Onset of the Carnian Pluvial Episode: a record by clay minerals from the Misurina stratigraphic succession (Dolomites, Italy)

The Carnian Pluvial Episode (CPE) was a climate and biological crisis that occurred in the early Late Triassic (~232–234 Ma) and has been linked to large igneous province volcanism, which would have injected large quantities of volcanic CO₂ into the Carnian atmosphere and triggered global warming and a strong enhancement of the hydrological cycle. In the marine sedimentary succession of the Dolomites, the CPE is marked by a crisis of the early Carnian microbial-dominated high-relief carbonate platforms and an increase of siliciclastic deposition. To understand weathering processes related to the more humid climate that occurred at the onset of the CPE, we present organic-carbon isotope data and clay mineralogical composition of 112 fine-grained samples taken from a basinal stratigraphic series outcropping near Misurina (Auronzo di Cadore, Dolomites, Italy). The succession encompasses the topmost San Cassiano (early Carnian) and the base of the Heiligkreuz (late early Carnian) formations, corresponding to the interval recording the onset of the CPE. Overall quantitative mineralogical composition was obtained from whole-rock samples XRD analysis using Rietveld method. Analyses on oriented mounts of the <2µm fraction in air-dried, ethylene-glycol saturated, and heated form, were also performed for the accurate identification of clay minerals. The Misurina section consists mainly of argillaceous and calcareous marls where two sharp δ¹³C_TOC excursions (CIE 1a and 1b), corresponding to the first global C-cycle perturbation of the CPE, were identified. The CIE 1b can be correlated to the negative shift previously recorded in the nearby Milieres reference section. At Misurina, the C-isotope perturbation can be divided in six phases: I) background; II) negative δ¹³C_TOC shift 1a; III) positive rebound 1a; IV) negative δ¹³C_TOC shift 1b; V) positive rebound 1b; VI) return to background. These C-isotope changes are linked to changes in clay mineralogy. We accurately quantified the clay mineral assemblages composed of Chlorite (0.26–4.33%), Kaolinite (0.70–14.46%), Illite (2.29–14.08%) and the major component Illite/Smectite (IS) mixed-layer (67.93–95.77%). From phase I to phase II, Kaolinite increases from average 1.52 % to 2.92 %, as well as Kaolinite/Illite ratio. Kaolinite and Kaolinite/Illite remain higher during phases III (avg 2.70 %) and IV (avg 3.26 %) with respect to background values (avg 1.52 %). A further increase in Kaolinite content (avg 5.53 %) is recorded in phase V. Phase VI records a relative increase of the siliciclastic component and a parallel relative increase of Chlorite, Kaolinite and Illite with respect to IS. The mineralogical variations during the C-isotope phases IV, V, VI at Misurina are parallel to variations in the relative abundance of Kaolinite, Kaolinite/Illite and Quartz + Feldspars at Milieres section. An increase of Kaolinite content during phases II-V are interpreted to reflect an increase of the intensity of silicate hydrolysis on land, which is in agreement with the CPE onset model. Our high-resolution dataset shows that the onset of the CPE was marked by repeated perturbations of the C-cycle and an increase of the chemical weathering as observed in other marine and terrestrial settings within the same stratigraphic interval.