Dynamic changes in sulfate sulfur isotopes preceding the Ediacaran Shuram Excursion
Large excursions in δ^(13)C and δ^(34)S 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 δ^(34)S_(CAS) requires the introduction of strongly ^(34)S-enriched sulfate, possibly from weathering of Cryogenian and earlier Ediacaran ^(34)S-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 δ^(34)S 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.
© 2015 Elsevier Ltd. Received 4 August 2014; accepted in revised form 28 July 2015; Available online 20 August 2015. This study was labor intensive and required the cooperation of many. We would like to thank the participants of the 2010 Agouron Advanced Field Course (John Ableson, JC Creveling, Tais Dahl, Lauren Edgar, David Fike, Woodward Fischer, John Grotzinger, Lindsay Hayes, Andy Knoll, Ben Kotrc, Tim Lyons, and Magdalena Osburn) for initial stratigraphic observations and sample collection, Haley Barnes, Sean Dembowski, and Emma Dodd for sample preparation, and Bob Criss, Ken Macleod, and Lora Wingate for carbon and oxygen isotope analyses. We are grateful in particular to Angela Gerhardt and Steve Bates for their efforts with the sulfur extractions and analyses at UC Riverside. This work would not have been possible without support from Petroleum development Oman (PDO) for field logistics and isotopic analyses. Bill Wilks, Gordon Forbes, Zuwaina Al-Rawahi, Joachim Amthor, and Salim Al Maskery (Shuram Oil & Gas) also provided helpful insight. We thank the Ministry of Oil and Gas, Sultanate of Oman, for permission to publish this work. The NSF GRFP supplied funding to MRO and KDB. Funds were also provided by the NASA Exobiology Program, the NASA Astrobiology Institute, and NSF-EAR (FESD and ELT programs) (TWL).
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