AAPG Annual Convention and Exhibition

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A High-Resolution Investigation of the Relationship Between the Steptoean Positive Carbon Isotope Excursion and the End-Marjuman Trilobite Extinction Within an Intrashelf Basin in the Southern Appalachians


The latest Cambrian and earliest Ordovician contain a series of trilobite extinctions which appear to coincide with perturbations in the carbon cycle. The first of these extinctions (the end-Marjuman) roughly coincides with the initiation of a large (4 to 5‰) and rapid excursion in the marine carbon isotope (dC) record called the Steptoean Positive Carbon Isotope Excursion or SPICE. This excursion, which is expressed in sedimentary successions globally in both carbonates and organic carbon, is thought to represent a large perturbation to the carbon cycle during this time. Additionally, a limited amount of carbon isotope data from the sedimentary succession in the Black Hills of South Dakota suggests that a small (1‰) negative dC excursion may occur immediately at the extinction boundary, preceding the initiation of the SPICE. Further it has been poised, based on paired carbon and sulfur isotope data, that the SPICE represents an interval of enhanced burial of organic carbon and pyrite in marine sediments deposited under euxinic (i.e. anoxic and sulfidic) conditions. This supports the notion that a rise and/or intensification of the oxygen minimum zone may be an extinction mechanism if the geochemical changes coincide with the extinction interval. Here we will present a high-resolution carbon isotope record to investigate the precise relationship between the disturbances in the carbon cycle, the extinction event and changes in the local sedimentary environment in the Appalachian Valley and Ridge Province of southwestern Virginia and eastern Tennessee. The sedimentary succession here is an excellent candidate for this study as it represents an intrashelf basin that developed on the regional, rimmed carbonate shelf and thus preserves a record of shallow to deeper water sedimentary environments. Additionally, published high-resolution biostratigraphy is available that precisely identifies the extinction interval in several stratigraphic sections. The onset of the SPICE appears to directly coincide with the extinction interval within our sections indicating that the environmental processes behind the carbon isotope excursion (likely enhanced anoxia and euxinia) resulted in the extinction event.