Earth and Environmental Science

Abstract:
Positive ∂13C excursions throughout the geological record have been linked to mass extinctions, and eustatic sea-level falls during icehouse conditions. Elevated concentrations of ∂13C in seawater may be caused by biological carbon-fractionation, and removal of ∂12C from the ocean due to an increased storage of organic carbon in deep anoxic oceans. The Steptoean Positive Carbon Isotope Excursion, better known as the SPICE excursion, is an exception to some of these trends. The SPICE is a 3%o-5%o positive ∂13C shift spanning 4 million years under greenhouse conditions during the Late Cambrian, representing a widespread ocean anoxic event and perturbation of the global carbon cycle. ∂13C and ∂18O isotopic data suggest that this excursion may correspond to events in sequence stratigraphy and an overall warming of seawater near its peak. The onset of the SPICE, however, is believed to have experienced a cooling of seawater due to upwelling of cool anoxic waters which coincides with, and may very well be the cause of a mass trilobite extinction at the base of the Pterocephaliid Biomere. Inferences from both isotopic data and this plateau in biodiversification between the Cambrian Explosion and the Great Ordovician Biodiversification Event suggest that ocean anoxia was prevalent throughout the Cambrian-Ordovician transition, allowing for redox-dependent deposits to be formed.