Moderate Uranium Isotope Fractionation in Weakly Euxinic Oceans During the Early Proterozoic Great Oxidation Episode

The Great Oxidation Episode (GOE) is the first permanent rise in atmospheric O2 concentration above a threshold of ~10-6-10-5 present atmospheric level (PAL) that occurred between ca. 2.43 and 2.06 billion years ago (Ga). Yet discussion continued regarding the magnitude and scale of Earth’s surface oxygenation across the GOE. Recently, the widely accepted view of a global deoxygenation in atmosphere-ocean system at the termination of the GOE has been challenged by a suggestion of a globally advanced uranium (U) biogeochemical cycle with large U isotope fractionations and high U concentrations in a single section from the Karelia Craton where black shales were deposited in the immediate aftermath of the GOE. Taken at face value, this interpretation implies that the oceans remained well-oxygenated for millions of years in the aftermath of the GOE, with modern-style biogeochemical cycles operating. Here, we present new results on U isotope composition of another black shale and carbonate succession deposited at the terminal stage and in the aftermath of the GOE on the Congo Craton. The studied black-shale U isotope data show a moderate isotope fractionation (δ238U values between -0.71 and +0.08‰) that is almost indistinguishable between the terminal-GOE and post-GOE stage black shales, combined with low molybdenum isotope values reported in coeval marine sedimentary successions in the literature, which together imply a limited U isotope fractionation under likely weakly euxinic oceans that remained moderately oxygenated throughout the GOE. Moreover, U isotope data of the terminal-GOE stage carbonates show positive δ238U values (up to +0.10‰), and a moderately negative fractionation in the end-GOE carbonate (δ238U=-0.48‰), relative to the modern U riverine input value (δ238U=-0.29‰), which together point to a deoxygenation of the atmosphere–ocean system during the final stage of the GOE. Our U isotope data thus strengthen the paradigm that the Earth’s surface oxygenation remained low in magnitude, well below the modern level, across the GOE. The large U isotope fractionations recorded by black shales from the Karelia Craton might in contrast reflect post-depositional processes, with limited global implications.