30 Clastics, Carbonates, and Soils (Posters)

Friday, 31 March 2017
1:00 PM - 5:00 PM
James River Ballroom

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

Paper number: 30-4
Program: Theme Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 3

doi: 10.1130/abs/2017SE-291176

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