Trace Gases Variability in the Oxygen-Depleted African Eastern Boundary

Bita Sabbaghzadeh1, Stefan Otto2, Michael Glockzin1, Marcus Dengler3 and Gregor J Rehder4, (1)Leibniz Institute for Baltic Sea Research, Marine Chemistry, Rostock, Germany, (2)Leibniz Institute for Baltic Sea Research, Rostock, Germany, (3)GEOMAR, Kiel, Germany, (4)Leibniz Institute for Baltic Sea Research, Department of Marine Chemistry, Rostock, Germany
Abstract:
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.