Carbon and Sulfur Isotope Records of the Ordovician Stairsian Extinction from Western Maryland, Usa Reveal Evidence for Widespread Marine Anoxia

The Early Paleozoic was marked by two major biodiversification events that were important turning points in the evolution of marine ecosystems. The Cambrian Explosion (545 to 525 Ma) was the first major diversification of skeletonized animals, followed by the Great Ordovician Biodiversification Event (GOBE; 471 to 458 Ma), during which marine biodiversity nearly tripled. Between these two evolutionary events was a 40-million year plateau in marine biodiversity that appears to be caused by several rapid global extinction events, the drivers of which are unknown. Deoxygenation has been proposed as a potential driver for these extinction events. Here we investigate the final extinction event of this interval, the Ordovician Stairsian extinction (~484 million years ago). Successions from the Yangtze platform in South China and the western US show positive excursions in carbonate carbon (δ13Ccarb) isotope. Additionally, samples from the Western US also display a positive carbonate associated sulfate sulfur (δ34SCAS) isotope excursion. These trends have been interpreted as representing global changes to the marine carbon and sulfur cycles caused by the increased burial of organic carbon and pyrite sulfur under anoxic conditions. The positive isotope excursions associated with the Stairsian extinction have only been identified in a handful of successions that were deposited in the Panthalassic Ocean and Paleotethys Ocean. To assess whether these are representative of a global change in carbon and sulfur cycles, we have analyzed carbonate samples that represent deposition on the southern margin of Laurentia in the Iapetus Ocean. Here we will present δ13Ccarband δ34SCAS data from a succession located in the Chesapeake and Ohio Canal Historical Park in western Maryland, USA. The δ13Ccarb record shows a +4‰ positive excursion in parallel with a +15‰ positive excursion in the δ34SCAS record. These transient excursions are consistent with the results of previous studies and support the hypothesis of global changes to the carbon and sulfur cycles caused by the increased burial of organic carbon and pyrite in sediments. We will also present sulfate oxygen isotope data (δ18O), as well as results from sulfur and carbon cycle box modeling that will help constrain the amounts, rates, and carbon to sulfur (C/S) ratio of the enhanced organic carbon and pyrite burial during the event. These will be used to interpret changes to ocean redox chemistry across this interval, improving our understanding of the environmental conditions that drove this extinction before the GOBE.