Controls on uranium isotope fractionation in the late Paleoproterozoic ocean

Uranium isotope data from Proterozoic carbonates (δ238Ucarb) and black shales (δ238Uauth) are enigmatic. Average Proterozoic δ238Ucarb (approximating Proterozoic seawater, δ238Usw) is similar to modern river/seawater δ238U, and Proterozoic black shales do not always record highly fractionated δ238Uauth compared to contemporaneous δ238Ucarb. However, very light δ238Ucarb and heavy δ238Uauth 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 δ238Uauth through a well-characterised multi-core transect in the late Paleoproterozoic Animikie Basin, North America. There is a wide range of δ238Uauth, from –0.52‰ to 0.53‰, that generally correlates with organic carbon enrichments. Heavy δ238Uauth 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 δ238Uauth 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 δ238Ucarb and δ238Uauth close to modern river/seawater values, whereas higher-productivity basins (e.g., Animikie Basin) resulted in lower δ238Ucarb and more variable δ238Uauth.