Tracking uranium (V) species in ancient sedimentary rocks: Advancing the paleo-redox prox

Uranium speciation controls its reactivity and transport in natural settings and is strongly modulated by redox fluctuations, but how its speciation varies in reducing marine environments and its significance for U sequestration processes in these settings remains unknown. The redox-sensitive behavior of uranium and its isotopic composition has emerged as a promising proxy to reconstruct the redox state of the oceans through Earth’s history, but interpretation of uranium concentration and isotope ratio records is based on poorly constrained assumptions of the phases and burial pathways of reduced uranium. To investigate the processes that control U redox in marine sediments that ultimately are recorded in sedimentary rocks, we propose to determine the U distribution at µm-scale resolution and characterize its speciation using HERFD-XANES within well-characterized organic carbon-rich fine-grained rocks (shales) that were deposited in a range of reducing marine environments.