Paleo-oxygenation and Uranium Isotope Behavior in Lower Mississippian Black Shales of North America


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Uranium isotopes in ancient sedimentary rocks have emerged as a powerful proxy for the oxygenation history of Earth surface environments. Proper quantitative interpretation of uranium isotope data hinges, however, on an understanding of isotope fractionation associated with uranium removal to sediments under different redox conditions. Whereas oxygenated environments are plentiful in the modern ocean, studying contemporaneous uranium isotope behavior under a range of low-oxygen conditions is difficult given the paucity modern anoxic basins. For example, it is not possible to simultaneously test uranium isotope fractionation under oxic, euxinic, and ferruginous marine conditions using modern analogues. Here, we present uranium isotope data from coeval Lower Mississippian shales of the Appalachian Basin (North America) that represent a range of redox conditions. Iron speciation and trace metal data indicate that the Tournaisian Sunbury Shale was deposited across a strong redox gradient, from oxic to equivocal conditions proximal to the Catskill Delta in the northeast, to ferruginous conditions in the basin trough, to euxinic conditions towards the basin-bounding Cumberland Sill in the southwest. Overall, we find that both euxinic and ferruginous environments are capable of imparting high degrees of U isotope fractionation, and that U isotope values correlate more strongly with total organic carbon (TOC) and trace metal proxies for productivity than they do with euxinic vs. ferruginous conditions. We also compared our data to coeval carbonates from Nevada, which can be used to estimate Early Mississippian seawater U isotopes values. We find that cores deposited under more stably anoxic conditions below the redoxcline and under consistently brackish salinities away from direct freshwater sources record an increasing U isotope value through time, which loosely mirrors global seawater values. By contrast, cores deposited close to the Catskill Delta and in a shallow mixing zone near the Cumberland Sill show a decreasing U isotope trend through time which is the opposite of global seawater, indicating entirely local controls on U isotope fractionation. Ultimately, these results suggest that highly productive environments with high organic carbon loading can exert a strong control on the global U isotope mass balance regardless of whether euxinic or ferruginous conditions are present. These results necessitate a partial reassessment of the processes controlling seawater U isotope variability through Earth history.