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 δ⁹⁸Mo_auth (from ~2.0‰ to 1.3‰ and from ~2.0‰ to 0.3‰) and increase in δ²³⁸U_auth (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.