δ13C, δ34S, AND I/CA EVIDENCE FOR ANOXIA AT AN EARLY ORDOVICIAN (TREMADOCIAN) MASS EXTINCTION
Here we examine one of the youngest extinction events interpreted to reflect a continuation of anoxia-caused extinctions. We sampled three Early Ordovician (Tremadocian) carbonate successions for δ13C, δ34S, and I/Ca trends and show evidence that anoxia was regionally widespread across the Laurentian margin at this time. Evidence for anoxic shallow subsurface waters is based on ~0 I/Ca values that roughly coincide with parallel δ13C and δ34S excursions. Geochemical evidence for anoxia also coincides with the extinction of local faunas where 30% of standing generic diversity went extinct. Indirect proxies for redox conditions using δ13C/δ34S broadly agree with direct proxies for anoxia (I/Ca), indicating that anoxia was the likely cause of this Tremadocian extinction event, although differences between I/Ca and δ13C/δ34S signals suggest regional variation in the extent of anoxia.