URANIUM ISOTOPES AS A PROXY FOR PRIMARY DEPOSITIONAL REDOX CONDITIONS IN REDEPOSITED SEDIMENTS OF THE NAMIBIAN CONTINENTAL MARGIN

Authigenic uranium (U) enrichments and U-isotope compositions are increasingly important tools for interpreting changes in organic carbon burial and the redox conditions of marine depositional environments. Their use as paleoproxies, however, requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and U-isotope composition of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt. %), each location displays a unique U to TOC relationship. Shelf sediments exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. Previous studies in the Namibian continental margin have shown that particle-rich nepheloid layers can rework seafloor sediment, moving organic-rich, uranium-laden deposits from within the OMZ through oxic water to be redeposited on the slope. This lateral movement may result in a release of the reduced U phases back into the water column through oxidation while preserving the organic carbon in the redeposited sediment. Oxidation of U during transport does not alter the average primary ²³⁸U/²³⁵U isotopic signature in redeposited sediment, and the combination of high TOC, low U content, and high δ²³⁸U values may become a useful tool for the identification of the boundaries of ancient OMZs.