REDOX GRADIENTS RECORDED IN LOWER MISSISSIPPIAN BLACK SHALES OF THE APPALACHIAN AND WILLISTON BASINS, NORTH AMERICA: A TEST CASE FOR URANIUM ISOTOPE BEHAVIOR

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 North America that represent a range of redox conditions. Previously published iron speciation and trace metal data indicate that the Sunbury Shale of the Appalachian Basin was deposited across a strong redox gradient, from oxic conditions proximal to the clastic wedge, to ferruginous conditions in the basin trough, to euxinic conditions towards the basin-bounding sill. In addition, coeval black shale of the Williston Basin (Upper Bakken Shale) was deposited under hypersulfidic conditions as evidenced by zinc and vanadium hyper-enrichment. Taken together, these deposits represent the perfect test case for uranium isotope behavior because a full range of redox conditions—oxic, ferruginous, sulfidic, and hypersulfidic—are recorded in coeval units. In this study, we will present uranium isotope data from five cores that intersect the Sunbury Shale in the Appalachian Basin and one core that intersects the Upper Bakken Shale in the Williston Basin. These data will provide valuable new insight on the uranium isotope system, which has emerged as a premier tool for deciphering Earth’s oxygenation history.