Living on the edge: The impact of protracted oxygen stress on life in the Late Devonian

The Late Devonian records one of the most dramatic series of taxonomic and ecological turnovers in the history of life. The precise controls over the elevated extinctions and depressed origination rates at this time are not fully resolved, but reduced oxygen conditions undoubtedly played a role. A combined geochemical and paleonto-logical dataset from 17 localities across New York and Ohio provide a cohesive record of prolonged and repeated oxygen stress in the Appalachian Basin associated with numerous biotic turnovers through the Late Devonian. The trace fossil signal, captured as ichnofabric index data, and trace metal proxies (Mo, U, V) in this study provide insight into bottom water conditions, while biomarker data, including hopane/sterane, sterane carbon number ratios and Chlorobi-derived carotenoid abundances, in combination with organic-walled microfossil (OWM) abundances inform the nature of biological communities and redox conditions in the water column. These collective proxies allow us to compare the Lower and Upper Kellwasser Events, and the Hangenberg Biocrisis, with background conditions through the Late Devonian across a range of depositional environments. The results reveal that each of these three extinctions has a unique signal. Interestingly, the Hangenberg exhibits the greatest departure from background environmental conditions without comparable extinction rates, suggesting that prolonged oxygen stress throughout the Late Devonian removed vulnerable populations and limited the impact of this event. Further, variable signals and responses between bioevents likely indicate different drivers for each. This work adds to the narrative that the Late Devonian extinction event was unique among the "Big 5" and sets the stage for a more global-scale dissection of triggers, kill mechanisms, and responses.