- GeoRef, Copyright 2004, American Geological Institute.
A number of natural events can cause ozone depletion, including asteroid and comet impacts, large-scale volcanism involving the stratospheric injection of chlorine, and close cosmic events such as supernovae. These events have previously been postulated to have been sole or contributory causes of mass extinctions. Following such events, UV-B radiation would have been elevated at the surface of the earth. The possibilities for detecting elevated UV-B as a kill mechanism in the fossil record are discussed. In the case of impact events and large-scale volcanism, the taxa affected by increases in UV-B radiation are likely to be similar to those affected by cooling and by the initial drop in irradiance caused by stratospheric dust injection. Thus UV-B may synergistically exacerbate the effects of these other environmental changes and contribute to stress in the biosphere, although UV-B alone is unlikely to cause a mass extinction. By the same token, however, this similarity in affected taxa is likely to make delineating the involvement of UV-B radiation in the fossil record more difficult. Cosmic events such as supernovae may produce smaller extinction events, but ones that are "cleaner" UV catastrophes without the involvement of other environmental changes.