99-13 Carbon and sulfur isotope records of Cambrian end-Sunwaptan extinction from Lawson Cove, Utah point to an expansion of marine anoxia

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

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Abstract:

The interval between the Cambrian Explosion and the Great Ordovician Biodiversification Event was marked by a series of marine extinction events. These events notably affected trilobites but also impacted brachiopods and conodonts. Among these, the end-Sunwaptan extinction that occurred at the end of the Sunwaptan North American Stage (~490 million years ago) is one of the least studied. Understanding this extinction is key to reconstructing early marine ecosystem dynamics and identifying environmental drivers of biodiversity changes during this transitional time. Some geochemical studies of better documented events, such as the Cambrian end-Marjuman extinction associated with the Steptoean Positive Carbon Isotope Excursion (SPICE) and the Ordovician Stairsian extinction, have implicated ocean deoxygenation and euxinia as key drivers of biodiversity turnover. The environmental conditions during the end-Sunwaptan extinction, however, are unconstrained. To investigate whether these conditions played a role in this extinction, we analyzed the carbonate carbon (δ¹³C_carb) and carbonate-associated sulfate sulfur (δ³⁴S_CAS) isotope records from the sedimentary succession at Lawson Cove in western Utah. These geochemical proxies provide insights into ancient carbon and sulfur biogeochemical cycling, which can be recorders of marine redox state.

Our geochemical records from the Lawson Cove section are well-suited for detailed analysis of the end-Sunwaptan extinction because of the voluminous pre-existing biostratigraphic work on this section. Near the base of the section, our data reveal a significant negative excursion (~ -2‰) that corresponds to the previously documented Hellnmaria Red Tops Boundary (HERB) carbon isotope excursion; this occurs in beds underlying the end-Sunwaptan extinction. During the end-Sunwaptan extinction, located in the Lava Dam Member of the Notch Peak Formation, there is no δ¹³C_carb excursion. In contrast, our δ³⁴S_CAS record shows a positive sulfur isotope excursion that initiates before the extinction, supporting the hypothesis of enhanced pyrite burial and oceanic anoxia as potential environmental drivers of this event. This excursion indicates a prolonged period of redox instability that could have stressed marine ecosystems. Together, our carbon and sulfur isotope data provide new constraints on redox changes in the Cambrian marine environment, offer context for evaluating the environmental drivers of the end-Sunwaptan extinction, and add to a growing body of evidence demonstrating that unstable marine redox conditions were extinction drivers in early Paleozoic oceans.

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

doi: 10.1130/abs/2025AM-10368

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