The Carnian Pluvial Episode: Timing and Mechanism

The Carnian Pluvial Episode (CPE) is marked by major changes in the climate to significantly more wet and humid conditions, followed by a return to an arid state. This episode is recorded in several stratigraphic sections around the world. The climatic shift is thought to have been driven by perturbation of the global carbon cycle, associated with the emplacement of the Wrangellian Terrain Large Igneous Province (WT-LIP) in the Northern Panthalssic ocean (~231-225 Ma). The event is often linked to the rise and diversification of dinosaurs, major biotic shifts on land along with the establishment of modern ecosystems making it a critical event in Earth's history. Detailed studies of the CPE that examine geochemical evidence of the link to the emplacement of Wrangellia are limited. Here, we present new high-resolution geochemical data, combined with lithological description, of the Carnian in the Knocksoghey Formation sampled throughout the Mercia Mudstone from the Carnduff-2 core, Northern Ireland. The Knocksoghey Formation represents continental sediments, marked by an abrupt shift to increased coarse-grained siliciclastics at the presumed onset of the CPE. These sandy sections are followed by an abundance of anhydrite nodules, with reddish brown mudstones displaying green reduction spots.  Multiple proxies, including changes in carbon isotope and elemental compositions, weathering proxies, Hg/TOC variations, and astrochronology, are utilized to assess the temporal link between the emplacement of the WT-LIP and the onset and pulses of the CPE and to determine the potential mechanisms driving the event. The lithological change to increase coarse-grained siliciclastics is preceded by higher Hg concentrations, as well as a negative carbon isotopic excursion in the range of ~3-4 ‰, which points to an increased volcanic activity. This interval, which appears to represent wetter conditions, is followed by sedimentological evidence of more arid conditions marked by increased gypsum followed by a second increase in Hg/TOC. Altogether, further evidence of wetter conditions close to the Carnian–Norian Boundary suggests that volcanism was closely linked to the alternations between an arid and humid climate during the CPE.