Weathering and sea level control on siliciclastic deposition during the Carnian Pluvial Episode (Southern Alps, Italy)

A higher precipitation regime during the early Late Triassic Carnian Pluvial Episode (CPE; 232–234 Ma) in many marine basins is evidenced by thick siliciclastic deposits, which are coeval with C-cycle perturbations and global warming. However, the mechanisms driving higher siliciclastic deposition are still not fully explored and could be linked to different effects of the climate change on depositional environments. Here, we present the first major, minor and trace elements, and mineralogical data from fine-grained sediments across the CPE in the Dolomites (Southern Alps, Western Tethys). Combining weathering indices (CPA and Rb/K2O), and qualitative and quantitative mineralogical analysis, we recognize two main phases during the CPE. Pre-CPE conditions show no significant weathering variations and some peaks of clay minerals referable to the weathering of Ladinian igneous material. The first C-isotope excursion of the CPE is coincident with an increase of CPA, Rb/K2O and kaolinite, with no change in the sediment-source and depositional setting, that indicate an enhancement of chemical weathering and confirm a shift to more humid conditions at the onset of the CPE. Such a signal from the Dolomites can be correlated with other similar observations from marine and terrestrial settings worldwide. After this first phase, a major sea level fall, probably linked to aquifer-eustasy and/or limno-eustatism, and following important transgression led to the erosion and recycling of older rocks marked by a decoupling of CPA and Rb/K2O and a substantial increase of K-feldspar from older volcanic rocks. The eustatic signal overwhelmed the humid climatic signal that is inferred by independent palynological and paleosol data within the same interval. After the CPE, data show high physical weathering under arid climate. Our results are compatible with the general view that the CPE was linked to injections of volcanic CO2 from Large Igneous Province activity and consequent global warming and enhanced hydrological cycle. This first intensified rock chemical weathering at the onset of the CPE, then promoted higher storage of freshwater on land and a substantial increase of erosion and transport of unweathered material into the basins of the western Tethys, that were rapidly infilled.