The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation

Abstract

Current understanding of secular changes in the carbon isotopic composition of mid- to late Ediacaran carbonates suggests a relatively long, steady recovery of the global ocean from the Shuram negative excursion, followed by a smaller negative excursion at the Precambrian-Cambrian boundary. New radiometric, stratigraphic, and carbon isotope data from thick exposures of the upper Johnnie Formation in the Panamint Range of eastern California, combined with data from carbonate-rich facies of the Stirling Quartzite in the Funeral Mountains, confirm an Ediacaran age for these strata and provide a more complete record of isotopic variations during this time interval than previously determined from SW Laurentia and other key sections around the globe. A siltstone in the lower part of the Johnnie Formation yielded a detrital zircon grain with an age of 640.33 ± 0.09 Ma, lowering the maximum radiometric age constraint on the Johnnie Formation by >400 m.y., consistent with an Ediacaran age based on chemo- and biostratigraphic data. In contrast to previous C isotope compilations from this region, which were generally based on relatively thin portions of the Cordilleran miogeocline near its depositional hinge, the more basinward exposures exhibit a recovery from values near –12‰ to 0‰ within the upper part of the Johnnie Formation. Details in the shape of the chemostratigraphic profile through the upper Johnnie Formation closely match those in profiles through the Wonoka Formation in South Australia (which lies above the basal Ediacaran global stratotype section and point) and the Shuram-Buah interval in Oman, confirming temporal correlation and suggesting genesis through changes in the isotopic composition of the global ocean. The Shuram excursion in SW Laurentia is followed by at least three smaller Ediacaran to earliest Cambrian isotopic excursions recorded within, from oldest to youngest, the uppermost Johnnie Formation, the middle Stirling Quartzite, and the lower Wood Canyon Formation. These data indicate that the negative excursion associated with the base of the Cambrian is not a unique post-Shuram event, and that post-Shuram, pre-Cambrian animal evolution occurred in an environment of repeated large-magnitude fluctuations in the carbon isotopic composition of the global ocean.

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