Controls on uranium isotope fractionation in the late Paleoproterozoic ocean

Abstract

Uranium isotope data from Proterozoic carbonates (δ²³⁸U_carb) and black shales (δ²³⁸U_auth) are enigmatic. Average Proterozoic δ²³⁸U_carb (approximating Proterozoic seawater, δ²³⁸U_sw) is similar to modern river/seawater δ²³⁸U, and Proterozoic black shales do not always record highly fractionated δ²³⁸U_auth compared to contemporaneous δ²³⁸U_carb. However, very light δ²³⁸U_carb and heavy δ²³⁸U_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 δ²³⁸U_auth through a well-characterised multi-core transect in the late Paleoproterozoic Animikie Basin, North America. There is a wide range of δ²³⁸U_auth, from –0.52‰ to 0.53‰, that generally correlates with organic carbon enrichments. Heavy δ²³⁸U_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 δ²³⁸U_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 δ²³⁸U_carb and δ²³⁸U_auth close to modern river/seawater values, whereas higher-productivity basins (e.g., Animikie Basin) resulted in lower δ²³⁸U_carb and more variable δ²³⁸U_auth.