CARBON AND SULFUR ISOTOPES, REDOX CONDITIONS, AND BIOLOGICAL EVOLUTION IN THE EDIACARAN DOUSHANTUO FORMATION

Recent geochemical data from Oman, Newfoundland, and the western USA suggest that stepwise oxidation of the Ediacaran oceans at ~ 580 Ma may have triggered the radiation of acanthomorphic acritarchs, macroscopic Ediacara organisms, and bilaterian animals. To further test the linkages between this oxygenation event and biological evolution, we report high-resolution chemostratigraphic and biostratigraphic data from the Doushantuo Formation (635–551 Ma) in South China.

Carbon and sulfur isotopic data can be interpreted in terms of a non-steady state system involving a large DOC reservoir in the deep ocean interacting with a comparatively small DIC reservoir in the surface ocean. Markedly stable δ¹³C_org values (~ -30‰), positive δ¹³C_carb values (~ +6‰), meter-scale variation in sulfur isotopes, and high total organic carbon (4-8%) and pyrite (up to 12%) concentrations indicate persistent anoxic conditions in a stratified basin. Two prominent negative δ¹³C_carb excursions in the middle and upper Doushantuo likely record episodic oxidation of the DOC reservoir by enhanced bacterial sulfate reduction during regression and increased sulfate supply from terrestrial oxidative weathering of sulfides. However, δ¹³C_org values remained buffered by the large DOC reservoir, except in the uppermost Doushantuo, where δ¹³C_org, δ³⁴S_CAS and δ³⁴S_py show a consistent negative shift coincident with transgression and upwelling of sulfidic deep waters.

Integrated biostratigraphy suggest that these oxidation events were integral to evolution of Doushantuo organisms. Acanthomorphic acritarchs and probable animal embryos, although first appear shortly after the 635 Ma Nantuo glaciation, show a pronounced radiation following the first oxidation event. Macroscopic algae radiated after the second oxidation event, followed by the appearance of frondose and fractal Ediacara organisms, bilaterian animals, and biomineralizing animals in the overlying Dengying Formation. Comparison to other Ediacaran basins suggests that early Ediacaran evolution may have been diachronous, tracking local redox conditions. Only after the second oxidation event were Ediacaran oceans pervasively oxidized, allowing for global biostratigraphic correlation of late Ediacaran successions.