DECOUPLING EXCURSIONS, EXTINCTIONS, AND ENVIRONMENTAL CHANGE: THE TIME-TRANSGRESSIVE AND FACIES-DEPENDENT CAMBRIAN SPICE EVENT

Biogeochemical data from the early Paleozoic indicate intermittent perturbations in sedimentary isotopic records, with several prominent positive carbon isotopic excursions attributed to a variety of secular oceanic and biotic changes. The late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) has been proposed as a globally synchronous event, thought to be contemporaneous with the biotic crisis at the Marjumiid-Pterocephaliid biomere boundary and an ocean anoxia event coupled with atmospheric oxygenation, to name a few. While conceptual models to explain the causes and effects of these secular changes are abundant, the synchronicity and regional variation of the SPICE event remain largely unresolved. The late Cambrian carbonate platform of southeastern Missouri, USA, presents an ideal locality in which to test the competing effects of global ocean chemistry versus local carbon productivity and burial, and their relationship to biotic turnover and facies architecture. From examination of five cores representing a transect of the paleodepth gradient in the Central Missouri Intrashelf Basin, we observe facies- and lithologic-dependence of the SPICE signal, as expressed by variation in rock-type and relative stratigraphic thickness over which the SPICE interval is recorded. The role of facies in controlling the δ¹³C signals observed here may also provide some insights into the origin of the large variability in the expression of δ³⁴S signatures observed in SPICE sections around the world. While the SPICE may represent a global signal, we suggest that the manner in which it is recorded in rocks should vary, as a consequence of variability in bathymetry, platform geometry, and local bioproductivity and water chemistry, which may collectively affect the magnitude, stratigraphic extent, and timing of the signal. Fundamentally, the SPICE is only a signal, one where any perturbations to the global carbon cycle are filtered through local depositional conditions. We need to shift the focus to understanding the larger, likely more complicated processes that are driving the ocean-carbonate system, of which the SPICE is only one component. The overarching processes involve ocean chemistry, biotic turnover, and transgression, among others.