EDIACARAN OCEAN OXIDATION TRIGGERED BY THE DISAPPEARANCE OF EUKARYOTIC PHYTOPLANKTONS

The second rise of atmospheric O₂ level and ocean oxidation in the Ediacaran Period (635-542 Ma) is supported by the global occurrences of Shuram excursion (SE), the largest negative carbon isotope excursion in Earth’s history, recording massive oxidation of dissolved organic carbon (DOC) in deep ocean. However, recent studies indicate that there was no significant change in atmospheric O₂ content from Ediacaran to Cambrian and Cambrian oceans were frequently punctuated by anoxia, suggesting the magnitude of the Ediacaran oxygenation might have been overestimated, and Ediacaran ocean oxidation might be sporadic and periodic. Here, we propose a new model for the causes of Ediacaran ocean oxidation. The new model was inspired by the observation that complex acanthomorphic acritarchs (CAA) disappeared right before the onset of SE. As the major eukaryotic phytoplanktons, disappearance of CAA reduces organic matter input into ocean interior, minimizes oxidant consumption, and accordingly results in the excessive supply of oxidants with respect to organic matter input. The surplus oxidants remineralize DOC in the deep ocean, followed by carbonate precipitation from DOC-derived alkalinity, driving carbonate carbon isotopes shift negatively. Decrease in the eukaryotic productivity also changes the composition of buried organic carbon, explaining the decoupled organic carbon and carbonate carbon isotope signals. Numerical simulation indicates that the disappearance of CAA is sufficient to generate SE. If this model is correct, ocean oxidation does not necessarily imply the oxygenation of atmosphere. And ocean oxidation was dominant in regions with high primary productivity. Finally, this study stresses that evolution of the Ediacaran marine redox landscape might be tightly associated with biological evolution, adding to the growing realization that life plays the central role in the evolution of a habitable planet.