DECIPHERING THE RECORD OF BIOLOGIC AND ENVIRONMENTAL CHANGE DURING THE LATER CAMBRIAN STEPTOEAN POSITIVE CARBON ISOTOPE EXCURSION (Invited Presentation)

Biogeochemical studies of the late Cambrian-early Ordovician transition have suggested that oxygen deficiency was widespread in the deeper oceans and potentially influenced evolutionary events during this time. Specifically, this time interval contains several discrete marine extinction events, the first of which occurs at the end of the Marjuman Stage of North America (base of the Paibian International Stage). The end-Marjuman extinctions occur in two phases and appear to approximately coincide with the beginning of the Steptoean Positive Carbon Isotope Excursion, or SPICE. The SPICE is a carbon isotope excursion of +4 to +5‰ in the marine record representing a perturbation to the carbon cycle during this time. It has been suggested, based on globally distributed geochemical datasets — specifically stratigraphic changes in sulfur and uranium isotopes and molybdenum concentrations — that the SPICE represents an event akin to Mesozoic oceanic anoxic events; a transitory event where the area of ocean anoxia expanded. However various aspects of the record of the SPICE have been disputed including the evidence for the expansion of marine anoxia and the coincidence of the geochemical changes with the end-Marjuman extinction events.

Here we will review the available records and present new geochemical data from globally distributed locations that record the SPICE to: 1) untangle the timeline of biologic and environmental change across the SPICE, and 2) to identify the potential drivers of the events. In stratigraphic sections where both phases of the end-Marjuman extinctions can be clearly resolved, the onset of the SPICE occurs at or before the stratigraphic interval where the second phase of the end-Marjuman extinctions occur. However, uncertainty in the placement of the beginning of the SPICE illustrates a clear need for a standard criterion to delineate the start of the isotopic event. New iron speciation data from North America, the United Kingdom and South Korea point to local expansion of anoxic conditions during and after the initiation of the SPICE. The most parsimonious explanation for these data are that the SPICE represents an interval of expanded marine anoxic and euxinic conditions and the expansion of these reducing marine conditions were likely an important mechanism for the end-Marjuman extinctions.