The Frasnian–Famennian boundary (Late Devonian) in Peri-Gondwana and South China. New data from the Carnic Alps (Italy) and Longmenshan (Sichuan, China): conodont biostratigraphy, stable isotope analysis, and correlation

Abstract We integrate conodont biostratigraphy, carbonate microfacies, and stable carbon–oxygen isotopes from two new Frasnian–Famennian (F–F) successions that bracket contrasting depositional settings: the Plan di Zermula D (PZD) section in the Carnic Alps (Peri-Gondwana) and the Qinglongxia (QLX) sections in the Longmenshan (South China). PZD captures a deep-water basinal record within the Pal Grande Formation, including a ~25cm dark pelitic bed and a continuous seven-zone conodont succession from FZ 13b through the Palmatolepis crepida Zone. The F–F boundary is fixed immediately above the pelite by the FAD of Pa. subperlobata and coincides with a +1.8‰ δ¹³Ccarb rise (peak 3.31‰), a typical Upper Kellwasser positive excursion. Conodont diversity collapses by ~80% at the boundary and recovers stepwise to ~20 taxa in the Pa. crepida Zone. In contrast, the shallow-water QLX A section (Shawozi–Maoba formations) lacks a diagnostic black shale but resolves a clear facies trajectory across the lower FZ 13b–Pa. crepida composite zone: shallowing to shoal-margin packstone (M4; samples QLXA 116–117) records the “linguiformis regression,” followed by deepening to M2 at sample G30, tentatively tied to the Upper Kellwasser transgression, and renewed regressive shoaling above. Against a high shallow-water δ¹³Ccarb baseline (~2.96‰), QLX A shows an anomalous negative shift (min 1.62‰ at G29; r = 0.391), paralleling several South China records. We interpret this as a regional expression of initial volcanic CO₂ degassing preceding the Kellwasser positive peak (Racki–Schobben model), with any subsequent positive overprint muted by facies-controlled baseline effects rather than wholly diagenetic resetting.