The discovery of a layer of iridium in samples of clay from the K-T boundary in Italy was interpreted by Alvarez et al. (1995) as having been caused by impact with a giant meteorite or bolide. Iridium is a rare element, often associated with extra-terrestrial bodies. It has subsequently been discovered at the K-T boundary in several other parts of the world. The environmental effects postulated by supporters of the bolide impact theory include severe reduction in light intensity due to a cloud of dust, meteorite debris and smoke from the wildfires engendered. In addition, temperatures would have dropped and acid rain fallen. In the original scenario, it was suggested that the earth had been plunged into darkness for 3 years. Toon et al. (1997), however, questioned whether dust from impact with an asteroid could have remained in the atmosphere for so long. Although dramatic sunsets after Krakatoa's eruption lasted for 3 years, Toon thought it probable that the dust would have fallen within 3 months at most, and that the striking sunsets observed after that time must have been due to sulphurous gases from the volcano. Alvarez (1997) therefore revised the period of darkness that he had postulated to one of between 3 and 6 months.
Even so, most biologists would find it difficult to accept the possibility that either planktonic organisms or, indeed, many terrestrial organisms could have survived for so long without light. It has been calculated that the microfauna of modern seas would consume all their food reserves within 10 to 100 days from the beginning of a blackout (Wilford 1985). According to some authorities, impact with a meteorite would have created gigantic tidal waves or tsunami, raised the temperature of the sea, and generated a super-hurricane or hyper-cane which would have blown dust and water high into the stratosphere,black-ing out the sun for several months, reacting with atmospheric gases and wrecking the ozone layer.
The meteorite hypothesis in no way explains the precise mechanism of the extinction, nor its apparently random selectivity. Was it simply the effect of prolonged darkness or was there environmental cooling as a result of the shielding effect of dust in the atmosphere? Was there environmental warming through the greenhouse effect, and did the impact and resulting fireball produce torrents of acid rain? Perhaps several of these factors were combined (Norman 1991). Frankel (1999) attempted to explain the selectivity of the K-T extinction by postulating mass killing of the surface plankton and survival of benthic forms. Partial replacement of flowering plants by pteridophytes might, he suggested, have taken place on land accompanied by the survival of several invertebrate but only of small vertebrate animals.
Evidence for a meteorite impact is, however, boosted by the presence in the K-T layer of shocked quartz grains (in which the crystal structure is disrupted by fracture planes that occur at extremely high pressures as would have been caused by an impact; Archibald 1996). Finally, the presumed impact crater itself has been located off the coast of Yucatan, Mexico, near Chicxulub (Raup
1992; Frankel 1999). Some scientists (mostly with a geological rather than palaeontological background) unquestioningly assume that impact from a bolide must have been the cause of the final Cretaceous extinction. A statistical correlation does not, however, necessarily prove a causal connection (Raup 1989) and most authorities are far more cautious.
The precision of dating events that occurred many millions of years ago is, at best, in the order of many thousands of years. It is, therefore, impossible to determine from palaeontological evidence whether the K-T extinctions took place well before any asteroid impact, contemporaneously with it, or some time afterwards. Moreover, if a bolide were indeed responsible for the Cretaceous extinctions, it is likely that the time interval during which the various species died out would not have exceeded a year or two. It is not possible to be certain, because particular groups of organisms became extinct at the K-T boundary in one place, that they did so everywhere. Sudden disappearance could well be an illusion created by sampling error (McGowan 1991). There is also considerable evidence that the dinosaurs died out at different times in different places, sometimes fossils post-date the event that produced the iridium anomaly (Charig 1989). It is ironical that, of all the groups that became extinct, the Dinosauria is probably the one with the poorest fossil record!
A massive extraterrestrial disaster would certainly have made conditions on earth even more unfavourable to the dinosaurs and other large reptiles that were already declining at that time. Extensive volcanic activity might, however, have been responsible for much of the iridium anomaly (McGowan 1991): alternatively, a change in the sedimentation rate of seawater could have engendered an abnormal concentration of iridium. In any case, the Chicxulub bolide could well have been considerably smaller than of a 6-mile diameter as estimated by Alvarez and his colleagues in 1980 (see discussion in Frankel 1999).
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