Abstract
The abundances and isotope compositions of molybdenum (Mo) and uranium (U) in ancient sediments are promising tracers of the redox state of the past ocean, whose basis lies in the environmentally dependent Mo and U isotope signatures in modern oceanic settings. Despite their dominance in oceanic budgets, the controls on the Mo-U systematics of upwelling margin sediments remain to be fully understood. Here we present a comprehensive sediment-porewater Mo and U isotope study in the Benguela upwelling system off Namibia, including the first dataset incorporating coupled Mo-U abundance and isotope analysis of both solid authigenic phases and porewaters.The investigated stations represent shelf-to-slope settings, which lie on the upper edge, within and below the oxygen minimum zone (OMZ) of the South Atlantic. The sediments across all stations share similar characteristics: both Mo and U show increasing authigenic enrichment with depth, coupled to an overall decrease in δ98Moauth (from ~2.0‰ to 1.3‰ and from ~2.0‰ to 0.3‰) and increase in δ238Uauth (from −0.18‰ to 0.05‰ and from −0.34‰ to −0.21‰). Nevertheless, the extents of Mo and U enrichment and associated isotopic fractionations display spatial variability across the OMZ, reflecting variations in local sedimentary redox conditions. Porewater Mo and U concentration patterns are more complex, exhibiting peaks in Mo and U abundance well in excess of seawater (up to 8 times seawater for Mo) associated with correlated shifts in isotope composition. As a result, porewaters exhibit a wide range in isotope compositions, between 0.90‰ and 2.79‰ for δ98Mo and between −1.74‰ and 0.26‰ for δ238U.Porewater gradients at the time of sampling are inconsistent with diffusion downwards across the sediment–seawater interface as a means of enrichment of the sediment-porewater system. Though these sampled conditions may represent only a snapshot, so that periodically more reducing conditions could lead to concentration gradients that do permit downward diffusion, the data are also readily explained by addition of Mo and U to the sediment-porewater system in particulate form, also under more reducing conditions than at the time of sampling. For example, sequestration of Mo and U to particulate matter as a result of the presence of intermittent sulfide, either in bottom water or in porewater right at the sediment–water interface, explains much of the geochemical and isotope data.The data thus suggest that the early diagenetic enrichment of Mo and U in sediments of upwelling margins is strongly governed by temporal redox fluctuations. Early diagenesis under these dynamic redox conditions on the Namibian upwelling margin are strongly reflected in Mo-U co-variation patterns, as well as anti-correlations between authigenic δ98Mo and δ238U in sediments. Overall, our new data demonstrate that early diagenetic processes on open-marine continental margins reproduce patterns previously observed for coupled Mo-U isotope systematics in restricted and semi-restricted basins, but via a different set of processes and with important implications for the use of such a coupled approach in the study of ancient marine anoxia.
Highlights
The sedimentary record of molybdenum (Mo) and uranium (U) abundances and their isotope compositions are widely used to investigate past global marine redox conditions (e.g., Kendall et al, 2011; Azrieli-Tal et al, 2014; Chen et al, 2015; Cheng et al, 2016; Clarkson et al, 2018; Tostevin et al, 2019; Zhang et al, 2020)
From the inner shelf through the oxygen minimum zone (OMZ), Fe/ Al ratios increase from values close to the upper continental crust (UCC) background for core 26010 (0.52–0.65) to values >2.0 in cores 26030 and 26070
The highest Fe/ Al ratios coincide with the lowest Fepy in sediments at the middle shelf site, within the OMZ, indicating sedimentary Fe enrichment controlled by a phase other than sulfide
Summary
The sedimentary record of molybdenum (Mo) and uranium (U) abundances and their isotope compositions are widely used to investigate past global marine redox conditions (e.g., Kendall et al, 2011; Azrieli-Tal et al, 2014; Chen et al, 2015; Cheng et al, 2016; Clarkson et al, 2018; Tostevin et al, 2019; Zhang et al, 2020). In strongly euxinic settings, where the aqueous H2S concentration exceeds 11 lM, the conversion of molybdate to tetrathiomolybdate (MoS24À) is nearly complete and Mo can be quantitatively scavenged into sulfidic sediments (Erickson and Helz, 2000) Sediments in these settings (for example, in the Black Sea) can evolve to isotope compositions that approach the seawater source (Nagler et al, 2005; Neubert et al, 2008)
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