A GRADUAL INCREASE IN PCO2 ACROSS THE ABRUPT END-TRIASSIC EXTINCTION

The End Triassic Extinction (ETE) is one of the “Big-5” mass extinctions of the Phanerozoic and has arguably among the best terrestrial stratigraphic records based on the cyclic lacustrine sediments of the Newark Supergroup, in eastern North America. In the Newark Basin, the ETE horizon is positioned stratigraphically above a short reverse magnetic polarity chron E23r and below the locally lowest flows of the Central Atlantic Magmatic Province (CAMP), a massive Large Igneous Province spread across four continents. We report on new and detailed magnetic stratigraphy and paleosol-based estimates of atmospheric pCO₂ from latest Triassic strata of the Newark basin from several cores that encompass the lowest CAMP lava flows and underlying strata. We systematically identify the short magnetic chron E23r in the upper Passaic Formation, tying our pCO₂ estimates to the established geomagnetic polarity timescale from the Newark, which constrains the time represented by the interval between E23r and the base of the CAMP basalts to be about 23 kyrs. We find a coherent and gradual increase in pCO₂ from ~1800 to 3000 ppm that begins significantly before, and may extend through, the ETE horizon. This gradual increase is noted in two basins (Newark and Hartford), data from which are separated by 200 km but are from demonstrably contemporaneous sediments. However, the exact placement of the ETE and E23r with respect to the pCO₂ change will be confirmed by more detailed sampling. Secondly, we note a sharp increase in pCO₂ to ~4000 ppm just above the ETE but just ~2 meters below the first CAMP basalts in both basins. In previous work we identified a spike in atmospheric pCO₂ immediately following emplacement of each CAMP volcanic unit from soils in stratigraphic superposition immediately above the basalts in the Newark and Hartford basins. We likewise attribute this increase below the first Newark basalts to the very first CAMP eruptions elsewhere, which based on recent high-resolution U/Pb dates, precede the earliest basalts in the Newark. Given the abrupt and rapid nature of the terrestrial faunal turnover, a gradual rise in pCO₂ through the ETE strata appears to preclude CO₂ as a primary trigger for the mass extinction, leaving volcanogenic sulfur emissions and multiple subsequent short-lived volcanic winters as the most likely kill mechanism.