Trace Metal Distribution in Redeposited Organic Carbon-Rich Sediments in a Modern Upwelling System

Abstract

To determine the depositional pathways and environments of organic carbon-rich material, sediment cores from the Benguela upwelling system offshore Namibia were investigated. Trace and major elements, organic carbon, and calcium carbonate were analyzed in sediments from three cores collected on the upper, middle, and lower continental slope. The results show that decoupled cadmium and zinc profiles as well as silver depletions in these sediments around the last glacial maximum suggest a change in the nutrient source in upwelled waters which may have impacted primary productivity. Vanadium accumulation correlates strongly with nickel and TOC accumulation in sediments on the lower and middle continental slope. This correlation is lost in sediments higher on the slope. The lack of correlation between nickel and vanadium with TOC in the upper slope core indicates redeposition of reworked, laterally transported sediments from the continental shelf. Additionally, at all three core sites, vanadium shows strong correlation with iron and manganese, indicating that oxic bottom waters have been present during much of the recorded time period. The proxies evaluated in this study suggest that during the last glacial maximum in the Benguela upwelling system around our study sites, intensified upwelling incorporated nutrients from a different source. An increase in primary productivity around that time period resulted in increased deposition of organic carbon-rich materials on the continental slope.