Uranium isotope evidence for extensive shallow water anoxia in the early Tonian oceans

Abstract

The Earth's redox evolution has been commonly assumed to have played a key role in shaping the evolutionary history of the biosphere. However, whether and how shifts in marine redox conditions are linked to key biotic events – foremost the rise of animals and the ecological expansion of eukaryotic algae in the late Proterozoic oceans – remains heavily debated. Our current picture of global marine redox evolution during this critical interval is incomplete. This is particularly the case for the Tonian Period (∼1.0 to ∼0.717 Ga), when animals may have diverged and when eukaryotic algae began their rise in ecological importance. Here, we present new uranium isotope (δ238U) measurements from Tonian carbonates to fill this outstanding gap. These Tonian carbonates (∼1000–800 Ma) record variable δ238U values, indicating temporal variation in global marine redox through this under-investigated time interval. Arguably the most interesting feature of this new δ238U dataset is an interval of anomalously negative δ238U values (<−1‰) that represent among the most negative stratigraphically continuous values reported to date. These low δ238U values are best explained by prevalent shallow-water anoxia, potentially driven by increases in productivity in a low-O2 Tonian Earth system. We thus provide compelling evidence for extensive shallow marine anoxia just prior to or coincident with Neoproterozoic ecological shifts.