Productivity and organic carbon loading control uranium isotope behavior in ancient reducing settings: Implications for the paleoredox proxy

Uranium isotopes in ancient sedimentary rocks have emerged as a powerful proxy for the oxygenation history of Earth surface environments. Proper quantitative interpretation of delta U-238 data hinges, however, on an understanding of isotopic fractionation associated with uranium removal to sediments under different redox conditions. Whereas oxygenated environments are plentiful in the modern ocean, euxinic settings are scarce today, and persistently ferruginous settings are not a feature in the modern ocean, leaving lakes as our best analogs for iron-rich waters. These challenges to studying delta U-238 behavior under contemporaneous oxic, euxinic (sulfidic), and ferruginous (iron-rich) marine conditions limit our ability to apply the isotope proxy to ancient oceans. Here, we present delta U-238 data from five coeval cores through the Lower Mississippian Sunbury Shale of the semi-restricted Appalachian Basin (North America), which were deposited across a strong redox gradient from intermittently oxic conditions proximal to the Catskill Delta, to ferruginous conditions in the central basin axis and euxinic conditions along the basin-bounding Cumberland Sill. Overall, we find that both euxinic and ferruginous environments are capable of imparting high degrees of uranium isotope fractionation and that delta U-238 values covary more strongly with total organic carbon (TOC) and trace metal abundances than they do with the presence of euxinic versus ferruginous conditions. We suggest uranium sorption or incorporation into sinking organic matter in the water column as a mechanism for generating positive covariation between delta U-238 and TOC, particularly at lower TOC concentrations. We also compare our delta U-238 data to broadly coeval carbonates from Nevada, which can be used to estimate Early Mississippian seawater delta U-238 values. We find that Sunbury Shale cores deposited under stably anoxic conditions (both euxinic and ferruginous) and under consistently brackish salinities away from freshwater sources record an increasing delta U-238 trend through time that loosely mirrors the record for carbonates (and thus estimates for seawater) with an offset of similar to 0.4 parts per thousand to 1.1 parts per thousand, depending on the inferred offset between carbonate and seawater. In contrast, cores deposited close to the Catskill Delta and in a shallow oceanic-restricted basin transition across the Cumberland Sill record a decreasing delta U-238 trend through time, opposite the inferred seawater trend and suggestive of entirely local controls on delta U-238 values. Ultimately, these data suggest that highly productive environments with high rates of organic carbon loading can exert a strong control on the global delta U-238 mass balance regardless of whether euxinic or ferruginous conditions are present. These results necessitate a partial reassessment of the processes controlling delta U-238 variability in seawater and sedimentary rocks through Earth history.