Stratigraphic variation of the late Cambrian spice event in upper Cambrian carbonates of southern Missouri
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Several positive carbon isotope excursions in lower Paleozoic rocks, including the prominent upper Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE), are thought to reflect intermittent perturbations in the hydrosphere–biosphere system. Models to explain these secular changes are abundant, but the synchronicity and regional variation of the isotope signals are not well understood. Examination of 5 cores across apaleodepth gradient in the upper Cambrian Central Missouri Intrashelf Basin (USA), reveals a time-transgressive, facies- dependent nature of the SPICE. Moreover, two outcrops where the appropriate strata are exposed, and two additional cores representing disparate environments outside the Central Missouri Intrashelf Basin (including a thinner stratigraphic section on the Lebanon Arch, and a thick stratigraphic section on a fault block of the St. Francois regional carbonate platform) corroborate this time-transgressive and facies-dependent nature of the SPICE. Magnitude of local δ13C excursions approach maximum values of > 4‰ V-PDB in deeper-water, intrashelf basinal rocks (alternating lithologies of limestone and shale, 40-60% shale) and ~ 2 to 3‰ V-PDB in shallower water, platform edge, shaley carbonates. Initiation of the positive excursion coincides with an up-section shift to shale-dominated lithologies, which correspond to the transition from Sauk II to Sauk III transgression. While the SPICE event may be a global signal, the manner in which it is recorded in rocks should and does vary as a function of facies and carbonate platform geometry. We call for a paradigm shift to better constrain facies, stratigraphic, and biostratigraphic architecture, and to apply these observations to the variability in magnitude, stratigraphic extent, and timing of the SPICE signal, and other biogeochemical perturbations, in order to elucidate the complex processes driving the ocean–carbonate system.
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