CONSTRAINING LONG-TERM REDOX CONDITIONS IN THE TROPICAL PANTHALASSIC OCEAN DURING THE LATE TRIASSIC THROUGH THE EARLY JURASSIC

The end-Triassic mass extinction (ETE) was one of the “Big 5” mass extinctions in the Phanerozoic and has been linked to the emplacement of the Central Atlantic Magmatic Province (CAMP). Volcanic outgassing from CAMP is thought to have led to a variety of environmental and climatic responses that led to severe deterioration of global environments. Marine deoxygenation is one such environmental response and has been implicated as a driver of the ETE. Evidence for deoxygenation across this time interval is limited, however. The handful of studies on marine redox are mostly from sedimentary successions representing the ancient Tethys Ocean, with few data available from other ocean basins, and have mainly focused on the interval immediately surrounding the ETE. To address these limitations, we present iron speciation data from the Upper Triassic–Lower Jurassic McCarthy Formation near Grotto Creek in the Wrangell Mountains of Southcentral Alaska. The thick (>500 m) succession represents a distal ramp environment that existed at tropical latitudes on the Wrangellia Plateau in the open Panthalassic Ocean (Veenma et al., accepted). Biostratigraphy, U-Pb age dates, and carbon isotope stratigraphy indicate that the succession represents a relatively complete upper Norian through lower Sinemurian record that includes the ETE (Caruthers et al. 2022). Iron speciation data suggest the McCarthy Formation was deposited predominantly under an anoxic water column that was predominantly ferruginous (iron-replete) but was punctuated by instances of euxinia near the ETE and the in latest Hettangian and earliest Sinemurian. Sedimentological and petrographic analysis also reveals bioturbated intervals that likely represent short-term oxygenation events not recorded by the geochemical data. Together, these data suggest that the Grotto Creek succession was deposited under a dynamic but predominantly anoxic water column, likely in the tropical oxygen minimum zone throughout the studied interval. Moreover, given the persistent anoxic conditions recorded there, this succession holds great potential for further redox proxies such as elemental concentrations and isotopes, which can be used to delineate regional to global marine redox conditions over the Late Triassic– Early Jurassic and the ETE.