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

Sulfate reduction rates in the surface sediments from 17 stations from an a long-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 Upwell ing system. Profiles at all sites in the upwelling area showed increasing s ulfate reduction rates from near zero at the surface to a peak at 2-5 cm (u p 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-integ rated sulfate reduction rates were greatest at 1,300 m and decreased expone ntially with water depth. Along the transect following the 1,300-m isobath, depth-integrated sulfate reduction rates were highest in the north Cape Ba sin (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 (0.3 1 +/- 0.23 mmol m(-2) d(-1)). Depth-integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and wer e also strongly correlated with surface organic carbon concentrations. Sulf ate reduction rates, both as a function of depth and in comparison with sed iment trap data, indicated that lateral downslope transport of organic carb on occurs. Sulfate reduction was estimated to account for 20-90% of the pub lished 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 sulf ate reduction rate profiles with the published diffusive oxygen uptake rate s showed that the kinetics of oxygen utilization in the surface sediments a re much faster than those for anaerobic organic carbon remineralization, al though the underlying cause of the difference was not clear.