SHALLOW WATER ANOXIA PRECEDING THE RISE OF EUKARYOTES (Invited Presentation)

Earth’s redox evolution has been commonly assumed to have shaped the course of biological evolution. However, whether and how shifts in marine redox conditions are linked to key biotic innovations–foremost the rise and diversification of animals in the late Proterozoic oceans–remains heavily debated. Our current picture of global marine redox evolution during this critical interval is incomplete fostering debate. This is particularly the case for the Tonian Period (~1.0 to ~0.717 Ga), where molecular clock data suggest animals may have emerged and where biomarker and planetological data indicate there was a significant increase in the abundance and diversity of eukaryotic algae. Here, we present >140 new uranium isotope (δ²³⁸U) measurements from Tonian carbonate rocks to fill this outstanding gap. Tonian carbonates from 1000-800 Ma are characterized by anomalously negative δ²³⁸U values—we report the most negative stratigraphically continuous observations to date. These low δ²³⁸U values can only be explained by an interval of intensive and widespread marine euxinia, including prevalent photic zone anoxic conditions.