Limnol. Oceanogr., 44(3), 1999, 650–661

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 cm23 d21) and then decreasing exponentially with depth to near background rates at 10–20 cm depth (,2 nmol cm23 d21). 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, depthintegrated sulfate reduction rates were highest in the north Cape Basin (1.16 6 0.23 mmol m22 d21), decreased over the Walvis Ridge (0.67 6 0.02 mmol m22 d21), and were lowest in the south Angola Basin (0.31 6 0.23 mmol m22 d21). 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. The benthic remineralization of organic carbon exerts an important influence on the flux and distribution of carbon and nutrients throughout the ocean, especially along continental margins (Jahnke et al. 1990; Hensen et al. 1998; Wollast 1998). The distribution of bottom-water oxygen, dissolved inorganic and organic carbon fluxes, nutrient fluxes, and the balance between water-column particulate organic carbon flux and burial of organic carbon in the sediments is determined by the rate and perhaps the pathway of organic carbon remineralization. In the deep ocean, dissolved O2 fluxes into the sediment account for the sum total of benthic remineralization processes (Cai and Reimers 1995; Jahnke 1996); however, anaerobic processes begin to play a quantitatively important role in organic carbon remineralization 1 Present address: National Environmental Research Institute, Department of Lake and Estuarine Ecology, Vejlsoevej 25, P.O. Box 314, DK-8600 Silkeborg, Denmark.