Meridional and Cross‐Shelf Variability of N2O and CH4 in the Eastern‐South Atlantic

B Sabbaghzadeh,G Rehder,D L Arévalo‐Martínez,M Glockzin,S Otto

doi:10.1029/2020jc016878

Abstract

AbstractUpward transport and/or mixing of trace gas‐enriched subsurface waters fosters the exchange of nitrous oxide (N2O) and methane (CH4) with the atmosphere in the Eastern‐South Atlantic (ESA). To date, it is, however, unclear whether this source is maintained by local production or advection of trace gas‐enriched water masses. The meridional and zonal variability of N2O and CH4 in the ESA were investigated to identify the contributions of the major regional water masses to the overall budget of N2O and CH4. The maximal sea surface N2O and CH4 concentrations and the main ESA upwelling cells co‐occurred with a strong negative correlation with the sea surface temperature (SST) (p < 0.05). The dominance of the central water masses in the winter and spring seasons and the interplay between shelf topography and wind regime are suggested to determine enhanced gas transfer toward the sea‐air interface or “capping” at midwater depth. These parameters are supposed to be critical in the local budget of N2O and CH4 in the ESA. Our findings also show that the shape of N2O and CH4 gradients is very similar both meridionally and zonally; however, the extent of the differences between the high‐end and low‐end members of the concentrations/saturations range is different. This suggests a more pronounced effect of local sources on CH4 than N2O distribution, in particular in the Walvis Bay area. With respect to N2O, however, low‐oxygen waters from the poleward undercurrent impinge in the shelf close to Cape Frio and often result in N2O concentrations significantly higher than off Lüderitz (p < 0.05).

Highlights

Nitrous oxide (N2O) and methane (CH4) are long-lived atmospheric trace gases which account for a large fraction of the total radiative forcing due to well-mixed greenhouse gases (GHGs; Myhre, 2013)
It is well documented that the along-shelf poleward undercurrent of South Atlantic Central Water (SACW) from the north is suppressed by strong cross-shelf circulation (Mohrholz et al, 2008), which results in a poleward decreasing SACW fraction (Siegfried et al, 2019)
We found that SACW was dominant off Cape Frio with ∼92.50% relative to only 7.50% of Eastern-South Atlantic Central Water (ESACW), while ESACW was mostly dominant in Walvis Bay and Lüderitz with 70.00% and 78.00%, respectively (Figure 8)

Summary

Introduction

Nitrous oxide (N2O) and methane (CH4) are long-lived atmospheric trace gases which account for a large fraction of the total radiative forcing due to well-mixed greenhouse gases (GHGs; Myhre, 2013). The trace gas distribution in the ocean is spatially and vertically heterogeneous due to biogeochemical (microbial) and physical (mixing, upwelling, and stratification) processes, acting either as a trace gas source or sink with respect to the atmosphere (Bates et al, 1996; Reeburgh, 2007). N2O is produced in intermediate waters via microbially driven nitrification and denitrification under oxic and suboxic conditions, respectively, and exchanged with the atmosphere in surface waters (Bakker et al, 2014). During bacterial and archeal denitrification, N2O is an obligate intermediate in the stepwise reduction of NO3− to N2, such that this pathway may represent a source or a sink for this gas, with the enzymes involved on each step being differently affected by environmental conditions, e.g., oxygen concentrations (Zumft, 1997). During nitrifier-denitrification, N2O can be produced by direct reduction of NO2− (Cantera & Stein, 2007)

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