Abstract:
Molybdenum (Mo) and uranium (U) concentrations and isotopic ratios are widely used to reconstruct the evolution of oceanic and atmospheric redox conditions. The application of Mo-U systematics generally relies upon our understanding of their behavior in modern oceanic settings. Although the mechanisms controlling sedimentary accumulation of Mo, U and the associated isotopic fractionations have been widely studied, little attention has been paid to the extent of Mo and U isotope variation of pore waters from continental margin marine sediments. Here we present coupled Mo and U data for the near-surface coexisting sedimentary solid phases and pore waters retrieved from the Benguela upwelling system off Namibia continental margin. The sampling sites represent shelf-to-slope settings with different redox conditions spanning from intermittently anoxic/euxinic to suboxic to fully oxygenated. The sediments are characterized by varying degrees of enrichment of Mo and U with depth associated with different isotopic signatures. With the site becoming more reducing, the average Mo concentration in the sediments increases from 2.6 ppm to 6.5 ppm with average 98Mo increasing from 0.85‰ to 1.5‰, approaching modern seawater value (2.3‰). Meanwhile, the average U concentration in the sediments decreases from 11 ppm to 8 ppm with average 238U increasing from -0.26‰ to -0.10‰, positively shifting from the modern seawater value (~ -0.40‰). At each site, sediment 98Mo values are lightest at the bottom of the core and become heavier upwards in the profile, recording changes of the redox conditions through time. In contrast, the variability in U isotopic compositions of the sediments is small in each site, indicating distinct response of U isotopes to the redox conditions. The extent of the 238U variations in the sediments can be explained in context of variable U isotope mass-balance during non-quantitative authigenic U sediment uptake. The pore water profiles demonstrate large variations in Mo and U concentrations and isotopic ratios. The pore water 98Mo correlate negatively with Mo concentrations, suggesting light Mo isotopes are preferentially released to the pore waters. We show that the Mo isotopic compositions of the sediments exert an important control on the 98Mo signatures of the pore waters. In contrast, pore water U data record both release of U from the sediments and uptake of U from the pore waters. U reduction and removal is accompanied by a progressive shift in 238U/235U towards isotopically light values in the pore waters as heavier 238U is preferentially exported to sediments over lighter 235U. This gives rise to apparent isotope enrichment factors of ε = -0.41‰ to -0.81‰ when U removal is modelled by Rayleigh isotope fractionation. These ε values fall within the range determined for bacterial U reduction experiments. Importantly, we have observed extensive re-oxygenation of the reduced Mo and U from the sediments, as evidence by the elevated pore water concentrations. The above Mo and U isotopic behaviors in the pore waters can provide important constraints on the use of Mo and U isotopes as a proxy of the dynamic redox state of ancient oceans.

Research experience:
High-temperature experiments on element partitioning between silicate mineral and melt
Applications of Fe, Cu and Zn isotopes in magmatic-hydrothermal ore deposits
Study of Fe-Zn-Ni-Mo-U isotopes in marine sediments and pore waters