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Geochemical Signatures for Redepositional Environments Beneath Oxygen Minimum Zones: The Benguela Upwelling System Offshore Namibia

Cofrancesco, Jessica
Due to changes in redox environment that occur during the deposition of marine sediments, trace metal paleo-proxies have become powerful tools for reconstructing paleo-depositional environments. While there are numerous proxies for understanding endmember redox conditions, there is still a significant gap in our understanding in regards to constraining specific signals for oxygen minimum zones (OMZ) and adjacent organic-rich settings found in ancient rocks. In this study, we explore the application of geochemical and sedimentological proxies in a modern organic-rich marine OMZ system � the Benguela upwelling system off the coast of Namibia. In this upwelling system, the lateral transport of the shelf mudbelt organic-rich sediments deposited under anoxic (sulfidic) bottom waters are being redeposited under an oxygen-rich water column at the upper slope, resulting in a secondary transported organic carbon depocenter. To assess the geochemical inventory of these two different areas of organic carbon accumulation, sediment cores were collected at the shelf mudbelt and slope depocenter, using the R/V MIRABILIS. Major elements and trace metal concentrations were determined for the aqueous phase and the solid phase samples. The solid phase was also analyzed for iron speciation and grain-size distribution. The results show significant enrichments above the lithogenic background for redox sensitive trace metals such as Mo and V, and for Ni - a bioproductivity indicator. The trace metal concentrations have highest enrichments in the sediments below the anoxic bottom waters; except for Ni, which shows a higher enrichment in the bioturbated and oxygenated slope sediments compared to the oxygen-depleted, finely laminated shelf sediments of the organic-rich shelf mudbelt. The positive correlation between nickel and total organic carbon that is observed in the sediments of upwelling zones is not maintained after lateral redeposition of the organic-rich sediments from shelf to slope in this system. The observed variations in trace metal enrichments in conjunction with high organic carbon concentrations at each site could provide a useful tool for identifying environments of organic carbon redeposition and ancient upwelling OMZs recorded in ancient rocks.