218-2 Widespread marine deoxygenation during Ordovician Stairsian extinction as indicated by marine carbonate carbon and sulfate sulfur isotope records

Abstract:

The Early Paleozoic was marked by two major biodiversification episodes that were important to the evolution of marine ecosystems. The Cambrian Explosion (545 to 525 Ma) was a major diversification of skeletonized marine metazoans, followed by the Great Ordovician Biodiversification Event (GOBE; 471 to 458 Ma), when marine biodiversity nearly tripled. Between these two events was a ~40-million-year plateau in marine biodiversity characterized by a series of extinction events. While the mechanisms behind these extinctions are debated, marine deoxygenation is one proposed driver. Here we investigate one of these extinctions, the Ordovician Stairsian extinction (~484 Ma). Sedimentary successions from the Yangtze platform in South China and the northern Laurentian margin in the Western United States show positive excursions in carbonate carbon isotope (δ¹³C_carb) records just after the extinction event. Records from Western US also display a positive carbonate-associated sulfate sulfur (δ³⁴S_CAS) isotope excursion. These trends have been interpreted as representing global changes to the carbon and sulfur cycles related to the increased burial of organic carbon and pyrite sulfur in sediments under anoxic marine conditions.

However, the isotopic excursions associated with the Stairsian extinction have only been identified in a handful of successions that were deposited in the Panthalassic and Paleotethys Oceans. We assessed whether these isotopic signals are representative of global changes in carbon and sulfur cycles by investigating a succession deposited in the Iapetus Ocean and located in the present-day Chesapeake and Ohio Canal Historical Park of Maryland, USA. Just after the Stairsian extinction, this section contains a δ¹³C_carb excursion of +4‰ and a δ³⁴S_CAS excursion of +15‰. These transient excursions are consistent with previous studies and support the hypothesis that they represent global changes to the carbon and sulfur cycles. Sulfur and carbon isotope box modeling confirms that transient increases in marine organic carbon and pyrite sulfur burial under anoxic conditions was the most likely scenario to generate these isotopic excursions. Together our new isotopic records and modeling results indicate that changes to ocean redox chemistry across this interval was a key environmental shift that drove this extinction before the GOBE.

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