A Shallow-Water Record of the Frasnian-Famennian Boundary from Longmenshan, Sichuan, South China

The Frasnian–Famennian transition of the Late Devonian represents a major interval of biotic turnover and environmental change in the Phanerozoic. In shallow-water carbonate platform successions, however, recognition of the boundary and correlation with the global standard remain difficult because of strong facies control, the scarcity of key zonal taxa, and the common occurrence of depositional breaks. In the Longmenshan area (Sichuan Province, China), although Upper Devonian strata are relatively well developed, conodont zonation across the Frasnian–Famennian transition has long remained unestablished, hindering detailed stratigraphic subdivision and regional correlation. This study examines the Qinglongxia sections through an integrated analysis of conodont biostratigraphy, carbonate microfacies, and carbon and oxygen stable isotopes. The Qinglongxia A section preserves a relatively continuous conodont succession from FZ 3 to the Palmatolepis gracilis expansa, but the Frasnian–Famennian boundary is constrained to a FZ 13b - Palmatolepis crepida indifferentiated interval. Microfacies data indicate that the boundary interval records a transition from low-energy inner-platform deposition to higher-energy platform-margin shoal conditions, accompanied by a regression–transgression–renewed regression trend. Stable-isotope data show that δ¹³Ccarbvalues remain at a relatively high background level through most of the section but display a localized negative shift near the boundary. The weak correlation between δ¹³Ccarb and δ¹⁸Ocarb suggests that this anomaly is unlikely to have resulted solely from diagenetic alteration and more probably reflects a local perturbation of the shallow-marine carbon cycle during the Frasnian–Famennian transition. Comparable negative shifts have also been reported from some South China sections across the Frasnian–Famennian transition, although positive excursions are more commonly documented in the region, indicating that the isotopic expression of the event was regionally variable. Together with previously reported Hg anomalies and evidence of volcanic activity, this negative shift is more likely to reflect a broader environmental disturbance related to climatic instability, sea-level fluctuations, and marine anoxia, with a possible contribution from volcanic activity. The Qinglongxia A section may represent a reference section for the Frasnian–Famennian boundary in shallow-water carbonate platforms of South China.