Catastrophes!: Earthquakes, Tsunamis, Tornadoes, and Other Earth-Shattering Disasters

11 Mass Extinctions: When Life Nearly Died

11 Mass Extinctions When Life Nearly Died Mass extinction is box office, a darling of the popular press, the subject of cover stories and television documentaries, many books, even a rock song . . . At the end of 1989, the Associated Press designated mass extinction as one of the “Top 10 Scientific Advances of the Decade.” Everybody has weighed in, from the economist to National Geographic. —David Raup, 1991 For every problem, there is a solution that is simple, neat and wrong. —H. L. Mencken Catastrophist Bandwagons In 1980, a scientific paper hit the professions of geology and paleontology like a blazing comet. After more than a century of speculating about how and why the dinosaurs had vanished, and why other great mass extinctions occurred, the authors had proposed a novel solution. In their scenario, about 65 million years ago, an asteroid 10 km in diameter had slammed into the earth and caused a global “nuclear winter” of cold and dark conditions that decimated Mass Extinctions 269 life. About 50 percent of the earth’s species had vanished, including dinosaurs, marine reptiles, pterosaurs, and many marine organisms, especially among the plankton and the Nautilus-like ammonites. The paper provided a simple and neat solution to a complex problem, but this was not how the research started. Ironically, the authors were looking for something else altogether and found the evidence of an impact by accident. In the late 1970s, a young geologist named Walter Alvarez was busy working on the geology of Italy, especially the Apennine Mountains that run down the spine of the peninsula. (I knew Walter when he was a postdoctoral student at Lamont-Doherty Geological Observatory of Columbia University and I was a graduate student.) Although Walter was primarily concerned with the way in which the mountain belt had grown and been deformed, he was aware that in some outcrops along a highway near Gubbio, Italy, there was an amazingly complete sequence of limestones that spanned the end of the Cretaceous Period (the final period of the Mesozoic or “age of dinosaurs”) and the first few million years of the Tertiary Period (“the age of mammals”). Sandwiched between the limestones at this boundary was a thin layer of clay that represented the time when the extinction took place (fig. 11.1). Walter took a sample of this Gubbio boundary rock home to his father, Fig. 11.1. The Gubbio boundary layer (dark band with the coin on it, just above the light-colored limestone). (Photo courtesy A. Montanari) 270 Catastrophes! Nobel Prize–winning physicist Luis Alvarez, at the University of California, Berkeley. They both were interested in finding a technique that would show how long it took the clay layer to form and, therefore, how rapid the mass extinction event had been. They looked for rare elements found primarily in extraterrestrial matter as a means of detecting how much cosmic dust had accumulated in the sample. If there was a lot of cosmic dust, the sample had accumulated slowly, but if there was very little, the time interval would be very short. They used the rare platinum-group element iridium as their tracer of cosmic dust and gave the samples to nuclear chemists Frank Asaro and Helen Michel to analyze. When the results came back, they were all stunned. The total amount of iridium (the iridium “anomaly”) was much larger than any of them expected and could not be just the product of a simple rain of cosmic dust. Eventually, they concluded that it could have been produced by the impact of an extraterrestrial body, presumably an asteroid, which blasted a huge amount of crustal material into space upon landing and caused a “nuclear winter” of cold and darkness that killed off plants on land and algae in the ocean and so on up the food chain. Naturally, the geological community immediately challenged such a bold and provocative hypothesis. All ideas in science must undergo the crucible of testing by other scientists and by peer review. At first, scientists thought it might be an artifact of the well-known and peculiar ability of clays to concentrate all sorts of rare materials or an artifact of unusual ocean chemistry, but those ideas were ruled out when the iridium anomaly was found in land sections . As the 1980s progressed, iridium anomalies were found at the boundary around the world, both in deep-sea cores and on marine sections that had...