STRATIGRAPHY OF LATE EDIACARAN SEDIMENTARY SUCCESSIONS: CHEMICAL HETEROGENEITY OF THE OCEANS AT THE DAWN OF ANIMAL LIFE?

High resolution time-series carbon and sulfur isotope analyses of carbonate-dominated Edaicaran successions in Namibia, Oman, India, China, and Siberia reveal a series of positive and negative excursions that have been used for both intra- and inter-basinal correlations, which most likely reflect changes in the redox state of the world ocean. Detailed comparisons of equivalent shallow marine strata, however, suggest that other isotopic events may be unique to individual basins. The isotopic discrepancies may be the result of 1) diagenesis, 2) stratigraphic omission at unconformity surfaces, or 3) chemical difference in deep and/or restricted (i.e., lagoonal or evaporitic) water masses. For example, carbon isotope correlation of late Ediacaran strata in arctic Siberia (Khorbusuonka Group including the Mastakh, Khatyspyt, and Turkut formations) and South China (comprising the Algal Dolomite, Gaojiashan, and Beiwan members of the Dengying Formation) paired with U-Pb zircon age constraints suggest a depositional window of less than 10 Ma. Each contain similar carbonate facies and Ediacara fossil assemblages. The Chinese strata is overall enriched in ¹³C (and in ¹⁸O) relative to time-equivalent strata in arctic Siberia. Both data sets (from micro-sample analyses of fine grained micrite) generally fall outside of a published diagenetic stabilization trend suggesting that the bulk of these samples have not equilibrated with meteoric fluids. Smooth secular trends in both carbon and oxygen isotopes defined by closely-spaced samples suggest that environment plays a more important role than diagenesis on the variance in isotopic compositions between ocean basins. The organic-rich portion of the arctic Siberian succession preserves three negative carbon isotope excursions not generally recorded elsewhere, as well as a profound 25‰ positive spike in sulfur isotopes that may be equivalent to the Ara anomaly in Ediacaran strata of Oman. In contrast, neither the section in Oman nor the South China strata reveal any negative carbon isotope anomalies through this time interval. These detailed observations complicate correlations, fossil ranges, and the reconstruction of general redox conditions for late Ediacaran and Early Cambrian oceans.