HYDROGRAPHIC EVOLUTION OF THE EPEIRIC SEAWAYS OF NORTH AMERICA DURING THE LATE DEVONIAN: IMPLICATIONS FOR HABITABILITY AND MASS EXTINCTIONS

The Late Devonian was a time of repeated mass extinctions of marine fauna and widespread deposition of organic-rich shales across North America, suggesting a linkage between extinction and marine anoxia. However, anoxia in epeiric seas has many potential causal mechanisms related to the hydrographic architecture of individual basins, including nutrient inputs and productivity, sea level, salinity, connectivity to the open ocean, and potential watermass stratification. These holistic hydrographic factors have long been overlooked in explaining the episodic reduction of habitability and triggering of biocrises in North American epeiric seas during the Late Devonian. Here, we present a detailed reconstruction of the hydrographic evolution of three North American epeiric basins through the Late Devonian—the Illinois, Appalachian, and Williston basins. We employ a multi-proxy approach that examines redox, productivity, restriction, and salinity in thirteen cores across North Dakota, Illinois, Indiana, Kentucky, Tennessee, and Ohio, with data presented for TOC, iron speciation, redox- and productivity-sensitive trace metal abundances and ratios, organic carbon, nitrogen, and pyrite sulfur isotopes, and paleo-salinity proxies (B/Ga, Sr/Ba, S/TOC). Overall, we find that the two largest pulses of Late Devonian extinction (the Kellwasser and Hangenberg events) were associated with major hydrographic reorganizations of these basins, leading to expansions of toxic euxinic waters into the shallow habitable zone. In the Illinois Basin, the Upper Kellwasser Event was associated with enhanced freshwater runoff and thus increased water-column density stratification, which resulted in the expansion of deep-water euxinia. Subsequently during the Hangenberg Event, sea-level rise in the Appalachian and Williston basins led to the transgression of euxinic, metal-rich waters into the photic zone, an event recorded in a spectrum of salinity facies from brackish to marine to hypersaline. Overall, our results highlight the complex hydrographic factors that influence the habitability of epeiric seas, suggesting that extinction intervals should be evaluated in the context of variation not only in redox conditions but also in salinity, watermass restriction, productivity, and other environmental variables.