RECONSIDERING ORDOVICIAN AND SILURIAN CARBON CYCLE PERTURBATIONS AND EXTINCTION EVENTS (Invited Presentation)

One of the most striking and intensively studied features of the Paleozoic record is the recurring association between large positive carbon isotope excursions and faunal turnover events. Recent work has made considerable progress in elucidating the mechanisms responsible for the former but many questions remain open regarding the relationship between carbon cycle perturbations and biotic impacts. Late Ordovician and Silurian carbon isotope perturbations are multifaceted events, potentially involving complex cascades of environmental disruption over timescales of 10³ to10⁶ years. It is not always clear which of these various disruptions are most implicated in causing elevated extinction rates. This uncertainty can be reduced by analyzing the selectivity of extinction, deriving a priori predictions about ecological, physiological, biogeographic or life history characteristics that would be expected to influence survivorship under different extinction scenarios and comparing these predictions to observed extinction patterns. I will review the current state of knowledge regarding the causes of Ordovician and Silurian isotope excursions and the environmental disruptions associated with them, with a particular focus on the Hirnantian excursion and mass extinction event. Changes in ocean redox state during excursion events are poorly understood, with some lines of evidence indicating oxygenation of shelf environments during excursions and others suggesting increasing global anoxia. I will discuss the timing and selectivity of extinctions in different clades and their implications for the role played by oxygenation changes and by coeval changes in climate and sea level.