--> Abstract: Neoproterozoic Carbon Isotopes of the Neoproterozoic Shuram and Wonoka Formations; #90063 (2007)

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Neoproterozoic Carbon Isotopes of the Neoproterozoic Shuram and Wonoka Formations

 

Le Guerroue, Erwan1, Martin Kennedy1 (1) University of California Riverside, Riverside, CA

 

Carbonates δ13C values provide a useful monitor of changes in the global carbon cycle because they can record the ratio of organic to carbonate carbon burial. Changes in the carbon isotopic composition of the carbonate through time can identify times of anomalous organic carbon burial and related climate events, important for recognizing source rock intervals. The most pronounced isotope excursions in the geologic record occur in the Neoproterozoic and have assumed a central role in the interpretation of biogeochemical events preceding the Ediacaran and Cambrian radiations. These excursions are less well understood than those in the Phanerozoic because of their magnitude, duration and the difficulty of correlation between basins to independently confirm that they do indeed record global change in the mixed ocean reservoir. The absence of carbonate secreting organisms and the poorly understood organic carbon cycling also adds to the uncertainty. Here we described some of the geologic and geochemical constraints on the Earth's most profound negative carbon isotope excursion which is recorded in the Shuram Formation of Oman and potentially equivalent Wonoka Formation of South Australia. Using Phanerozoic carbon cycle models, the -12permil excursion sustained over many millions of years implies a prolonged interval of complete oxidation of the DOC carbon reservoir, no organic carbon burial and complete consumption of atmospheric oxygen. It is apparent from the geological record that these conditions are not met. Alternatives to this scenario include a diagenetic origin for these values, or more methanogens mode of carbon cycling with longer DOC residence times.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California