Reconstruction of Local and Global Marine Paleoredox Conditions during Deposition of the Devonian-Mississippian Exshaw Formation (Black Shale Member) and the Evaluation of Hydrocarbon Maturation Effects

Abstract

The end-Devonian Hangenberg event represented one of the biggest Phanerozoic mass extinctions. However, the potential mechanism(s) for triggering this event, including climate cooling, sea-level change, glaciation and ocean anoxia, is still an ongoing subject of debate. In this study, geochemical paleoredox proxies (redox-sensitive trace metals as well as Mo and S isotopes) preserved in black shales of the Exshaw Formation were used to reconstruct local and global ocean redox conditions during the Hangenberg event. The effect of hydrocarbon maturation on the paleoredox proxies was first assessed. Statistically, an unpaired t-test of S and Mo isotope data showed no significant difference between thermally immature, mature, and overmature samples. Large sulfur isotope fractionations (–15‰ to –65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction (MSR) during deposition and early diagenesis rather than thermochemical sulfate reduction (TSR). Hence, it suggested that the geochemical data were minimally affected by hydrocarbon maturation and retained their depositional signatures. Eastern immature shales and central mature shales have a wide range of total organic carbon (TOC; 1.0–15.3 wt%; 2.2–19.3 wt%, respectively) and Mo (1.2–116.7 ppm; 18.3–129.3 ppm, respectively), U (2.4–44.5 ppm; 6.1–66.1 ppm, respectively), and Re (3.9–214.1 ppb; 11.2–316.0 ppb, respectively) concentrations, implying rapidly fluctuating redox conditions at each locality. By contrast, the stratigraphically higher shales from the western overmature area are marked by lower TOC (1.3–2.3 wt%) and trace metal contents (Mo: 1.3–23.5 ppm; U: 2.1–6.5 ppm; Re: 6.7–34.1 ppb), suggesting oxic to suboxic depositional conditions, whereas the stratigraphically lower shales have moderate trace metal enrichments (TOC: 1.3–3.5 wt%; Mo: 10.7–62.7 ppm; U: 5.5–20.7 ppm; Re: 21.5–89.3 ppb), indicating a suboxic to euxinic environment. The basin restriction is no more than moderate, based on moderate Mo/TOC and Mo/U EF (enrichment factor) ratios. The Mo isotope values in the most Mo-rich shales (most likely to be deposited from locally euxinic bottom waters) from the three areas range from 0.3‰ to 1.1‰. Samples with low δ98Mo (0.3–0.4‰) have high V enrichments, suggesting the operation of an Fe-Mn particulate shuttle that delivered isotopically light Mo to sediments. Samples with lower V enrichments may record local seawater-sediment Mo isotope fractionation because of weakly euxinic conditions. Applying Mo isotope fractionation factors of 0.3–1.0‰, global seawater δ98Mo at the Devonian-Mississippian boundary may fall in the range of 1.4‰ to 2.1‰. The Re and Mo enrichment data constrain the extent of anoxic and euxinic seafloor, respectively, to be <2% in both cases using mass-balance models. The Mo isotope mass-balance model suggests <5% euxinic seafloor. Hence, compared to modern ocean seafloor (anoxia: 0.35%; euxinia: 0.021–0.063%), the global ocean at the Devonian-Mississippian boundary likely had more anoxia/euxinia, particularly in intracratonic and continental margin regions, which may have contributed to the Hangenberg mass extinction.