Tuesday, 7 October 2008: 9:25 AM
George R. Brown Convention Center, 361DE
The decoupling of carbonate and organic carbon isotopes have been used as evidence for the existence of a large dissolved organic carbon pool in the Ediacaran ocean and for the interpretation of a stepwise, irreversible oxidation event at ca. 580 to 550 Ma. In this interpretation, the coupling of carbonate and organic carbon isotopes would be expected in the Early Cambrian. Integrated carbonate and organic carbon isotope analyses of the Early Cambrian strata in south China, however, show a strong decoupling of δ13Ccarb and δ13Corg values. In the eastern Yangtze platform, carbonate carbon isotope analyses of three sections in the Yangtze Gorges area and in Changyang reveal two regionally persistent, negative δ13Ccarb excursions of Nemakit–Daldynian and Tommotian ages, with minimum values down to -5‰. For the majority of the Nemakit–Daldynian negative δ13Ccarb excursion, δ13Corg remains at -26‰ to -28‰. Only until the end of the negative δ13Ccarb excursion, δ13Corg values abruptly drop down to -36‰ to -37‰ and shift back to -33‰ to -32‰ within a < 3 m interval. The temporal carbonate and organic carbon isotope trends are similar to those of the Ediacaran Shuram and Doushantuo δ13C anomalies. However, in contrast to the Ediacaran δ13C anomaly, the decoupling of δ13Ccarb and δ13Corg in the Early Cambrian continues upward, with δ13Corg values remain constant at -32‰ across the positive and negative δ13Ccarb shifts to the Tommotian δ13Ccarb excursion. The isotope trends challenge the interpretation of a stepwise, irreversible oxidation of the Ediacaran oceans. Instead, they seem to support periodic anoxic and stratified oceans possibly induced by increased production under high atmospheric CO2 and/or localized restrictions of sedimentary basins.