Sulfate reduction in surface sediments of the southeast Atlantic continental margin between 15°38'S and 27°57'S (Angola and Namibia)

Abstract

Sulfate reduction rates in the surface sediments from 17 stations from an along‐slope transect (1,300 m) and from a cross‐slope transect (855–4,766 m) were determined in the continental margin sediments of the Benguela Upwelling system. Profiles at all sites in the upwelling area showed increasing sulfate reduction rates from near zero at the surface to a peak at 2–5 cm (up to 29 nmol cm−3 d−1) and then decreasing exponentially with depth to near background rates at 10–20 cm depth (<2 nmol cm−3 d−1). Depth‐integrated sulfate reduction rates were greatest at 1,300 m and decreased exponentially with water depth. Along the transect following the 1,300‐m isobath, depth‐integrated sulfate reduction rates were highest in the north Cape Basin (1.16 +/− 0.23 mmol m−2 d−1), decreased over the Walvis Ridge (0.67 +/− 0.02 mmol m−2 d−1), and were lowest in the south Angola Basin (0.31 +/− 0.23 mmol m−2 d−1). Depth‐integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and were also strongly correlated with surface organic carbon concentrations. Sulfate reduction rates, both as a function of depth and in comparison with sediment trap data, indicated that lateral downslope transport of organic carbon occurs. Sulfate reduction was estimated to account for 20–90% of the published rates of total oxygen consumption for the sediments at 1,300 m depth and 3–16% of sediments from 2,000 to 3,000 m depth. Comparison of the sulfate reduction rate profiles with the published diffusive oxygen uptake rates showed that the kinetics of oxygen utilization in the surface sediments are much faster than those for anaerobic organic carbon remineralization, although the underlying cause of the difference was not clear.