Connection of sulfide species and sedimentary trace metal cycles in the Namibian shelf – implications for paleo-marine redox proxies

The identification of redox-states and -cycling in the paleo-ocean is typically based on the application of geochemical proxies in the geological record. In particular total organic carbon (TOC) and total sulfur (TS) contents, trace metal (TM) enrichments, elemental ratios (e.g. TM/Al, Fe/Al) and stable isotope compositions (e.g. of sulfur and molybdenum) in siliciclastic sedimentary rocks are used to distinguish between anoxic-ferruginous and -sulfidic conditions of the past [e.g. 1]. For the understanding of depositional and diagenetic mechanisms that may affect these proxy signatures, their detailed calibration in modern marine environments is, however, required. Notably, the connection of TM accumulation, TOC content and sulfide production is still not fully understood. In this context, open-marine shelf sediments connected to upwelling regimes are of high importance given that high organic carbon burial fuels sulfate reduction and the accumulation of TMs in the sediment.

Here we investigate sediment cores from different redox-regimes along the Namibian shelf, which are controlled by the Benguela upwelling system. Sediments were deposited under anoxic-sulfidic to oxic conditions and show a correlation of TOC (average: 3.57 to 8.93 wt-%) and TS concentrations (average: 1.02 to 1.42 wt-%). In contrast, TM enrichments are mainly uncorrelated with TOC and TS contents. Indeed, the extraction of different sulfide species and their sulfur isotopic compositions reveal variabilities in the sulfur cycling between the investigated settings and an influence on the accumulation of TMs. We discuss the importance of different sulfur species on TM redox-proxies.

[1] Calvert and Pedersen (1993) Marine Geology 113, 67-88.