Controls on uranium isotope fractionation in the late Paleoproterozoic ocean

• Wide range of δ 238 U authigenic (–0.52 to 0.53‰) in the 1.84 Ga Rove and Virginia Formations. • Large isotopic fractionations related to microbial U reduction, slow reaction kinetics, and/or aqueous U speciation. • Deeper water ferruginous-anoxic environments record limited isotopic fractionation. • Small areas of highly productive Paleoproterozoic continental margins coupled with non-productive anoxic deep oceans can produce δ 238 U seawater near modern values. Uranium isotope data from Proterozoic carbonates (δ 238 U carb ) and black shales (δ 238 U auth ) are enigmatic. Average Proterozoic δ 238 U carb (approximating Proterozoic seawater, δ 238 U sw ) is similar to modern river/seawater δ 238 U, and Proterozoic black shales do not always record highly fractionated δ 238 U auth compared to contemporaneous δ 238 U carb . However, very light δ 238 U carb and heavy δ 238 U auth for the widely anoxic Proterozoic oceans was expected because large isotope fractionations accompany U reduction in anoxic environments. To address this enigma, we report black shale δ 238 U auth through a well-characterised multi-core transect in the late Paleoproterozoic Animikie Basin, North America. There is a wide range of δ 238 U auth , from –0.52‰ to 0.53‰, that generally correlates with organic carbon enrichments. Heavy δ 238 U auth in organic-rich shallow shelf sediments within and near a euxinic wedge are attributed to enhanced productivity with vigorous sediment microbial activity, neutrally charged aqueous U species that slow reaction kinetics, and redoxcline fluctuations. In less organic-rich sediments of the anoxic-ferruginous deep shelf environment, characterized by lower productivity and plentiful reduced iron availability, light δ 238 U auth may reflect rapid U reduction or adsorption to solid Fe/Mn species. Hence, for the widely anoxic Proterozoic oceans, we propose that large isotopic fractionations (0.4 to 1.2‰) were associated with highly productive areas on anoxic continental margins, and muted isotopic fractionations (–0.1 to 0.4‰) occurred in anoxic deep-ocean environments. Low-productivity Proterozoic oceans yielded sediments with δ 238 U carb and δ 238 U auth close to modern river/seawater values, whereas higher-productivity basins (e.g., Animikie Basin) resulted in lower δ 238 U carb and more variable δ 238 U auth .