中国科学院南京地质古生物研究所机构知识库

The Ediacaran Shuram negative carbon isotope excursion (SE) records major paleoceanographic changes during the late Neoproterozoic, possibly linked to a global oceanic oxygenation event, yet its cause(s) remain uncertain. Earlier studies of the upper Ediacaran Doushantuo Formation in South China based on local redox proxies have documented strong spatial redox heterogeneity along shelf-to-basin transects, but variations of delta U-238 (a global redox proxy) have not yet been examined in deep-water SE carbonates. In this study, we examined delta U-238 variations through the SE in the upper slope Siduping section. Similar to platform SE sections, Siduping exhibits a shift toward higher delta U-238 values correlative with the peak of the SE (i.e., maximum negative delta C-13(carb)), confirming inferences of global ocean oxygenation during the SE. This raises an apparent paradox, because a global negative carbon isotope excursion implies net oxidant consumption, requiring an ocean-based oxygenation mechanism. We hypothesize that an increase in the efficiency of phosphorus burial due to a plankton-driven shift from dominantly dissolved organic matter (DOM) cycling to greater particulate organic matter (POM) export depleted the ocean of nutrient phosphorus. By producing a steep redox gradient close to the sediment-water interface, we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light delta C-13(carb) was precipitated in authigenic carbonate minerals. This scenario can account for delta U-238 differences between shallow-water and deep-water carbonates, which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow-water settings, generating both a larger negative delta C-13(carb) shift and a larger early diagenetic delta U-238 offset. (C) 2020 Published by Elsevier Ltd.