EUSTASY, EUXINIA, AND EXTINCTION: GLOBAL REORGANIZATION ACROSS THE DEVONIAN-CARBONIFEROUS TRANSITION

The Devonian-Carboniferous transition (~359 million years ago) marks a fundamental reorganization of the Earth system, which resulted from a ‘perfect storm’ of events, including the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering, stepwise glacioeustatic fluctuations, eutrophication and anoxic expansion in epicontinental seas, and mass extinction of marine and terrestrial biotas. Here, we present an expansive new redox and biogeochemical dataset from the world-famous Bakken Shale (North Dakota, USA) from two drill cores, as well as the distribution of redox-sensitive elements from an additional 90 cores (~11,000 data points) that provide an ultra-high resolution spatiotemporal view of sea level, climate, redox, and extinction across the D-C transition. Iron speciation indicates persistently euxinic conditions throughout the deposition of the organic-rich shales below and above carbonaceous sandstone associated with glacial regression of sea level. The latest Devonian Lower Bakken Shale (LBS) below the sandstone records three discrete pulses of redox-sensitive trace metal (RSTM) enrichments that are accompanied by distinct positive excursions in δ¹⁵N compositions. When interpreted in a sequence stratigraphic framework, these data suggest that three pulses of expanded water column euxinia were associated with stepwise marine transgression, which is supported by the stepwise decreases in δ³⁴S_py through the LBS. Our compilation of RSTM data across the entire basin clearly indicates that the locus of euxinia was in shallow, nearshore environments. Conodont biostratigraphy indicates that the three pulses of marine transgression and expanded euxinia coincide directly with three Late Devonian biotic crises—the Annulata, Dasberg, and Hangenberg events. We therefore suggest that stepwise transgression of toxic euxinic waters into the shallow oceans was a major driver of Late Devonian extinction, culminating in the Hangenberg Event that fundamentally altered the composition of Earth’s biosphere.