URANIUM SPECIATION IN MARINE PHOSPHORITES AND THE IMPLICATIONS ON THE PALEO-REDOX PROXY

The amount of oxygen in the ocean has exerted a major control on the development of complex life throughout Earth’s history, making it important to understand the timing and frequency of oxygenation changes. Because past oxygen contents cannot be directly measured, paleo-redox proxies are used to track changes in oxygen content through time. Records of uranium (U) concentrations and isotopes are invaluable for reconstructing long-term and transient changes in the redox state of oceans. The redox-controlled solubility and isotope fractionation of U make it an applicable proxy for revealing these changes. However, interpreting U concentrations and U isotopes from marine sedimentary rocks as a paleo-redox proxy assumes that burial and isotope fractionation are associated with the reduction of U(VI) to U(IV). Without knowing the oxidation states or phase associations of U in marine sedimentary rocks, interpretations of the U isotope proxy remain uncertain. We address this by characterizing U speciation within anoxic marine sedimentary samples.

We paired X-ray fluorescence (XRF) maps with X-ray absorption near edge structure (XANES) spectroscopy at the L3 edge to study U speciation in marine phosphorites from the Monterey Formation (Miocene) and the Phosphoria Rock Complex (Permian). Elemental correlations from the XRF mapping suggest U is associated with both phosphate and organic matter. Linear combination fitting results from XANES measurements indicate a mixture of U(IV), U(V), and U(VI) oxidation states in both formations. Taken together, these new insights draw a more complicated picture where multiple U redox pathways occur simultaneously in phosphorus rich environments. The U oxidation states in marine sedimentary rocks, including the apparent persistence of U(V), could impact the interpretation of the U isotope signature.