COUPLED CARBON AND SULFUR ISOTOPE EXCURSIONS IN CARBONATES OF CABORCA, MEXICO: IMPLICATIONS FOR EDIACARAN MARINE OXYGENATION

Many Ediacaran localities worldwide record a single extreme negative δ13C excursion (down to ca. –12‰) in carbonates above Marinoan glacial deposits. In the western USA the anomaly is constrained to older than ca. 580 Ma, but in China there are two similar, discrete events between 635 and 550 Ma. Models related to the oxidation of various organic carbon pools (methane, deep ocean DOC, and sedimentary organic matter) have been proposed, yet the direct cause(s) of these carbon isotope excursions remains unclear. Sulfur isotopic data across the carbon isotope anomalies have been reported from Oman, Death Valley, and China, but the data sets differ in the magnitude of the negative δ34S excursions, possibly because of local metamorphic or diagenetic effects. In this study, we present new carbonate associated sulfate concentration (CAS) and isotopic (δ34Scas) data from equivalent carbonates near Caborca, Mexico that are remarkably similar to those in the western USA, and are likely also to be 580 Ma or older. Caborca carbonates record a decrease in δ34Scas from +20 to +12‰ (V-CDT) in conjunction with a ~9‰ negative δ13C excursion. Additionally, CAS abundances increase from less than 200 to more than 1000 ppm, which is very similar to events recorded in the Johnnie Formation in Death Valley, California. Above and below the excursion facies, CAS concentrations typically range between 50 and 200 ppm. δ34Scas ranges between 20 and 26‰ prior to the excursion and increases to between 25 and 32‰ afterwards. Contemporaneous negative excursions in δ13Ccarbonate and δ34Scas indicate that a source of 12C and 32S was introduced to the oceanic carbonate and sulfate reservoir. Reduced phases of carbon and sulfur are typically generated through biologic cycling and thus have characteristically-light isotopic signatures. Oxidation of biologically-derived organic carbon and sulfide would produce the observed negative isotopic excursions and the increase in CAS concentration. The specific oxidant (i.e., O2, NO3-, etc.), is presently unknown, but is not likely sulfate due to parallel excursions in δ13Ccarbonate and δ34Scas. However, increased oceanic O2 is thought to be necessary to support large body plans, so the radiation of metazoans subsequent to the isotopic excursions may be a response to elevated oceanic O2.