AN AUTHIGENIC ORIGIN FOR THE MIDDLE EDIACARAN SHURAM EXCURSION: THE VIEW FROM NAMIBIA AND SOUTH CHINA

The enigmatic Shuram Excursion (ca. 580 to > 550 Ma) is characterized by carbonate rocks preserving the largest, and perhaps longest, negative carbon isotope excursion in Earth history, coupled with changes in oxygen, sulfur, and strontium isotope compositions of marine proxies. The origin of this carbon cycle anomaly has been a puzzle that may be solved if the coupled variations are due to the proportional mixing of primary carbonates that accumulated in shallow marine environments and authigenic calcite cements that formed contemporaneously near the sediment-water interface. Field observations of the Mara Member in the Nama Group of southern Namibia and in the upper Doushantuo Formation of South China reveal the presence of silica-lined authigenic calcite nodules along bedding planes in carbonates associated with the Shuram Excursion. In Namibia the nodular calcite has carbon isotope compositions as low as ca. -20‰, while in South China we have measured values down to -37‰. The ¹³C depleted signatures of the nodules coupled with the presence of 5-15 mm pyrite framboids embedded within the silica linings suggest that methane and other simple organic compounds were oxidized within sedimentary pore waters through microbial sulfate reduction. Given the global distribution of the Shuram Excursion and our observation of these unique authigenic fabrics on two continents, we suggest that authigenesis of these minerals was driven by a significant flux of sulfate and nutrients to the oceans, likely driven by enhanced continental weathering associated with a marked rise in Ediacaran atmospheric oxygen levels. Our observations of methane-derived authigenic calcite provides a solution – through proportional mixing of primary and authigenic sedimentary components – to the enigmatic environmental event that subdivided the Ediacaran Period and paved the way for the evolution and diversification of large and complex multicellular animals.