Paired carbonate and organic carbon isotope variations of the Ediacaran Doushantuo Formation from an upper slope section at Siduping, South China

Abstract

Intensive carbon isotope analyses from the Doushantuo Formation (ca. 635–551 Ma) in South China reveal decoupled carbonate and organic carbon isotopes (δ13Ccarb and δ13Corg) that have been interpreted as recording a large dissolved organic carbon (DOC) reservoir in the Ediacaran ocean or chemoautotrophic-methanotrophic recycling/modification of organic matter in a redox-stratified ocean. Existing paired δ13Ccarb–δ13Corg data, however, are mostly from sections of an intrashelf lagoon near the Yangtze Gorges area where basin restriction may have resulted in local isotope departure from the open ocean. To better understand the spatial and temporal variations of the Ediacaran δ13C excursions, we have conducted high-resolution, paired δ13Ccarb–δ13Corg analyses of the Doushantuo Formation from an upper-slope section at Siduping, Hunan Province, South China. The data reveal three negative δ13Ccarb excursions at the basal, middle, and upper Doushantuo Formation. Two prominent negative δ13Corg anomalies are also observed in black shales and shaly dolostones at the base and top of the Doushantuo Formation. For the majority of the Doushantuo Formation, however, δ13Ccarb and δ13Corg values are decoupled, similar to those observed in the Yangtze Gorges area. Both δ13Ccarb and δ13Corg show large (up to 10‰) variations among shallow- and deep-water sections that are consistent with a large carbon isotope gradient in a stratified Ediacaran ocean. The large δ13Ccarb gradient between shallow- and deep-water sections implies the availability of sulfate in deep-water environments where the addition of authigenic carbonate films/cements into surface-ocean-derived carbonate particles in anoxic water column or pore-water led to more negative δ13Ccarb values. Significant spatial δ13Corg variations suggest that the decoupled δ13Ccarb–δ13Corg may not have been resulted from direct buffering of a large DOC reservoir but from recycling of organic matter by chemoautotrophs and methanotrophs. Compilation of available paired δ13Ccarb–δ13Corg data from the Yangtze platform reveals alternating intervals of coupled and decoupled δ13Ccarb–δ13Corg values up to the early Cambrian (≤520 Ma). Intervals with coupled δ13Ccarb and δ13Corg coincide with evidence for ocean oxygenation events. This phenomenon may be explained by episodic growth and oxidation of a moderate DOC reservoir 6–12 times of the modern ocean. The upper Doushantuo δ13Ccarb anomaly temporally crosses a major stratigraphic unconformity, indicating that part of the upper Doushantuo-Shuram δ13Ccarb excursion was formed during sea-level regression. Large spatial and temporal variations of the upper Doushantuo δ13Ccarb excursion may record a mixed signature of open-ocean seawater overprinted by facies-dependent isotope variation, stratigraphic truncation and authigenic/diagenetic modification along the unconformity. The real primary δ13Ccarb excursion may be shorter-lived than previously thought.