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

In estimating the biotic effects of large terrestrial impacts we are greatly reliant upon apparent crater diameter as a proxy for impact magnitude. This underlies the "kill-curve" approach where crater size is graphed directly against likely percentage of lost taxa. However, crater diameter is a complex product of 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 4 of our largest and best documented Phanerozoic impacts to explore this more holistic size-time-place relationship. Only the c.180km end Cretaceous Chicxulub crater (Mexico) links to any substantial immediate extinction and some of the worst effects stem from where it struck the planet (a continental margin carbonate platform site) and when (a time of high regional and global biodiversity). Both the c.100km late Triassic Manicouagan crater in NE Canada (arid continental interior, low regional and world biodiversity) and the c.35Ma 100 km Popigai crater, Siberia (continental arctic desert) provide much less damaging scenarios. However the c.90km Chesapeake Bay crater, Eastern USA (also c.35Ma) marks a far more sensitive site, potentially Chicxulub - like in important respects, but it also proved relatively benign. Here the rheologically varied shallow marine target site generated an anomalously broad crater and the scale of the impact seems to have been overestimated. We offer a new approach to the graphical prediction of biotic risk using 3 dimensions, where both crater diameter and a generalised time/place-related factor we term "vulnerability" are variables.