Ichnofabric Signatures of Palaeozoic Bioevents: A Key to Understanding Life–Environment Interactions from the Cellon Section (Carnic Alps, Austria)

The Ordovician–Devonian interval was punctuated by four major biotic crises (the Silurian Ireviken, Mulde, and Lau events, as well as the Silurian-Devonian Klonk Event) yet their impact on burrowing benthic communities remains poorly constrained. Here, we present an ichnofabric-based record of this critical interval from the Cellon Section (Carnic Alps, Austria), a stratigraphical succession spanning the Upper Ordovician to Lower Devonian. Despite the presence of a few stratigraphical gaps, the section is characterised by robust biostratigraphical control mainly based on conodonts and, to a lesser degree, on graptolites. A total of 160 rock samples were systematically collected throughout the section. These samples were cut, polished, and subsequently analysed for ichnofabrics. Three ichnofabrics (IFs) with distinct bioturbation intensities (BI) have been identified: (1) a high-bioturbation IF, featuring a mottled texture with BI>90%, indicative of stable, oxygenated conditions; (2) Chondrites IF, indicating chemosynthetic adaptation to low-oxygen environments; and (3) a low-bioturbation IF, with BI<5%, linked to anoxic conditions. Low-oxygen proxies (Chondrites and low-bioturbation IFs) are documented in the Ludfordian Cardiola Formation at Cellon and appear in the early phases of the onset of the Lau Event. The observed pattern indicates that the progressive expansion of a low-oxygen zone preceded faunal turnover in the water column. This suggests that hypoxic seafloor conditions generated benthic dead zones, which subsequently affected pelagic ecosystems. Our results support a bottom-up scenario in which the expansion of hypoxic seafloor conditions triggered a progressive collapse of benthic ecosystems, followed by disruptions in pelagic communities, while simultaneously emphasizing the need for targeted methodological approaches to resolve ichnofabric signals within black-shale successions.