LATE DEVONIAN IMPACTS, EVENTS, AND EXTINCTIONS — A PARADIGM FOR PRE-MESOZOIC EARTH HISTORY

Pre-Mesozoic extraterrestrial bombardment of Earth, which included large single impacts, showers of comets and meteorites, and incoming high-energy electromagnetic radiation, was a pivotal mechanism causing extreme environmental changes and driving concurrent mass extinctions within the evolving biosphere. Interdisciplinary research during the past 25 years, however, has produced several caveats: (a) ancient impacts, which may not be evidenced by preserved craters, must be confirmed by strict geological, petrographic, and/or geochemical criteria (i.e., presence of shock metamorphism and/or traces of meteoritic matter); (b) evidence for impacts or other extraterrestrial events must be rigorously and judiciously applied to accurately construct a cause-and-effect link to observed geologic and biotic phenomena, which could also have resulted from Earth-bound processes; and (c) the mode and duration of an observed geologic or biotic event must be consistent with the proposed mechanism, be it extraterrestrial or endogenic in origin. Above all, an integrated, high-resolution data set is crucial to link correctly an observed phenomenon to its causal mechanism.

Late Devonian geologic and biotic events typify patterns and proposed causes documented for other critical pre-Mesozoic episodes. The Late Devonian was a time of major sea-level changes and catastrophic events, some impact related, and two mass extinctions, one probably impact related. The stepwise, late Frasnian (so-called F-F) mass extinction, which was probably initiated by multiple, subcritical impacts, is directly associated with major global cooling that culminated in Southern Hemisphere glaciation during the Famennian. Biotic responses to the F-F mass extinction included marked biodiversity reduction; biomass loss; low-latitude, shallow-marine ecosystem collapse and reorganization; and short-term proliferation of aberrant mutations within both doomed and surviving lineages. Although extraterrestrial forces undoubtedly promoted Late Devonian climate and biotic changes, endogenic factors, such as plate tectonics, volcanism, submarine hydrothermal activity, continental weathering, and nutrient cycling, also played important roles in amplifying environmental fluctuations.