High Resolution Carbon Isotope Stratigraphy and Palynology across CPE-Event1 (Late Triassic)

During the Carnian (⁓ 237–227 Ma; Late Triassic), the C-cycle was repeatedly perturbed leading to widespread environmental changes and biological turnovers during the Carnian Pluvial Episode (CPE). In the sedimentary record, these C-cycle perturbations are documented by negative carbon isotope excursion (NCIEs) recorded worldwide in both terrestrial and marine settings, and in different substrates (i.e., organic matter and marine carbonates). These NCIEs indicate injections of 13C-depleted CO2 into the Carnian exogenic reservoirs of the C-cycle and have been linked to the emplacement of Wrangellia Large Igneous Province. Box models indicate that the pulses of CO2 occurred under relatively low background atmospheric pCO2 levels and temperatures, and at the peak amalgamation of Pangea. These conditions likely amplified the environmental response to CO2-driven global warming. Oxygen isotope records (δ18O) from conodont apatite show a warming of 4º–7°C in seawater temperature. Widespread higher siliciclastic input and sedimentation rates, along with hygrophytic sporomorph assemblages, indicate a more humid climate and an enhancement of continental runoff during the CPE. However, the CPE represents a relatively long interval of up to max. 2.5 Myr, whereas estimations of the duration of each NCIE indicate much shorter timescales (ten to hundreds of years), comparable to other major events of the Mesozoic (i.e. the Toarcian Oceanic Anoxic Event). Based on this observation and supported by evidence of discrete sedimentological and paleontological changes linked to those NCIEs, it has been recently proposed to consider the phenomena occurring in correspondence to each NCIE of the CPE as a distinct “event”. This approach facilitates a more detailed understanding of the processes related to each C-cycle perturbation, and enables more consistent comparisons with other major extinction events. To investigate the NCIE marking the first event of the CPE, i.e., CPE-Event1, which occurs across the boundary between the Trachyceras aonoides and Austrotrachyceras austriacum ammonoid Zones, we performed high-resolution organic C-isotopes (δ13CTOC) analyses integrated with palynological data on two marine sections of the Dolomites (Southern Alps, Italy). δ13CTOC data shows that NCIE of CPE-Event1 is composed of two-discreate δ13CTOC isotope shift (NCIE1α and NCIE1β) with a duration likely less than 50 Kyr. Biostratigraphic correlations demonstrate that these two δ13C shifts are also present in other relative high resolution Tethyan δ13C records, suggesting a global extend of this two-stepped pattern. This feature may reflect short-lived volcanic pulses associated with the emplacement of Wrangellia LIP.