived QuikSCAT wind speed data, while fluxes at the offshore stations averaged 0.04 μmol/m2/d. Comparison of the isotopic composition of the deeper N_2O in the South Atlantic (800m 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_2O to the mesopelagic South Atlantic via the Southern Ocean. This deeper N_2O was isotopically and isotopomerically enriched (δ~(15)N~(bulk) - N_2O = 8.7 ± 0.1‰, δ~(18)O - N_2O = 46.5 ± 0.2‰, and Site Preference = 18.7 ± 0.6‰) relative to the shallow N_2O source, indicating that N_2O consumption by denitrification influenced its isotopic composition. The N_2O concentration maximum was observed between 200mand 400mand reached 49 nM near the Angolan coast. The depths of the N_2O concentration maximum coincided with those of sedimentary particle resuspension along the coast. The isotopic composition of this N_2O (δ~(15)N~(bulk) - N_2O = 5.8 ± 0.1‰, δ~(18)O - N_2O = 39.7 ± 0.1‰, and Site Preference = 9.8 ± 1.0‰) was consistent with production by diffusion-limited nitrate (NO_3~-) reduction to nitrite (NO_2~-), followed by NO_2~- reduction to N_2O by denitrification and/or nitrifier denitrification, with additional N_2O production by NH_2OH decomposition during NH3 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_2O.