An early Carnian carbon-cycle perturbation before the Carnian Pluvial Episode in the Nanpanjiang Basin (South China)?

The Carnian Pluvial Episode (CPE) was a major climatic and environmental perturbation during the Late Triassic, commonly linked to emplacement of the Wrangellia Large Igneous Province (LIP). Negative carbon isotope excursions (NCIEs) are key chemostratigraphic markers of the CPE, but their number, magnitude, and timing remain debated, particularly for the early phase of the event. Here we present an integrated stratigraphic study of the Carnian succession at the composite Chenjiabao (CJB) section in the Nanpanjiang Basin, based on ammonoid and conodont biostratigraphy and whole-rock organic- and carbonate‑carbon isotope records. The biostratigraphic framework indicates that the studied succession spans the Julian 1 to Julian 2 interval. A ∼ 1.3‰ negative shift in δ13Corg (NCIE-0) occurs in the middle Zhuganpo Formation without a corresponding excursion in δ13Ccarb. A larger negative excursion (NCIE-1), marked by declines of ∼2.3‰ in δ13Corg and ∼ 4.7‰ in δ13Ccarb, is recorded from the uppermost Zhuganpo Formation to the basal Wayao Formation. Two alternative biostratigraphic constraints are available for the Julian 1/2 boundary in the CJB section. In the ammonoid-based framework, NCIE-0 predates the conventionally recognized CPE, whereas in the conodont-based framework it represents the first carbon-isotope excursion of the CPE. NCIE-1 can be more confidently correlated with the nearby Longchang section and western Tethyan records, and may be linked to 13C-depleted carbon input associated with emplacement of the Wrangellia LIP. These results support the view that carbon-cycle perturbations during the Carnian were not a single event, but rather evolved through a multi-stage, stepwise pattern, and also highlight the need for further refinement of Carnian biostratigraphic frameworks in the Nanpanjiang Basin to better constrain the timing and onset of the event.