First discovery of the Frasnian–Famennian (F–F) boundary with black shale deposition in the Carnic Alps, Italy

Abstract

The Frasnian–Famennian (F–F) boundary mass extinction, associated with the Kellwasser events, triggered one of the most severe biodiversity crises of the Phanerozoic. The Plan di Zermula D (PZD) section in the Carnic Alps of northeastern Italy provides the first record of a deposition of black shale level at the boundary in this region. The rich conodont association allows to identify seven conodont zones from FZ 13b to the Palmatolepis crepida Zone, yielding a high resolution biostratigraphic framework for the F–F interval. Conodont diversity dropped abruptly by about 80% at the boundary, with only a few taxa surviving. Post-crisis recovery was delayed and gradual, with diversity remaining low in the earliest Famennian and rising significantly in the Palmatolepis crepida Zone. Comparison with nearby sections lacking the black shale layer at the boundary confirms the peculiar depositional conditions in the Carnic Alps, which could explain the slow recovery in the PZD section compared to other areas.