Uranium Isotope Evidence for Persistent Global Ocean Anoxia Across the Ediacaran-Cambrian Transition in Siberia.

The enigmatic Ediacara biota represents Earth's earliest example of macroscopic, complex life, yet the driving forces promoting their mid-Ediacaran evolution and subsequent extinction across the Ediacaran-Cambrian boundary remain speculative. If these were true animals requiring oxygen for aerobic respiration, their rise has often been linked to dramatic changes in the redox state of seawater. Their disappearance from the fossil record-whether gradual or abrupt-and replacement with Cambrian animals has similarly been linked to redox fluctuations, suggesting an environmental driver, or alternatively to biological factors such as predation of the predominantly soft-bodied organisms and bioturbation that disrupted the microbial mats in which they were intimately associated. Insofar as Ediacara extinction is linked to a major perturbation of the global carbon cycle (the BAsal Cambrian carbon isotope Excursion or BACE), an environmental driver seems likely. On the one hand, their demise might reflect the spread of seawater anoxia if these organisms had aerobic metabolisms, but on the other, widespread ocean oxygenation may have been the proximal cause of their extinction and the BACE event if the biota was tolerant and even thrived under low-oxygen conditions. To test between the two alternative redox scenarios, we present a suite of geochemical data including uranium isotopes (δ238U) from carbonate and evaporite-rich successions that span the BACE on the Siberian Platform. Like conditions before and after the event, consistently low δ238U values from both sections (median reconstructed seawater δ238U = -0.74‰ to -0.79‰) suggest that widespread ocean anoxia persisted across the Ediacaran-Cambrian transition. The integrated data indicate that neither a rise nor fall of seawater oxidation state led to the demise of the Ediacara biota. Lacking evidence for a redox driver, we hypothesize that sea-level regression and the widespread development of evaporative conditions in shallow-water environments negatively impacted the largely immobile, diffusion-dependent, and potentially stenothermal Ediacara biota by subaerial exposure, dehydration through osmosis, and/or large temperature variations in shallow water. Thereafter, normal salinity and subtle rises in shallow ocean O2 levels in concert with tectonic and ecological factors could have driven the Cambrian radiation in oceans that remained O2-depleted well into the Paleozoic Era.