Redox Landscape of Ediacaran and Cambrian Evolutionary Events
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Cherry, Lucas
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Abstract
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 during the Cambrian Explosion of animals. This study aims to constrain the extent to which ocean oxygenation (or lack thereof) played a role in the extinction of the Ediacaran 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 monitor the oxidation of seawater and also contain abundant carbonaceous compressions of Ediacaran fossils in the same succession. To test the redox sensitivity of these enigmatic organisms and generate a multi-proxy picture of the redox landscape of evolutionary events, we present a new high-resolution uranium isotope (δ238U) and cerium anomaly (Ce/Ce*) dataset from carbonate rocks spanning the Ediacaran-Cambrian transition in the Olenek Uplift region of northeastern Siberia. δ238U of marine carbonates represents a powerful proxy for constraining the global extent of seafloor euxinia, with lower seawater δ238U values corresponding to expanded euxinia in the global oceans. By contrast, Ce/Ce* is indicative of redox conditions in the local water column, with the lack of a negative Ce anomaly fingerprinting locally anoxic conditions. In our dataset, δ238U has a median value of -0.58‰, which is substantially lower than carbonates deposited in the modern oxic ocean but is comparable to δ238U values in carbonates deposited during the Permo- Triassic mass extinction, which represents one of the largest known expansions of anoxia in the Phanerozoic Eon. These low δ238U values are also consistent with exceptionally low δ238U values recorded in coeval terminal Ediacaran carbonates of Namibia and South China, suggesting that widespread marine euxinia was indeed a global phenomenon. In addition, the lack of a negative Ce anomaly in our samples indicates that local redox conditions in the Olenek Uplift depositional basin were persistently anoxic. Ediacaran fossils of the Avalon assemblage are notably preserved in situ in the same carbonate strata that record local anoxia, suggesting that Ediacara-type organisms were tolerant of low-oxygen conditions. Additionally, the Ediacaran fossils of multiple assemblages persisted globally through this period of intense global euxinia, indicating that redox conditions may not be responsible for the ultimate demise of these organisms. Taxa with more definite metazoan affinities such as cloudinids, however, are notably absent from the anoxic settings of the Olenek Uplift but are present in coeval oxic settings of South China, suggesting that the habitable zone for these early worm-like biomineralizing animals may have been redox-dependent. Ultimately, this study provides critical new insight on the oxygenation state of the terminal Ediacaran to Early Cambrian oceans, as well as the relationship between ocean redox and the early evolution of animals.
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Keywords
Uranium, Cambrian, Ediacaran, Redox