EARLY JURASSIC PALEOENVIRONMENTAL RECONSTRUCTION IN THE WESTERN CANADA SEDIMENTARY BASIN AND INFERENCE OF EXPANDED OCEAN ANOXIA/EUXINIA ACROSS THE TOARCIAN OAE FROM GLOBAL RE AND MO MASS BALANCE MODELS

Early Jurassic black shales are globally distributed and provide a record of climate and ocean variations across the Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma). Most OAEs coincide with episodes of enhanced volcanic activity; the Karoo–Ferrar large igneous province (LIP) is dated to ~183 Ma and is considered a trigger for climate shifts during the T-OAE. Here, we provide additional insight on these changes using redox-sensitive elemental and organic carbon isotope (δ¹³C) data from black shales of the Gordondale Member and overlying Poker Chip Shale (PCS) in the Western Canada Sedimentary Basin (NE British Columbia). We use δ¹³C to identify the T-OAE by a negative carbon isotope excursion (N-CIE); redox-sensitive trace metal (RSTM) concentrations ([Mo], [U], [Re], [V]) to infer local bottom-water paleoredox trends; and RSTM covariations (Re/Mo, Mo/U) to verify paleoredox trends and infer paleohydrography. We determine the basin’s candidacy for application of global Re and Mo mass balance models to constrain the global area of sea floor anoxia and euxinia, respectively.

The T-OAE was identified over 1.5 m around the Gordondale–PCS boundary from a N-CIE up to -7‰ from baseline δ¹³C values. A Mo/U covariation diagram suggests the depositional locality was open to global ocean water mass exchange, so RSTM enrichments are used to infer global paleo-ocean redox conditions. RSTMs are enriched in the Gordondale and T-OAE interval, suggesting deposition from anoxic/non-sulfidic and euxinic waters, whereas generally lower enrichments and elevated Re/Mo ratios for the PCS suggest deposition from suboxic to anoxic/non-sulfidic waters. There is no stratigraphic shift in the Re concentration of anoxic shales across the Gordondale/T-OAE, and an average area of sea floor anoxia up to a few percent is implied. Local euxinia is indicated in the lower Gordondale and uppermost Gordondale + T-OAE intervals by high Mo concentrations, high Mo/U and low Re/Mo. The lower Gordondale Mo data suggests a euxinic seafloor area of <0.2%; the upper Gordondale + T-OAE contain lower [Mo] and suggest a euxinic sea floor area up to a few percent. An expansion of euxinic conditions during the T-OAE in response to LIP activity is also indicated by previous Mo and Tl isotope studies and likely contributed to the Pliensbachian–Toarcian extinction event.