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Pliensbachian–Toarcian (Early Jurassic) extinction in western North America Caruthers, Andrew Harry


The Pliensbachian–Toarcian marine extinction is observable at the species and generic levels. Ammonite diversity data from Europe and parts of the Arctic suggest a multi-phased event with diversity declining over six separate intervals. The main-phase of decline begins at the Pliensbachian–Toarcian boundary and extends into the Early Toarcian to a level correlative with the Tenuicostatum / Serpentinum Zone boundary. To date only this main-phase has been demonstrated as being global in extent, affecting multiple taxonomic groups. The entire Pliensbachian-Toarcian extinction has been attributed to regional and global controlling mechanisms associated with the Volcanic Greenhouse Scenario, an hypothesis linking eruption of the Karoo–Ferrar large igneous province (LIP) to global warming and mass extinction, specifically involving the release of methane hydrate from shelf reservoirs and a global marine anoxic event in the Early Toarcian (the T–OAE). The study presented herein uses paleontology and isotope geochemistry to investigate the duration and potential controlling mechanisms of this protracted extinction. A primary objective is to compare new data from western North America with previously established records in Europe, testing: 1) the multi-phased nature of this extinction, 2) its magnitude within two taxonomic groups (ammonoids and foraminifera) in western North America and 3) its controlling mechanisms, relating to methane hydrate release and geographic extent of the T-OAE. Results show that all six phases of species decline are recognizable in western North America, even the oldest episode which was previously thought to be an event restricted to the Tethys Ocean area of Europe. This research strongly supports a correlation between the timing of the entire multi-phased extinction and formation of the Karoo igneous province. The study also provides one of the first records of the Early Toarcian ‘negative carbon-isotope excursion’ outside the Tethys Ocean area (concomitant with the main-phase of extinction) which implicates global methane hydrate release. Lastly, geochemical results do not support the presence of an anoxic water mass in the northeast paleo Pacific Ocean at the time of the so-called global Toarcian event (T-OAE).

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