Carbon Isotope Fractionation in the Early Cambrian Ocean

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 δ¹³C_carb and δ¹³C_org 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 δ¹³C_carb excursions of Nemakit–Daldynian and Tommotian ages, with minimum values down to -5‰. For the majority of the Nemakit–Daldynian negative δ¹³C_carb excursion, δ¹³C_org remains at -26‰ to -28‰. Only until the end of the negative δ¹³C_carb excursion, δ¹³C_org 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 δ¹³C_carb and δ¹³C_org in the Early Cambrian continues upward, with δ¹³C_org values remain constant at -32‰ across the positive and negative δ¹³C_carb shifts to the Tommotian δ¹³C_carb 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.