WAS THE SHURAM CARBON ISOTOPE ANOMALY DRIVEN BY AN OXIDATIVE PULSE OF ALKALINITY AND SULFATE TO THE EDIACARAN OCEAN?

A basin-wide analysis of the Ediacaran Doushantuo Formation in South China provides a unique geological framework for the study of the enigmatic Shuram carbon isotope anomaly. High resolution time-series C, S, and Sr isotope analyses in the inner shelf Jiulongwan section compared with outer shelf Zhongling and Yangjiaping sections document important contrasts that support the likelihood of strong geochemical gradients in both the abundance and isotopic composition of carbon and sulfur. New time-series data from the uppermost Doushantuo in the outer shelf section at Zhongling reveal that the Shuram anomaly is poorly developed in dolomicrites and associated calcite cements and nodules are remarkably depleted in ¹³C. Sulfur and carbon isotope compositions of pyrite and organic matter, respectively, in samples from Zhongling and Yangjiaping are enriched in both ³⁴S and ¹³C relative to equivalent horizons in the inner shelf section at Jiulongwan. In contrast to the outer shelf sections, the Shuram carbon isotope anomaly is well expressed in carbonates at Jiulongwan. Combined these observations suggest that both alkalinity and sulfate abundance were higher in the proximal environment, which is the expected consequence of the oxidation of organic matter and pyrite on exposed continental shelves triggered by a dramatic rise of oxygen in the atmosphere. In this biogeochemical model, sulfate and fossil organic matter delivered to the near shore environment at Jiulongwan stimulated sulfate reducing bacteria within the anoxic water column, resulting in the formation of additional ¹³C depleted alkalinity and ³⁴S depleted sulfide, which rained down and accumulated as carbonate and pyrite at the basin floor. In the outer shelf environment at Zhongling and Yangjiaping we consider the possibility that sulfate was more limited and that the anaerobic oxidation of methane by sulfate reducers in pore waters resulted in both the ³⁴S enrichment of pyrite and the formation of authigenic calcite cements strongly depleted in ¹³C. This analysis suggests that the worldwide expression of the Shuram anomaly is strongly linked to enhanced terrestrial chemical weathering, which is further supported by a rise in ⁸⁷Sr/⁸⁶Sr during this interval, stimulated by a pulsed rise in atmospheric oxygen.