A new Rhaetian δ13Corg record: Carbon cycle disturbances, volcanism, End-Triassic mass Extinction (ETE)

Abstract

The links between large-scale volcanism, carbon cycle perturbations and the biotic crises at the End-Triassic Extinction event (ETE) are not well understood. The ETE seems to be marked by three carbon isotope excursions (CIEs) likely triggered by different eruptive phases of the Central Atlantic Magmatic Province (CAMP). These three CIEs appear to occur during the late Rhaetian, close to the Triassic/Jurassic boundary (TJB), but the relative timing connecting the volcanism, carbon perturbations and extinctions is still subject to debate resulting from: i) the difficulties in long-distance biostratigraphic correlations of Rhaetian successions due to the step-like extinction pattern characterizing the ETE, and to the lack of globally correlatable key-fossil group(s) across the system boundary; ii) the limit of coverage of available δ13C profiles to the system boundary interval, rather than the entire Rhaetian; iii) the inherent difficulties in correlating CIEs without clear biostratigraphic markers. Here we present a δ13Corg curve from the Lombardy Basin which covers for the first time the entire Rhaetian. Using this chemostratigraphy, we propose two possible correlations (option 1 and option 2) with other late Rhaetian to early Hettangian successions worldwide. These two possible correlations rely on a combination of paleontological (ammonoids, pollens, conodonts), lithostratigraphic and geochemical constraints. Option 2 in particular has important implications for the causality and geochronology of the ETE events by suggesting that the sharp negative peak usually considered to be the “initial” CIE in the Lombardy might represent a carbon perturbation within the negative trend of the “main” CIE. This implies that a reconsideration of the succession of biotic events occurring during this time interval is required.