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IMPACTS AND MASS EXTINCTIONS REVISITED

¹ Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California, 92182-1020, USA

The first 300 words of the full text of this article appear below.

On several occasions at impact geology- and paleontology-related meetings during the past 10 years, I have heard the statement from colleagues that major extinctions must be linked to large asteroid or comet impact events because "what else could it be?" (i.e., what other known mechanisms, terrestrial or extraterrestrial, could drive the geologically rapid loss of a majority of Earth's species?) Although I am strongly pro-impact in my view of Earth history, I think that it is time to reappraise our state of knowledge within impact geology and mass-extinction research, and to examine the widely postulated link between impacts and mass extinctions more critically. It has been over 25 years since the landmark study of Alvarez et al. (1980) launched the global-scale search by the geological and paleontological communities for quantitative, testable evidence tying impact to extinction. Where then do we stand today? In this SPOTLIGHT, I highlight some recent advances in impact geology that have direct bearing on our ability to recognize the signature of ancient impact events and on our ability to link these events to mass killings.

The continuing challenge (and frustration!) facing any hypothesized link between impact and extinction is well illustrated in Figure 1. Major biotope events documented in the fossil record may have been driven by a number of very different, potentially interacting causes, both telluric (terrestrial) and cosmic (extraterrestrial) in ultimate origin. These could lead via very different pathways to results that are, at least superficially, very similar. Of the proposed cosmic mechanisms responsible potentially for driving biotic crises, large-body hypervelocity impact remains the most important and the most readily testable in the geological record.

View larger version (41K): [in this window] [in a new window]   FIGURE 1—Flow chart showing the possible driving mechanisms and complex interactions that could lead ultimately to a global bioevent (after Walliser, 1996)