The Devonian-Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean-atmosphere oxidation states 1,2 , resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering 3,4 , glacioeustasy 5,6 , eutrophication and anoxic expansion in epicontinental seas 3,4 , and mass extinction events 2,7,8 . Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biodiversity., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)