A DIVERSE EDIACARA ASSEMBLAGE SURVIVED UNDER ANOXIC CONDITIONS

The Ediacaran-Cambrian transition marks an explosion of complex life, 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 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. 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 U isotope (δ²³⁸U) dataset from carbonates spanning the Ediacaran-Cambrian boundary in the Olenek Uplift of northeastern Siberia. δ²³⁸U of marine carbonates represents a powerful proxy for constraining the global extent of seafloor euxinia. In our dataset, δ²³⁸U values are substantially lighter than modern carbonates with a median value of −0.58‰ and a minimum of −0.92‰. These values are similar to those recorded in terminal Ediacaran carbonates of Namibia and South China, suggesting widespread marine euxinia. In addition, local redox conditions in the Olenek Uplift section were assessed using Ce anomalies. In our section, Ce/Ce* values range from 0.99 to 1.55 with a median of 1.30, indicating that the local environment was anoxic. Despite these anoxic conditions, a diverse Ediacara assemblage is notably preserved in situ as carbonaceous compressions in limestones, allowing us to directly correlate geochemical and paleontological observations. Based on these data, we hypothesize that the Ediacaran organisms survived through a period of intense global euxinia and, locally, under anoxic conditions. This could suggest a chemoautotrophic metabolism for some members of the Ediacara biota. Additionally, the locally anoxic conditions could explain the absence of fossils with a clear metazoan affinity (e.g., Cloudina) in the Olenek Uplift.