FIRST REPORT OF THE ARCHAEOCYATHID EXTINCTION AND THE REDLICHIID-OLENELLID EXTINCTION CARBON ISOTOPE EXCURSIONS (AECE AND ROECE) IN EASTERN LAURENTIA: IMPLICATIONS FOR PERTURBATIONS IN THE LATE EARLY CAMBRIAN CARBON CYCLE

Soon after the biodiversification event of the Cambrian ‘explosion’, two major marine extinctions occurred. During these events in the late Early Cambrian (513 – 508 Ma), the widespread archaeocyathid-Renalcis reef ecosystem collapsed, and redlichiid and olenellid trilobites went extinct. These two biotic crises were accompanied by perturbations to biogeochemical cycles recorded in the geochemistry of sedimentary rocks from this time interval. Specifically, two negative carbon isotope excursions (CIEs) are seen in the chemostratigraphic record: One CIE, the AECE (Archaeocyathid Extinction Carbon isotope Excursion), coincided with the earlier archaeocyathid extinction and the other CIE, the ROECE (Redlichiid-Olenellid Extinction Carbon Isotope Excursion), is contemporaneous with the later trilobite faunal turnover. Both the CIEs and extinction events have been tentatively linked to the roughly coeval eruptions of the Kalkarindji Large Igneous Province (LIP) in Australia. It has been hypothesized that the massive release of ¹³C-depleted carbon from the eruptions drove the negative CIEs in the marine record and led to the biotic crises through development of marine anoxia or other environmental changes. However, evidence for this extinction scenario remains tenuous.

Here we present carbon isotope stratigraphy constructed from the Early to Middle Cambrian Shady Dolomite Formation in southwestern Virginia. The carbonate carbon isotope (δ¹³C_carb) data was generated from the drill core in the periplatform facies of the Shady that captures two negative δ¹³C excursions. Based on their stratigraphic positions and biostratigraphic data, the two negative CIEs likely represent the local appearance of the AECE and the ROECE. Locating the AECE and the ROECE in the Shady provides an eastern Laurentian record of these perturbations and strengthens the argument that they represent global changes in the late early Cambrian carbon cycle. This study also builds the foundation for employing other geochemical proxies for anoxia and volcanism such as Ca/I, Hg, and iron speciation to further understand the paleoenvironmental changes at the end of the early Cambrian and to bridge the gap between the two extinction events and their proposed driving mechanisms.