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Around 150 confirmed Phanerozoic bolide impact craters with a diameter of greater than 1 km survive on the Earth's surface, of which three are in the 100 km range or greater. Despite the dramatic and extensive immediate effects of impacts, most leave little to no permanent geological record other than a crater, so that links to extinction statistics are commonly speculative. The impact-kill curve generated by Raup (1992) (Fig. 1) used an exponential curve based on crater diameter to predict extinction, and craters in the order of 100 km and above were regarded as purveyors of mass extinction. This was modified by Poag et al. (1997, 2004) who moved the ascent point on the kill curve beyond 100 km (Fig. 1) in recognition of craters in the 90-100 km diameter range (e.g. Manicouagan, Popagai and Chesapeake, Fig. 2), that evidently lack any immediate extinction. Further modification by Kring (2002) took account of possible changes in survival thresholds through time.
Fig. 1.
The Raup 'Kill Curve' with subsequent modifications. Note that Sudbury and Vredfort are Precambrian impacts not further considered in this paper. Manicouagan would plot next to Popigai and Chesapeake Bay.
Fig. 2.
Generalized cross sections of four large terrestrial impact craters. Chicxulub (Hildebrand et al. 1998a,b) (bottom), Manicouagan (Grieve & Head 1983), Popigai (Masaitis et al. 1999) and Chesapeake Bay (Poag et al. 2004) (top).
Direct linkage between crater diameter (energy) and loss of taxa, over-simplistically implies rigid processes and a quantifiable critical threshold below which events should be benign. This approach, however, only allows for one variable - crater size - which although seemingly quantifiable for any given extraterrestrial bolide impact, ignores a plethora of terrestrial variables. Chicxulub, at c. 180 km (Yucatan, Mexico, c. 65 Ma, Fig. 2) and Manicouagan at c. 100 km (Quebec, NE Canada, 214 Ma, Fig. 2) are two of the largest and best-known terrestrial impact craters, and both relate to known distal ejecta layers that confirm their widespread effects (Smit 1999; Walkden et al. 2002). However, whilst of a similar order of magnitude in size, and having struck the Earth at similar latitudes (c. 26-29°N), these two impacts represent opposite extremes in terms of their evident biotic effects....