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Nitrous Oxide Dynamics in the Southern Benguela Upwelling System
Author(s) -
Mashifane T. B.,
Bourbonnais A.,
Fawcett S. E.
Publication year - 2022
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2022jc019129
Subject(s) - upwelling , oceanography , bay , geology , atmosphere (unit) , downwelling , flux (metallurgy) , biogeochemical cycle , advection , atmospheric sciences , chemistry , environmental chemistry , thermodynamics , physics , organic chemistry
St Helena Bay in the southern Benguela upwelling system (SBUS) is characterized by seasonal upwelling, water mass and nutrient retention, and persistent cyclonic circulation that limits oxygen exchange, creating ideal conditions for subsurface N 2 O production and subsequent ventilation at the ocean‐atmosphere interface. However, due to a paucity of observations, little is known about N 2 O dynamics in the SBUS. Here we use a coupled physical‐biogeochemical model and new observations to investigate the magnitude and seasonality of the N 2 O source and ocean‐atmosphere flux terms along a transect from St Helena Bay into offshore waters. Both the model and observations indicate that significant N 2 O production occurs at depth in nearshore waters, with ΔN 2 O concentrations (i.e., the difference between the simulated or observed concentration of N 2 O and the equilibrium value) exceeding 14 μmol m −3 . Equatorward advection of this N 2 O occurs at a maximum rate of 4.50 μmol N 2 O m −2 s −1 in the upper 200 m. By contrast, the SBUS poleward undercurrent hosts low N 2 O concentrations on the shelf slope. Modeled fluxes range from −0.02 to 0.2 nmol m −2 s −1 , consistent with reports from other upwelling systems. The ocean‐atmosphere N 2 O flux reaches 0.21 g N m −2 yr −1 in nearshore St Helena Bay, and follows a distinct seasonal cycle driven by ΔN 2 O disequilibrium in winter and prevailing south‐easterly winds and associated upwelling in spring and summer. We calculate a mean N 2 O flux for the whole SBUS of 4 ± 2 × 10 −3 Tg N yr −1 , representing 0.1% of the estimated global ocean annual flux.