OCEAN REDOX CONDITIONS AT THE DEVONIAN–MISSISSIPPIAN BOUNDARY (HANGENBERG EVENT) INFERRED FROM REDOX-SENSITIVE ELEMENTAL ABUNDANCES AND MOLYBDENUM ISOTOPE COMPOSITIONS OF THE EXSHAW FORMATION, ALBERTA, CANADA

Molybdenum isotope, sulfur isotope and elemental data are reported for organic-rich mudrocks from six cores of the Exshaw Formation (NW Alberta, Canada). Hydrocarbon thermal maturity of the samples ranges from immature to overmature. No significant change is observed for bulk Mo and pyrite S isotope compositions with increasing thermal maturity; pyrite S isotope data are best explained by microbial sulfate reduction rather than thermochemical sulfate reduction. We infer that alteration of depositional signatures in the overmature rocks is limited, consistent with concordant Re-Os mudrock and U-Pb zircon depositional age constraints previously published for the Exshaw Formation. Hence, these samples are used to place constraints on ocean redox conditions at the time of the Hangenberg mass extinction.

The Mo, Re, and U concentrations as well as Re/Mo and U/Mo ratios of the mudrocks suggest local bottom water redox conditions ranged from mildly oxygenated to euxinic and varied spatially and temporally. Many samples have high V (>500 μg/g) but variable Mo (18–129 μg/g) concentrations, suggesting a pronounced flux of Fe-Mn particulates to sediments overlain by both non-euxinic and euxinic waters. The lowest δ⁹⁸Mo values (0.3–0.4‰) are from samples with high V enrichments, suggesting enhanced delivery of isotopically light Mo to the sediments by Fe-Mn particulates (Mo isotope data are reported relative to NIST SRM 3134 = 0.25‰). The highest δ⁹⁸Mo (1.0–1.1‰) comes from Mo-rich samples with lower V enrichments and provide a minimum estimate for global seawater δ⁹⁸Mo during the Hangenberg event. A Mo isotope fractionation of at least 0.3‰ may have affected the samples with the highest δ⁹⁸Mo (i.e., global seawater was ≥1.4‰) because of non-quantitative Mo removal from euxinic bottom waters in an epeiric sea during a time of high eustatic sea-level. Mass-balance models for Mo isotopes (assuming global seawater δ⁹⁸Mo ≥1.4‰), Mo concentrations (from euxinic samples), and Re concentrations (from anoxic samples) collectively suggest up to ~5% of the global seafloor was covered by anoxic/euxinic waters. Our findings are consistent with multiple triggers for the Hangenberg mass extinction, including expanded ocean anoxia/euxinia in continental margin and intracratonic regions, glaciation, and possibly volcanism.