URANIUM AND MOLYBDENUM ISOTOPE EVIDENCE FOR GLOBALLY EXTENSIVE MARINE EUXINIA ON CONTINENTAL MARGINS AND IN EPICONTINENTAL SEAS DURING THE DEVONIAN-CARBONIFEROUS HANGENBERG CRISIS

The end-Devonian Hangenberg mass extinction event was characterized by significant loss of marine invertebrates and vertebrates. In this study, the molybdenum (Mo) and uranium (U) isotope systems were applied to Exshaw Formation black shales from six drill cores to test the hypothesis that expanded ocean anoxia/euxinia contributed to the Hangenberg Crisis. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Sulfur isotope ratios for our samples and accurate Re-Os ages previously reported for the Exshaw Formation suggest that the redox-sensitive metal data, including for thermally overmature shales, were not adversely affected by hydrocarbon generation and reflect depositional conditions. Paleosalinity proxies suggest brackish and marine conditions with significant water exchange between the basin and open ocean. Redox-sensitive metal concentrations and ratios (e.g., Mo/U), and Fe speciation indicate locally suboxic (with sulfidic pore waters) to euxinic environments. Authigenic δ⁹⁸Mo and δ²³⁸U for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to +0.39‰, respectively. The δ⁹⁸Mo of the Exshaw shales is likely lower than coeval global seawater because of a local particulate Fe-Mn shuttle, incomplete molybdate to tetrathiomolybdate conversion in weakly sulfidic conditions, or non-quantitative removal of Mo from bottom/pore waters. Comparison of δ²³⁸U and U enrichments for the euxinic black shales suggests that U isotopic offsets between the sediments and global seawater (heavier ²³⁸U is preferentially removed to sediments) ranged from 0.4–0.8‰ (as observed in modern euxinic basins) to 0.8–1.2‰ (enhanced U removal into organic floccule layers at the sediment-water interface). After correcting for seawater-sediment isotopic offsets, the most probable global seawater δ⁹⁸Mo and δ²³⁸U at the Devonian-Carboniferous boundary are inferred to be 1.4‰ to 1.9‰ and −0.8‰ to −0.5‰, respectively, which are lower than modern seawater values of 2.3‰ and –0.4‰, respectively. A Mo and U isotope mass balance model suggests euxinic seafloor expanded and was common along continental margins and in epicontinental seas during transgression, supporting expanded ocean anoxia/euxinia as an important kill mechanism for the Hangenberg Crisis.