Authigenic uranium isotopes of late Proterozoic black shale

The evolution of early life is intimately related to environmental changes on Earth, and in particular, the accumulation of oxygen in the atmosphere and oceans. However, the record of environmental O-2 abundance in the middle to late Proterozoic Eon, during which many new eukaryotic lineages emerged, is sparse and controversial. Here we present a uranium (U) isotope record from late Proterozoic shales from northwestern Canada, Arctic Canada (Baffin Island), Svalbard, and Greenland, coupled with a novel approach for inferring authigenic U isotope values (delta U-238(authigenic)). A compilation comprising our new data and available literature data (854 delta U-238 auth i gerde values) through geologic time indicates a consistent rise in delta U-238 auth i geff i c values following the Great Oxidation Event. This gradual increase in delta U-238 can be interpreted as an increase in the frequency of transient oxygenation events and also as a variation of U isotope fractionation factors between authigenic uptake and seawater (Delta U-2(38)) associated with different redox conditions occurring over the Earth's history. In conjunction with the U isotopic signature, we used previously published Fe speciation data from our samples to infer local controls on U incorporation and isotopic fractionation. The results suggest that late Proterozoic oceans were dominantly ferruginous, punctuated by periods of transient oxygenation. During these transient oxic conditions, high U isotope fractionation resulted in delta U-238 values as high as similar to 1.2 parts per thousand relative to the delta U-238(crust). However, under ferruginous conditions, smaller isotopic fractionation led to Delta U-238 values <0.6 parts per thousand. Integrating conclusions from our study with other geochemical studies suggests the occurrence of several spatially localized oxygenation events across the globe during the late Proterozoic. These conclusions help to better integrate geochemical and fossil records in the context of early evolution of complex life.