Ichnological response of macrobenthic fauna to Late Devonian anoxic pulses: new evidence from North America and a global synthesis

Abstract

During the Late Devonian, several anoxic pulses triggered severe biotic crises such as the Kellwasser Event, profoundly affecting marine ecosystems. These episodes are well-documented in the North American Seaway (NAS), in which accumulated thick deposits of dark-gray/black shales generally lacking biogenic structures. This study applies an ichnological approach to assessing the impact of anoxic pulses on macrobenthic tracemakers at five sites in the NAS (one from the Anadarko Basin, three from the Illinois Basin, and one from the Appalachian Basin). The study sections consist of dark-gray to black shale beds interbedded with light-gray to greenish shale layers. Ichnological analysis identified five ichnoassemblages dominated by: (1) Planolites, (2) Chondrites, (3) Planolites, Teichichnus, and Cylindrichnus, (4) Zoophycos, Chondrites, and Planolites, and (5) vertical spreiten-bearing structures along with Planolites, Teichichnus, and Cylindrichnus. In the NAS, Givetian deposits exhibit the highest ichnodiversity (Ichnoassemblage 5). Stratigraphically upwards, several anoxic pulses are recorded by laminated black shale deposits, correlated with globally-recorded biological crises (e.g., Rhinestreet, Kellwasser, Dasberg): lower Frasnian sediments exhibit alternating unbioturbated/laminated intervals and Ichnoassemblages 1a and 1b along with mottled textures, whereas the Famennian is characterized by more persistent anoxia interrupted by oxic intervals with Ichnoassemblage 4 or 1 and common mottled textures. In contrast, the Anadarko Basin shows only mottled textures alternating with laminated facies in the Frasnian-Famennian transition, followed by a persistent decline in biogenic structures during the Famennian (only sparse records of Ichnoassemblages 1a and 2). Notably, in inner areas of the NAS, oxic conditions were prevalent during the Upper Kellwasser Event, characterized by Ichnoassemblage 3. Our ichnological study, compared with the global record, indicates that Late Devonian anoxic pulses had a greater impact on open-marine settings than on the restricted areas of the NAS, highlighting that regional features heavily conditioned global effects of the Late Devonian biocrises, leading to highly variable patterns in benthic recovery and oxygenation dynamics.