Trace Gases Variability in the Oxygen-Depleted African Eastern Boundary

The productive African Eastern boundary, including the Benguela upwelling system, is one of the largest oxygen-depleted zones of the world. The high surface productivity triggered by nutrient-rich upwelled waters results in significant fluxes of organic carbon to sub-surface waters. Hence microbial O₂-consuming processes are promoted, driving oxygen depletion that favors N₂O and CH₄ production at relatively shallow depths. During upwelling, N₂O and CH₄-rich subsurface waters are also transported towards the surface waters, enhancing trace gas sea-air fluxes. To investigate the similarities and differences in variation of trace gases (i.e. CH₄, N₂O, and CO₂), several hydrographic sections perpendicular to the coast and also offshore transects were carried out between ~10°S and 25°S. The surface trace gas concentrations were also continuously monitored along and between the transects.

Water mass expression on surface water revealed cold and less oxygen-depleted surface waters in the more southern latitudes, as well as evidence of upwelled waters with an indication of filaments kilometres away offshore. In general, relatively moderate surface trace gas concentrations with a clear relation to sea surface temperature (i.e. SST) were revealed. The surface partial pressure of CO₂ (i.e. pCO₂) showed a quite uniform pattern, with the higher pCO₂ in nearshore waters and the maxima around 23°S. N₂O showed a comparable pattern to pCO₂ with the higher inshore concentrations towards the southern transects (i.e. between 15°S and 25°S). CH₄ concentrations also followed almost the same pattern to pCO₂ and N₂O, with gradients closer to the coast, and the maximum concentration of 250nM in inshore upwelled waters around 23°S.

N₂O water column profiles revealed depleted surface waters in relation to underlying waters along all transects. The sub-surface waters to a depth of about 1000m were enriched in N₂O with maxima of 50nM and became less concentrated in deeper waters. Also, a strong correlation between N₂O and apparent oxygen utilization (AOU) was found during all transects. CH₄ depth profiles, on the other hand, showed high concentrations through the water column in the stations closer to the shore.