The Frasnian–Famennian biotic crisis: How many (if any) bolide impacts?

The prime causation of the mid-Late Devonian mass extinction near the Frasnian-Famennian (F-F) boundary remains uncertain. Nevertheless, geochemical evidence has been presented recently as decisive evidence of a giant bolide impact occurring precisely at the F-F boundary, which promoted the global mortality episode. Palaeobiological data, however, imply a gradual global change, which is otherwise seen as a record of either multiple extraterrestrial catastrophes or of impact-triggered Earth-bound mechanisms. Sedimentological (mega-tsunami), physical (craters, microtektites), and geochemical records remain either elusive in many aspects, or incompatible with the predicted impact crisis pattern. Biotic succession across the F-F horizon is still poorly known, especially in continental domains, to evidence a synchronous ("bedding-plane") killing event at the close of the crisis. Instead, the commonly documented stepwise loss of biomass and an unproved distinctive "dead zone" are hard to explain simply as sampling artifacts. The assumed mass mortality precisely at the F-F boundary may be limited mainly to the pelagic realm. The underestimated role of early Variscan tectonism and associated volcanic-hydrothermal processes, resulting in thermal and nutrient pulses, as possible prime controls of the F-F crisis is suggested, as well as resemblances to the superplume-conditioned eventful mid-Cretaceous interval, exemplified in the Cenomanian-Turonian mass extinction. Additional shocks, generated by minor cometary strikes, are not excluded but may have affected some F-F biotas or areas.[PUBLICATION ABSTRACT]