Figure 1. Poag's (1997) modifications of the Raup (1991) "kill curve" showing that only impacts producing craters much greater than 100 km in diameter are capable of producing mass extinction. (From Poag, 1997.)

The overstatements of the impact advocates have prompted a detailed study of the impact and extinction record. Alroy (2002) and Prothero (2004b) both showed that there was no correlation between any of the many well-documented Cenozoic impacts and the extinction events of fossil mammals. The end-Triassic extinctions were once touted as impact caused (Olsen et al., 1987, 2002a, b). But the iridium anomaly near the Triassic-Jurassic boundary (TJB) is only 285 ppt, compared to the 6,300 ppt at the K/T boundary at Gubbio. In addition, the supposed culprit, Manicuoagan Crater in Canada, has turned out to be 14 million years too old (Keller, 2005). Instead, the recent work on the Triassic-Jurassic boundary has shown that the extinctions were spaced out over many different pulses spanning the later Triassic. The apparent "mass extinction" at the end of the Triassic was actually an artifact of compiling large low-resolution databases, where every last occurrence of a fossil is treated as occurring at the end of the binned interval, when in actuality their last occurrences were spaced throughout the interval (Tanner et al., 2003; Lucas and Tanner, 2004). As Lucas and Tanner (2004, 37) wrote: "two hundred years of fossil collecting failed to document a global mass extinction at the TJB, yet another 20 years of literature compilation and the 'compiled correlation effect' did."

In recent years, attention has shifted away from the K/T event to the "mother of all mass extinctions," the Permian-Triassic event, in which 95% of marine species may have died out (Hallam and Wignall, 1997; Benton, 2005; Erwin, 2006; Ward, 2006a, b). This event was so profound that the seafloor communities were completely reorganized, from the crinoid-brachiopod-tabulate-rugosid-bryozoan-dominated "Paleozoic fauna" to the mollusk-echinoid dominated "Modern fauna" that has prevailed since the Triassic (Sepkoski, 1981). Once the abruptness of the Permian extinction became apparent (Bowring et al., 1998; Jin et al., 2000), it was natural that someone would try to find evidence of impacts. Becker et al. (2001) claimed to find evidence of fullerenes at the Permo-Triassic boundary, but this evidence has been disputed (Farley and Mukhopad-hyay, 2001; Braun et al., 2001), as have other claims for evidence of a Permo-Triassic impact (Koeberl et al., 2004; Zhou and Kyte, 1988). The supposed Bedout impact crater in Australia (Becker et al., 2004) has not proven to be the right age or size or have the right kind of shocked quartz grains (Glikson, 2004; Wignall et al., 2004), nor have the alleged shocked quartz grains in Antarctica (Retallack et al., 1998) convinced the scientific community.

Finally, the Devonian extinctions have sometimes been blamed on impacts as well (McGhee, 1996, 2001), with impact debris reported from a number of Late Devonian sections. Two of the largest Late Devonian craters (Alamo and Woodleigh) reach 44-65 km in diameter (Glikson et al., 2005), yet the Alamo impact is at least 5 million years older than the late Frasnian extinction, and the Woodleigh impacts occurred in the middle of the Famennian, so no impact crater has dates that coincide with the major extinctions at the end of the Frasnian or the end of the Famennian (Keller, 2005). However, the Siljan crater is dated at 377 ± 2 Ma (Reimhold et al., 2005), close to the Frasnian-Famennian boundary of 376 Ma, so it potentially coincident with this extinction event. Unfortunately, the crater diameter is only 65-76 km, too small to be the principal "killer" in the Late Devonian extinction.

In summary, most of the recent evidence has shown loud and clear that impacts do not cause most major mass extinctions. In fact, there is no impact crater associated with any mass extinction except the K/T boundary, and even that is not the sole factor involved (the Deccan traps and global regression occurred at the same time as the impact). In contrast to the views voiced by McLaren in 1989 and Raup in 1991 cited above, impacts seem to have little to do with mass extinctions. The earth and its biosphere are apparently more resilient to the effects of an impact that was once widely believed.

0 0

Post a comment