Impact catastrophism versus mass extinctions in retrospective, perspective and prospective: Toward a Phanerozoic impact event stratigraphy

Abstract

Despite several, sometimes prominent propagators, meteorite impact research had a long period of peripheral status until the 1980s. Since then, there has been an intense search for impact-extinction pairs, driven by the rapid acceptance of Alvarez's hypothesis of a catastrophic Chicxulub impact at the end of the Mesozoic era. However, substantial errors have occurred for incompletely identified and/or indirectly dated impact craters in the context of purportedly coeval mass extinctions. For example, supposed giant craters based only on geophysical studies, such as those alleged as evidence of impact-driven end-Permian and Late Ordovician extinctions, are not supported by any real impact evidence (e.g., catastrophic sedimentation) in adjacent areas.

The updated three-step methodology presents an accurate approach to cause-effect inference in impact catastrophism. It begins with (1) conclusive recognition of impact craters and ejecta, followed by (2) their precise radiometric or biostratigraphic dating, and concludes with (3) assessing the impact's “kill” potential. The impact contribution to widely defined mass extinctions has been falsified based on the latest crater information from the global database and the updated ages of stratigraphic boundaries. In the Phanerozoic, two contrasting collision phenomena occurred: the Chicxulub asteroid mega-impact and a prolonged asteroid shower from a shattered chondritic body in the Middle to Late Ordovician. Accordingly, a distinction has been proposed between steady background conditions (impacts occurring singly and rarely in clusters) and perturbation (bombardment) intervals. Current evidence for an impact trigger has been reviewed in detail for the other four Big Five mass extinctions, but no confirmation has been found. The probability of a prolonged impact-enhanced Late Eocene to Early Oligocene crisis, caused by an asteroid shower, is considered, as well as biotic changes accompanying other major cratering events: the mid-Norian Manicouagan and the end-Jurassic Morokweng structures. In particular, for the Popigai asteroid swarm, implied from paired 100-km-sized craters, and the possible Morokweng-Mjølnir coincidence, the relationships between impact signatures and likely stepwise biotic events are far from conclusive. Even if medium-sized bolide impacts, recorded in ∼40-km-diameter craters, may have initiated near-global climatic hazards, the killing effect is unpredictable due to the diversity of cataclysm severity controls. Also the Ordovician cosmic bombardment did not have any negative influence on the great biodiversification. However, the asteroid swarms may have (by unusual dustiness of the inner Solar System) ultimately triggered or accelerated ice ages in the Late Ordovician and Oligocene, respectively. Overall, this implies a continuum in the biosphere's response to extraterrestrial stimuli.

Furthermore, a first attempt was made to explain the hidden record of predicted additional Chicxulub-type mega-events. ‘Lost’ oceanic impacts in the Middle Ordovician, Late Devonian, and Late Triassic were traced in the context of previously suggested records of mega-tsunamis and seismicity. The Frasnian-Famennian transition seems to be the most likely case of such a cryptic cataclysm, manifested in the worldwide “top-Frasnian reworking event.” In summary, of the 18 extinctions, one confirmed impact-induced mass extinction and 3–4 possible impact-enhanced biotic crises can now be considered in terms of extraterrestrial forcing. This tentative conclusion is only superficially consistent with the simplistic assertion in recent literature of four ‘mass extinctions’ associated with the four largest impacts, as much substantial evidence is still needed. In fact, well-documented volcanic cataclysms currently shape the mainstream neocatastrophic geology.

Many proposals, mostly by non-geologists, of periodic causal connections between extraterrestrial factors and biosphere turnovers are shown once more to be totally inconclusive. In this context, the future of actualistic impact catastrophism and Alvarez's ‘bolide theory’ remains open to many fascinating topics. In contrast, ‘non-bolide’ catastrophic concepts, such as the triggering role of intergalactic dark matter, are too questionable to provide real evidence in the fossil record for these ‘invisible’ phenomena.