中国科学院南京地质古生物研究所机构知识库

Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the early and middle Ediacaran Period. Various hypotheses have been proposed to attribute these evolutionary events to changes in ocean redox conditions. To date, published models of the Ediacaran ocean in South China have largely been focused on the Upper and Middle Yangtze blocks, as opposed to the slope and basin sections of the Lower Yangtze Block, where the oldest complex macrofossils of the Ediacaran (i.e. the similar to 600 Ma Lantian Biota) are preserved. To achieve a holistic understanding of the record of ocean chemistry in the entire Yangtze Block, we carried out carbon (delta C-13(carb) and delta C-13(org)) and sulfur (delta S-34(CAS) and delta S-34(pyr)) isotopic investigations of four new sections (i.e., Wujialing, Meishuxia, Wangfu and Zhushuwu) in the Lower Yangtze Block, in addition to one previously reported drill core at Lantian. The Doushantuo and Lantian formations at these sections represent shelf margin, upper slope, lower slope, and deep basin environments. In conjunction with previous work on sections of the Upper and Middle Yangtze blocks, our study reconstructs the redox structure of the entire Ediacaran ocean in South China. The new results show profound delta C-13(carb) negative excursions in the slope and deep basinal sections, which can be correlated to the EN3 and Shuram excursions at a global scale. Notably, the delta C-13(carb) and delta S-34(pyr) profiles from shelf to basinal sections show distinct spatial and temporal patterns, which can be explained by model of a redox stratified ocean during the early and middle Ediacaran Period. The finding of isotopically light (S-34-depleted) pyrite from this time suggests that the oceanic chemocline extended to intermediate depth. Integrating this study with existing ecological and taphonomic data, it is likely that marine redox stratification in the early Ediacaran limited the spatial distribution of early metazoans in the Lower Yangtze Block.