Carbonate associated uranium isotopes as a novel local redox indicator in oxidatively disturbed reducing sediments

Abstract

The interpretation of local redox indicators in marine sediments relies on their preservation. By their nature, such biogeochemical tracers are susceptible to diagenetic alteration, particularly through post-depositional re-oxidation of the sediment. This can result in the mobilization and loss of distinctive redox dependant signatures, leading to the classical problem that the presence of oxygenated conditions cannot be robustly inferred by the absence of evidence for anoxia. Authigenic uranium enrichments, and their isotope signatures (δ238U), are widely used to infer bottom-water and pore-water redox conditions but are susceptible to later oxidative diagenetic disturbance. Here we explore the preservation of authigenic δ238U signatures in sediment samples from Sapropel S1 at Site 64PE406-E1 in the Eastern Mediterranean Sea (~1760 m water depth), which was originally deposited under reducing conditions but severely affected by later oxidative diagenesis and the variable loss of authigenic U(IV) from the upper sapropel. To this end, we compare U isotope signatures from bulk measurements (δ238Ubulk) with detrital corrected authigenic U (δ238Uauth) and carbonate associated U (δ238UCAU). In contrast to open ocean carbonates deposited under fully oxic conditions, sapropelic carbonate leachates yield δ238UCAU similar to calculated δ238Uauth, thus recording (predominantly) redox dependant isotope fractionation. There is no evidence for significant isotope fractionation resulting from post-depositional oxidative diagenesis, but associated authigenic U(IV) removal results in larger relative detrital contributions to δ238Ubulk, and hence large uncertainties on δ238Uauth estimates. By contrast, δ238UCAU successfully avoids detrital phases, and the uncertainty associated with detrital corrections, providing a primary record of changes in local redox conditions. Thus, we propose that δ238UCAU can be used for accurate reconstructions of benthic de-oxygenation in sediments with low U enrichments, including sediments with post-depositional U(IV) loss and less reducing environments.