Dynamic changes in sulfate sulfur isotopes preceding the Ediacaran Shuram Excursion

摘要

Large excursions in delta C-13 and delta S-34 are found in sedimentary rocks from the Ediacaran Period that may provide detailed mechanistic information about oxidation of Earth's surface. However, poor stratigraphic resolution and diagenetic concerns have thus far limited the interpretation of these records. Here, we present a high-resolution record of carbon and sulfur isotopes from the Khufai Formation, leading up to and including the onset of the Shuram carbon isotope excursion. We document large coherent excursions in the sulfur isotope composition and concentration of carbonate-associated sulfate (CAS) that occur both independently and synchronously with the carbon isotope excursion. Isotopic changes appear decoupled from major stratigraphic surfaces and facies changes, suggesting regional or global processes rather than local controls. Our data suggest that very low marine sulfate concentrations are maintained at least through the middle-Khufai Formation and require that the burial fraction of pyrite and the fractionation factor between sulfate and pyrite necessarily change through deposition. Reconciliation of simultaneous, up-section increases in marine sulfate concentration and delta(34)SCAS requires the introduction of strongly S-34-enriched sulfate, possibly from weathering of Cryogenian and earlier Ediacaran S-34-enriched pyrite. Our analysis of the onset of the Shuram carbon isotope excursion, observed in stratigraphic and lithologic context, is not consistent with diagenetic or authigenic formation mechanisms. Instead, we observe a contemporaneous negative excursion in sulfate delta S-34 suggesting linked primary perturbations to the carbon and sulfur isotope systems. This work further constrains the size, isotopic composition, and potential input fluxes of the Ediacaran marine sulfate reservoir, placing mechanistic constraints on possible drivers of extreme isotopic perturbations during this critical period in Earth history. (C) 2015 Elsevier Ltd. All rights reserved.