The biotic effects of large bolide impacts: size versus time and place

摘要

In estimating the biotic effects of large terrestrial impacts we are reliant upon apparent craterdiameter as a proxy for impact magnitude. This underlies the ' kill-curve' approach which graphs craterdiameter directly against likely percentage losses of taxa. However, crater diameter is a complex productof syn- and post-impact processes that can be site-dependent. Furthermore, location (global positioning)and timing (moment in geological history) also strongly influence biotic effects. We examine four of ourlargest and best-documented Phanerozoic impacts to explore this more holistic size—time—placerelationship. Only the c. 180 km end-Cretaceous Chicxulub crater (Mexico) links to any substantialimmediate extinction and some of the worst effects stem from where it struck the planet (a continentalmargin carbonate platform site) and when (a time of high regional and global biodiversity). Both thec. 100 km late Triassic Manicouagan crater in NE Canada (arid continental interior, low regional andworld biodiversity) and the c. 35 Ma 100 km Popigai crater, Siberia (continental arctic desert) providemuch less damaging scenarios. However the c. 90 km Chesapeake Bay crater, Eastern USA (also c. 35 Ma)marks a far more sensitive (Chicxulub-like) site but it also proved relatively benign. Here therheologically varied shallow marine target site produced an anomalously broad crater, and the scale ofthe impact has evidently been overestimated. We offer a new approach to the graphical predictionof biotic risk in which both crater diameter and a generalised time/place factor we term `vulnerability'are variables.