International Geologiical Congress - Oslo 2008

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MPC-03 Precambrian isotope chemostratigraphy

 

Overturn of anoxic deep ocean in the Ediacaran Period - Evidences from isotopes and trace elements

 

Xuelei Chu, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
Jing Huang, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
Tonggang Zhang, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
 

 

In the Ediacaran Period, a large dissolved organic carbon (DOC) reservoir prevented deep ocean from oxidation and potentially retarded evolution of metazoan, although atmospheric O2 concentration had progressively rosen since the Neoproterozoic glaciations. Three negative δ13Ccarb excursions of the Doushantuo Formation (635-551 Ma) of South China may reflect complicated oxidation processes or redox fluctuations of the Ediacaran oceans. However, the nature and timing of deep oceanic oxidation remain uncertain.
The most prominent negative δ13Ccarb excursion occurred at the near Doushatuo-Dengying boundary (∼551 Ma). At the Tianjiayuanzi-Jijiapo section in the Yangtze Gorges area, the negative δ13Ccarb excursion exceeds 15‰, with a decrease from +4‰ of the upper Doushantuo Formation down to −13‰ of the Doushantuo-Dengying boundary and then an increase to +‰ of the lower Dengying Formation. Interestingly, the δ34SCAS (carbonate associated sulfate) values show a synchronous negative excursion of exceeding 40‰ and extraordinary negative values less than −10‰ at the base of the Dengying Formation. The synchronous negative δ13Ccarb and δ34SCAS excursions suggest the oxidation of both DOC and H2S produced by BSR, accompanying the upwelling of euxinic deep water. Redox-sensitive elements in sedimentary carbonate, U, V, Mo, Cu, Ni, Cd, Zn, and Mn, also show a rapid variation at or beyond the Doushatuo-Dengying boundary that their concentrations (normalized by PAAS) increased first and then followed by a decrease. Furthermore, (Ce/Ce*)SN (normalized by PAAS) generally exceeds 1.0 at the the Doushatuo-Dengying boundary, proving that anoxic and sulfidic water had arrived at shallow shelf at ∼551 Ma due to an overturn of deep ocean.
Post-glacial stratified ocean, high biologic productivity, and atmospheric O2 increase may sustain anoxia and higher H2S concentration of deep ocean. Our data document that an overturn of deep ocean happened in the Doushantuo-Dengying transition period, suggeting that the overturn of deep ocean may play a major role to reduce DOC reservoir and attain progressive oxidation of the Ediacaran oceans.

 

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