δ13C, δ34S, AND I/CA EVIDENCE FOR ANOXIA AT AN EARLY ORDOVICIAN (TREMADOCIAN) MASS EXTINCTION

Changing environmental conditions, particularly atmospheric oxygen levels, are thought to be important drivers of major biotic events (e.g. mass extinction and diversification). The early Paleozoic represents a key interval in the oxygenation of the atmosphere/ocean system and evolution of the biosphere, yet few direct geochemical proxies exist that characterize oxygen levels in deep time. Instead studies use indirect proxies (carbon (δ¹³C) and sulfur (δ³⁴S) isotope trends) to reconstruct oxygen levels and infer causes of environmental change and biotic turnover. The Cambrian–Ordovician contains several episodic trilobite extinctions attributed to anoxia based on parallel positive δ¹³C and δ³⁴S excursions but have yet to be coupled with direct redox proxies (e.g. I/Ca) to confirm whether these extinctions were caused by oxygen crises.

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.