In June of 1980 Luis W. Alvarez and his colleagues introduced a theory for the cause of the extinction that occurred between the Cretaceous and Tertiary periods. Alvarez proposed that an asteroid estimated at over 100km in width struck the earth off the coast of the modern day Yucatan peninsula. The impact sent up a plume of dust and debris large enough to block out the sun for years resulting in cold dark conditions that ultimately drove the dinosaurs to extinction (Alvarez 1980). Alvarez' original hypothesis has since been expanded to include everything from massive wildfires to global volcanic activity but most subscribers to this theory, otherwise known as catastrophists, agree that some sort of wintry darkness was the cause for the ultimate demise of the dinosaurs. An unusually high level of iridium appears at this time in the geologic record and is typically the biggest piece of evidence used in support of this theory.
The most popular alternative to catastrophism is the gradualistic approach. Few scientists will argue that the remnants of an ancient crater, dubbed the Chicxulub impact basin and hundreds of kilometers wide, exists below the surface of the Gulf of Mexico. But many believe that the impact, while notably large, was by no means the largest or most extraordinary of the time. Gradualists believe that the cause of the demise of the dinosaurs was overcompetition by mammals and gradual climatic changes caused by plate tectonics. The iridium layer commonly used to support catastrophism fits into this theory as evidence for the extreme volcanism that would have been produced by the eruption of Larasia and Gondwana.
Much evidence has been produced in the past decade to support catastrophism; however two basic problems prevent the acceptance of such a theory. The first is a simple consideration of the ecological results of environmental conditions produced by such an impact compared to the fossil record. The blockage of the sun for an extended period would have caused global temperatures to plummet and photosynthesis to all but cease. The cold dark anaerobic environment that catastrophists would like to believe killed only the dinosaurs would probably have wiped out all life on earth within three to five years at most. Stratigraphic resolution of late Cretaceous sediments is limited to around 100,000 years at best (Pough et al 1989). Based on this knowledge it is easy to see that this three to five year period is well below the usual level of resolution and the fossil record should show a distinct break for all species. The actual record shows a gradual decline in species that occurs over several thousand years.
One of the first studies to support this gradual decline was conducted in Montana, Wyoming, and Alberta, Canada (Sloan et al 1986.) This area is one of places where the last dinosaurs are known to have existed and contains a geologic formation known as Hell Creek where researchers sifted for dinosaur and mammalian teeth over a period of ten years. The teeth were then dated using other fossils, geographic magnetic data, carbon dating, and zonal marine shale comparison. Paleocene pollen and seven genera of dinosaurs were identified at the top of the formation located 1.3 meters above the base of the highest possible position of the iridium layer that defines the K/T boundary. The existence of these fossils alone casts serious doubt on the catastrophic theory and points toward gradual decline.
Alvarez himself had previously sampled in the area claiming to have found an abrupt end to all dinosaur species. Further investigation of this data revealed that Alvarez had used only the vertical position of specimens as a guide to abundance and had not sampled horizontally (Sloan et al 1986.).
Consider also the types of species that would be effected by the conditions created under the catastrophic theory. Even if the sun were only partially blocked by a giant cloud of debris and dust, snowfalls would have been extensive. For years the entire globe would have experienced acid rain, severe water pollution, air pollution and lethal airborne toxic chemicals. It is easy to see how most species could not survive under these conditions. Yet in actuality a fair number of reptilian and avian species survived the K/T impact. The survival of bird species is most notable since their high metabolism and rapid action lungs make them more susceptible to toxic conditions (Kerr 1987).
The second problem posed when considering catastrophism is the earth's past impact history. Many collisions of considerable size and periods of supervolcanism occurred early in the Mesozoic era. These events would have produced clouds comparable in size to the one predicted by catastrophists and should have had some effect on the dinosaurs of the time. However a review of these impacts and volcanic episodes revealed that through this period the dinosaurs maintained a relatively high level of diversity (Paul 1989). Gregory Paul, in a 1989 article, compared known impacts and super eruptions of the Mesozoic with radiations of the major pterosaur, dinosaur, and bird groups to see if a pattern could be identified. Paul proposed that these impacts should correlate with extreme diversity and size bottlenecks that would limit life to a handful of small-bodied representatives. Instead he found no evidence of any abnormalities relating to the impacts even when the events were assigned time spans that extended 0.3-3.0 million years. In his conclusion Paul states, "If the impact of a 10 kilometer meteorite or great volcanic eruptions extinguished all the dinosaurs at the K/T boundary, then at least some of the similar events earlier in the dinosaur era should have strongly effected populations of these animals."
Finally consider the island of Krakatoa. This island volcano, located off the coast of Java in Indonesia is the largest recorded terrestrial explosion in recent times. On August 26-27 1883, Krakatoa erupted and continually sent enormous plumes of ash and dust into the sky. It is estimated that around 4 cubic km of material settled into the atmosphere where it stayed for 2 to 3 years (Getis 1995). This eruption was a massive occurrence with environmental impacts that were felt globally however it did not produce mass extinction of any widespread group of organisms.
It seems catastrophism is an unstable theory with serious problems concerning the magnitude of the event and its repercussions. Alvarez did well in providing evidence for a Cretacious\Tertiary impact but left much to be desired when considering its results.
Alvarez, Luis W. Alvarez ,Walter. Asaro,Frank. Michel, Helen V. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1107.
Getis, Arthur. Getis, Judith. Fellman, Jerome D. 1996. Introduction to geography. Wm.C.Brown Publishers Dibuque Ia. 73-76p.
Kerr, R.A. 1987. Searching land and sea for the dinosaur killer. Science 237: 856-857.
Paul, Gregory S. 1989.Giant meteor impacts and great eruptions: Dinosaur killers? Bioscience 39: 162-171.
Pough, Harvey F. Heiser, John B. Mcfarland, William N. 1989. Vertebrate life. Macmillan publishing Co. New York N.Y.
Sloan, Robert E. Rigby, Kieth Jr. Van Valen, Leigh M. Gabriel, Diane. 1986. Gradual dinosaur extinction and simultaneous ungulate radiation in the Hell Creek formation. Science 232:629-633.