Covariation between molybdenum and uranium isotopes in reducing marine sediments

The isotopic compositions of redox-sensitive elements such as uranium (U) and molybdenum (Mo) have been extensively used to track the evolution of redox conditions on Earth???s surface. However, additional investigation of the behaviors of U and Mo in modern marine sediments???particularly in reducing sediments underlying oxic seawater???can bolster understanding and improve application of these systems as biogeochemical tracers. Here we present elemental and isotopic Mo and U data from reducing sediments from various sites in the Bering Sea where the extent of bioturbation is well characterized. Total Mo concentrations and total organic carbon contents (TOC) are positively correlated, as are authigenic Mo isotopic compositions and TOC, indicating the presence of authigenic Mo enrichments in these sediments. Similar to what has previously been observed in euxinic settings, we observe a positive correlation between authigenic U and Mo concentrations, and a negative correlation between the isotopic compositions of authigenic U and Mo (??238Uauth and ??98Moauth). These patterns likely reflect close associations of U and Mo reduction to organic carbon degradation. Observed variation in ??98Moauth values could reflect variations in Mo removal efficiency or Mo isotopic fractionation between particles and seawater related to porewater H2S levels. Conversely, we interpret variation in ??238Uauth values in these Bering Sea samples to be related to varying extents of U reduction and removal, coupled with anaerobic carbon degradation. There is no significant correlation between the extent of bioturbation and Mo or U isotope values, suggesting that bioturbation would likely not have a significant influence on sedimentary Mo and U records. Further, the array of U and Mo isotopic data from these sediments is distinct from those observed in anoxic sediments below euxinic seawater???suggesting that a combination of U and Mo isotopic compositions could help to more accurately resolve the redox state of depositional environments from past intervals of Earth???s history.

Automated summary

The study found enrichments of isotopically altered molybdenum and uranium in reducing sediments from the Bering Sea, likely indicating a close association between the reduction of U and Mo and organic carbon degradation.