Evaluating the importance of sulfurized organic matter for the sedimentary cycling of molybdenum and tungsten in upwelling regimes: a case study on the Namibian shelf

• Applicant: Dr. Sümeyya Eroglu
• Subject Area: Geology ; Mineralogy, Petrology and Geochemistry ; Oceanography
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495849770

One of the major sinks of trace metals in the marine environment are anoxic open-marine settings, which form along highly productive continental margins, where about 70% of global microbial sulfate reduction takes place. Resulting sulfidic conditions are highly relevant for the immobilization and fixation of trace metals in marine sediments via reduction, adsorption and incorporation into sulfidic phases of the reduced sulfur pool. The formation of this sulfur pool, which is mainly composed of iron-sulfides, e.g. pyrite, and sulfurized organic matter, is the result of a complex cycling of reduced and oxidized (intermediate) sulfur species. While enrichment mechanisms of trace metals are mainly connected to pyrite formation, the relevance of sulfur dynamics and in particular of sulfurized organic matter for trace metal accumulation remains enigmatic, although it is potetially the key to explain wide ranges of concentrations and isotope composition of certain redox-sensitive elements in anoxic open-marine settings. This includes molybdenum (Mo) and tungsten (W), which are both bio-relevant and redox-sensitive trace metals, behave conservative in oxic seawater and have an affinity to adsorb onto Mn- and Fe-oxides. However, under sulfidic conditions, at the presence of hydrogen sulfide, Mo rapidly changes its geochemical behavior, becomes particle-reactive and enriched in sediments, which makes it a valuable proxy to trace sulfidic conditions. In contrast, W remains soluble even at elevated sulfide concentrations and its enrichment mechanisms in anoxic sediments of upwelling regimes are still poorly constrained. Nevertheless, W concentrations and isotope signatures of marine sediments emerge as valuable redox proxies, complementing the established Mo isotope redox proxy. Given the importance of anoxic open-marine settings for the global mass balance of Mo and, presumably, W, this study aims to close major knowledge gaps and to improve the application of Mo- and W-based redox proxies.For this purpose, sediments from different redox regimes along the Namibian shelf will be investigated. The Namibian shelf is controlled by the Benguela upwelling system, which drives primary production in the surface water, resulting in enhanced oxygen utilization and the formation of anoxic bottom water overlying the organic-rich shelf sediments. The plan is to investigate sulfur speciation and redox dynamics in these surface sediments, covering redox conditions from anoxic-sulfidic to fully oxic. This will be achieved by sequential extraction and stable isotope analyses of different sulfur species. These results will be coupled with Mo and W isotope analyses in order to gain an understanding of enrichment mechanisms of these redox-sensitive trace metals. The overall aim is to constrain Mo- and W-based redox proxies in anoxic open-marine settings in more detail, focussing on sulfurized organic matter.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Olaf Dellwig; Dr. Florian Kurzweil, Ph.D.; Professor Dr. Florian Scholz