WIDESPREAD OCEAN OXIDATION IN THE EDIACARAN YANGTZE PLATFORM: CARBON ISOTOPE AND FE GEOCHEMISTRY EVIDENCE FROM DEEP WATER SECTIONS

It is widely accepted that the Ediacaran Period (635-541 Ma) had experienced an increase in atmospheric oxygen level and diversification of eukaryotes. In the Yangtze Platform, Ediacaran biological evolution is associated with at least four pulses of ocean oxidation events. The first oxidation event (OE1) is recognized by a negative δ¹³C_carb excursion during the deposition of cap carbonate, and the second one(OE2) occurs in the basal Member II of the Doushantuo Formation. The negative δ¹³C_carb excursion between the Member II and Member III (EN2) is the third oxidation event (OE3), while last and dramatic one (OE4) is recorded by an up to 12‰ negative excursion in δ¹³C_carb (EN3), which could be correlated with Shuram Excursion globally. Therefore, the redox landscapes in the Yangtze Platform might fluctuate during the Ediacaran Period, and the triggers of the oxidation events remain obscure. In order to constrain the paleogeographic extent and the mechanism of Ediacaran ocean oxidation events, in this study we report carbon isotopes and iron speciation data of the Doushantuo Formation from two distal sections deposited in the basin environment of the Yangtze Platform. Two prominent negative δ¹³C_carb excursions, recognized from the carbonate units of the 37301 core, could be correlated with OE2 and OE4, respectively. The Fe speciation data of the Shangdiping section show a clear ferruginous trend, suggesting the shrinkage of the sulfidic wedge in the intermediate depth. The ferruginous trend occurs in the top of the Doushantuo Formation, and accordingly can be compared with OE4 or the upper δ¹³C_carb negative excursion in the 37301 core. Thus, OE4 or Shuram Excursion might have extended to the basin facies, suggesting an intense oxidation from the shelf to basin environment of the Yangtze Platform. If our correlation is correct, OE4 might have caused the withdrawn of sulfidic wedge at intermediate depth and concurrent oxidation of DOC in deep water. Finally, we speculate that OE4 might have been triggered by the decrease of organic matter input from the surface water. With lower organic matter input, the sulfidic wedge cannot be sustained due to insufficient H₂S production. On the other hand, lower organic matter input consumes less terrestrial oxidant, resulting in the excessive supply of oxidant to oxidize DOC in deep water.