147-1 Carbon and sulfur isotope records of the Ordovician Stairsian extinction from Western Maryland reveal global changes to the marine carbon and sulfur cycles

Session: Climate, Ocean and Environmental Changes Through Earth History: From Marine and Terrestrial Proxies to Model Assessments (Posters)

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The early Paleozoic saw two major biodiversification events that were important turning points in the evolution of marine ecosystems. The Cambrian Explosion (545 to 525 Ma) the first major diversification of skeletonized animals, was followed by the Great Ordovician Biodiversification Event (GOBE; 471 to 458 Ma) that saw a tripling of marine biodiversity. Between these evolutionary events was a 40-million-year interval of lower marine diversity that corresponds to an interval with several rapid extinction events. While the drivers of these events are still largely unknown, deoxygenation in the oceans has been posed as a potential explanation. Here we examine the carbon and sulfur isotope record of the final extinction event of this interval, the Ordovician Stairsian extinction (~484 million years ago). Previous studies of sedimentary successions in Nevada and Utah (USA) deposited along on the northern margin of Laurentia show positive excursions in both carbonate carbon (δ¹³C_carb) and carbonate associated sulfate sulfur (δ³⁴S_CAS) isotopes. The excursions have been interpreted as representing global changes in the carbon and sulfur cycles caused by transient increases in the burial of organic carbon and pyrite sulfur under expanded ocean anoxia.  

The positive carbon and sulfur isotope excursions associated with the Stairsian extinction have only been identified in a handful of successions that were deposited in the Panthalassic Ocean. Thus, to assess whether these signals represent global changes in the carbon and sulfur cycles, we are investigating whether they are preserved in other paleo-ocean basins. Here, we present δ¹³C_carb and δ³⁴S_CAS data from a succession located in the Chesapeake and Ohio Canal National Historical Park in western Maryland. This succession represents deposition on the southern margin of Laurentia in the Iapetus Ocean. The δ¹³C_carb record shows a +4‰ positive excursion consistent with the results found in the western US and suggest that the excursion was indeed the result of a global increase in organic carbon burial. The δ³⁴S_CAS data are forthcoming and will test the global nature of the accompanying positive sulfur isotope excursion. Using paired carbon and sulfur isotope records we will interpret changes in ocean redox chemistry across the Stairsian extinction as well as testing its global extent prior to the GOBE.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10361

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