T240 Excursions, Extinctions, and Environmental Change: New Advances in Geochemical Tools Linked to Critical Intervals of Environmental and Biotic Change in Earth History (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

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

Paper number: T240-3
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 δ13Ccarb excursion during the deposition of cap carbonate, and the second one(OE2) occurs in the basal Member II of the Doushantuo Formation. The negative δ13Ccarb 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 δ13Ccarb (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 δ13Ccarb 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 δ13Ccarb 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 If our correlation is correct, OE4 might have caused the withdrawn of sulfidic wedge at intermediate depth and concurrent oxidation of DOC in deep water.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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284557

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