REDOX LANDSCAPE OF EDIACARAN AND CAMBRIAN EVOLUTIONARY EVENTS

The Ediacaran-Cambrian transition marks an explosion of complex life on Earth, potentially linked to stepwise changes in the redox landscape of the ocean-atmosphere system. Evolutionary events progressively include the diversification and extinction of the Ediacara biota, the first appearance of bioturbation and biomineralization, and ultimately, the appearance of long-lived Paleozoic lineages. This study aims to constrain the extent to which ocean oxygenation (or lack thereof) played a role in the extinction of the Ediacara biota and subsequent rise of Cambrian organisms. Whereas many previous studies have sought to link redox conditions with biological evolution, a major challenge is the lack of localities that contain rocks suitable for a wide array of geochemical analyses that also contain abundant Ediacaran fossils in the same succession. To test the redox sensitivity of Ediacaran organisms, and generate a holistic picture of the redox landscape of evolutionary events, we present a new high-resolution uranium isotope (δ²³⁸U) dataset from carbonate rocks spanning the Ediacaran-Cambrian boundary in the Olenek Uplift region of northeastern Siberia. δ²³⁸U of marine carbonates represent a powerful redox proxy for constraining the global extent of seafloor anoxia. In our current dataset, δ²³⁸U values range from −0.48 to −0.76‰ with a median value of −0.58‰, which are similar to those in terminal Ediacaran carbonates of Namibia and South China, suggesting widespread marine anoxia. In addition, local redox conditions in the Olenek Uplift section will be assessed using nitrogen isotopes and cerium anomalies. In the Olenek Uplift, Ediacaran fossils are notably preserved as carbonaceous compressions in limestones, allowing us to directly correlate geochemical measurements and paleontological observations. Based on the emerging data, we hypothesize that the Ediacaran organisms persisted through a period of intense global anoxia. Ultimately, this study provides new insights on the redox state of the terminal Ediacaran oceans and suggests that anoxia was not the sole driving force in the extinction of the Ediacara biota.