A burial diagenesis origin for the Ediacaran Shuram–Wonoka carbon isotope anomaly

Marked negative δ13C excursions in Ediacaran-age carbonate sediments have been identified in several sections globally, but are not recognized in all sections of similar age. The presence of δ13Ccarb values as low as −12‰ has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today. The δ13Ccarb anomalies are strongly correlated with δ18Ocarb values but are not represented in δ13Corg records. While no primary depositional processes have been identified that can produce the correlated δ18O–δ13C arrays, simulations show that fluid–rock interaction with high-pCO2 fluids is capable of producing such arrays at geologically reasonable pCO2 and water–rock ratios. Variations in the Mg/Ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs. calcite and anhydrite in the resulting mineral assemblage. Incorporation of an initially aragonitic mineralogy demonstrates that high Sr, low Mn/Sr and modest alteration of 87Sr/86Sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the δ13C anomalies, and do not necessarily imply preservation of primary values. The profound Ediacaran negative δ13C anomalies can be adequately explained by well-understood diagenetic processes, conflated with the difficulty of correlating Precambrian sections independently of chemostratigraphy. They are not a record of primary seawater variations and need not have independent stratigraphic significance.