MULTIPLE SULFUR ISOTOPIC RECORDS ASSOCIATED WITH THE “SHURAM EXCURSION” FROM SOUTH CHINA

The Ediacaran carbonates record the largest carbon isotopic excursion in Earth history globally. This anomaly implies a dramatic change of the Ediacaran oceanic chemistry, which was closely related to the oceanic/atmospheric oxidation and early animal evolution. The low δ¹³C values (down to -12‰) in the carbonates were thought to have derived predominately from the oxidation of organic carbon reservoir, and sulfate would be one of the most important oxidants. We report multiple sulfur isotopic data (Δ³³S and δ³⁴S) of pyrite from the Doushantuo carbonates in South China. In our study, we find a Δ³³S negative excursion from about 0.04‰ to -0.04‰ coupled with a positive δ³⁴S excursion in the upper Doushantuo Formation where the Shuram excursion is exhibited. Our data of multiple S-isotopes suggest that the sulfate level in the ocean may have greatly declined as a result of the long-term oxidation of organic carbon during the “Shuram excursion”. As well, the negative Δ³³S compositions imply potential mixing of sulfide from two sources: sulfide produced by sulfate reduction in the relatively open shallow water, with negative δ³⁴S values and positive Δ³³S values, and sulfide produced by quantitative sulfate reduction in the relatively closed bottom/pore water, with isotopically similar to seawater sulfate.