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 O2 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 (δ238Uauthigenic). A compilation comprising our new data and available literature data (854 δ238Uauthigenic values) through geologic time indicates a consistent rise in δ238Uauthigenic values following the Great Oxidation Event. This gradual increase in δ238U 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 (Δ238U) 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 Δ238U values as high as ~1.2‰ relative to the δ238Ucrust. However, under ferruginous conditions, smaller isotopic fractionation led to Δ238U values <0.6‰. 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.