What causes mass extinctions? Several putative causes provide a choice of catastrophes (Table 7.1). The chief among these are bolide impact, volcanism, methane hydrate release, climate change, marine regression, and anoxic events with or without marine transgression. Other processes might lead to widespread extinctions, but possibly not to extinctions on a global scale. It is feasible that a single factor, such as periodic bolide impacts, accounts for mass extinctions. It is equally feasible that each mass extinction event has a different cause or causes (for the contributory processes may act in tandem). Anthony Hallam and Paul Wignall's (1997) review of the evidence points to the latter interpretation (Table 7.2), and it is probably futile to seek a single cause covering all mass extinction events. Admittedly, all mass extinctions do have some features in common; for instance, in the marine realm they usually occur at times of reduced ecosystem productivity (e.g. Paul and Mitchell 1994), and presumably this would be the case in terrestrial mass extinctions, too. Reduced productivity suggests a general deterioration of the environment during mass extinctions. However, this simply begs the question of what causes the environmental to deteriorate. The list of possibilities is long (Table 7.1). Take the case of the end-Permian event, which
0.70.60.50.126.96.36.199 -00-0.5-1.0-1.5-2.0-2.5-3.0-188.8.131.52.184.108.40.206.1 -o-
End-Permian (Guadalupian and Djhulfian)
End-Ordovician (Late Ashgillian)
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