Geochemical signatures of redepositional environments: The Namibian continental margin

Abstract

Trace metal abundances in marine sediments have been used extensively to interpret periods of elevated primary productivity and ancient ocean redox conditions. However, sediment reworking that results in post-depositional oxidation, such as bio-irrigation or lateral sediment transport through oxic water, can modify the primary geochemical signal of the sediment, which in turn may impact paleo-redox and/or -productivity interpretations. In the case of sediments on the Namibian Continental Margin (NCM), lateral transport and redeposition contribute to the accumulation of organic matter on the margin slope. To better constrain the geochemical effects of lateral transport on the NCM, we examined the trace metal signature (including solid-phase Fe, Mo, V, Ni, Cu and Ag, and pore-water Fe, Mo, and V) in surface sediments (up to 25 cmbsf) along a transect from shelf to slope through the primary (shelf) and secondary (upper slope) depositional zones of the margin. Despite varying bottom water redox conditions ranging from seasonally anoxic (upper shelf), suboxic (shelf break), and oxic (upper slope), each site has elevated total organic carbon contents (average TOC content of 9.1, 3.2 and 6.8 wt%, respectively), due to high surface water primary productivity and lateral transport of organic-rich material from the shelf to the upper slope. Our results show that the solid-phase contents of the productivity proxies Ni, Cu, and Ag parallel the organic carbon accumulations largely irrespective of the local redox conditions. In contrast, the solid-phase contents of V, Mo, and Fe respond to the local bottom water redox conditions at each site, being enriched under strongly reducing conditions and less-enriched under oxic bottom waters despite enhanced TOC at all three investigated sites. Thus, the enrichment of trace metal and TOC at each site on the NCM can not only be used to reconstruct primary depositional bottom water redox conditions but also to identify zones of sediment redeposition. Using an offshore transect allows for the identification of intense lateral transport and redeposition of organic-rich sediments that are occurring along the margin. The relative concentrations of both redox-sensitive and productivity-related trace metals suggest that the decoupling of trace metals and organic carbon enrichments occasionally observed in the geological record could be explained by the process of lateral transport and redeposition.