Regional distribution of diffusive phosphate and silicate fluxes through the sediment–water interface: the eastern South Atlantic

From pore water concentration profiles and calculations of the corresponding diffusive fluxes, regional distribution patterns of benthic phosphate and silicate release rates were studied in the eastern South Atlantic. Comparisons among nutrient recycling rates and their control parameters, such as primary production and water depth, give evidence for significant differences in the benthic environment. Varying influences of these control parameters to decomposition and dissolution within surficial sediments are used to characterize single biogeochemical provinces within the study area. Maximum pore water concentrations (>0.4 mmol/l H4SiO4 and >10 μmol/l) and diffusive release rates (>400 mmol/m2 a H4SiO4 and >4 mmol/m2 a PO3-4 were observed in sediments off Namibia. Compared to benthic recycling rates in this upwelling regime, those in other regions of intensified primary production seem to be minor. While early diagenetic phosphate release is related to water depth and organic carbon flux to the sea floor, the benthic silicate flux is influenced by the general primary production and other parameters, such as the chemistry and rain rate of biogenic opal. These, again, depend on the oceanographic situation. With respect to the oceanic nutrient cycling and the role of marine deposits as geochemical sink, the presented data confirm the importance of degradation and dissolution processes at the sediment–water interface. At approximately 6.2×1011 mol/a the benthic silicate re-flux is about three times greater than the dissolved silicate discharge of the Zaire River, which is the dominant terrestrial source in this region.