Of the seven major geologic boundaries (the Big Five, the Eocene-Oligocene, and the Jurassic-Cretaceous), only the Ordovician-Silurian lacks a sizable impact crater of approximately the same age, though in some instances the age fit is not good. Five of the six boundaries that are 200 million years old or older have craters of nearly the same age, which is remarkable considering that most craters that old have been obscured or removed by erosion. Shocked minerals or microtektites have been reported at six of the seven boundaries, though the claim for the Permian-Triassic is new and controversial. All seven have iridium concentrations that appear to be higher than background, though often by only a little.
While many of these individual pieces of evidence are weak, taken as a set they are impressive. Not enough firm evidence is available to corroborate the claim that impact is responsible for any other mass extinction boundary than the K-T, yet more than enough exists to justify a continuing investigation, providing an opportunity for earth scientists.
d ID i MPACT c AUSE ALL e XTINCTIONS?
David Raup, always trying to see the big picture, could not restrain himself from asking a question that even he had to admit was rash and seemingly ridiculous: Could all extinctions of significant numbers of species—not just the major mass extinctions—have been caused by impact?16 Since there are so many geologic boundaries to investigate, this impertinent question cannot be answered by going into the field to examine rocks and collect fossils, but perhaps it can be answered in theory. Naturally, where angels and other geologists would fear to tread, Raup rushed in. And his distinguished record shows him to be no fool.
One way to describe events spaced out in time is to refer to the mean waiting time between those events of a given size, as we do for the 100-year flood and the 100-year wildfire. On average, a flood the size of the 100-year flood shows up every 100 years; in practice two can occur in successive years, or none can appear for several hundred years. Raup, who calls a spade a spade, used Sepkoski's compilation of the extinction records of genera, and the concept of waiting time, to build what he called a "kill curve" (Figure 2 3).'7■" It shows how much time passes on the average between extinction events of various sizes: the 1-million-year extinction, the 10-million-year extinction, the 100-million-year extinction, and so on.
Extinctions that destroy 5 percent of species occur about every million years. Interestingly, this is the approximate length of the "biostratigraphic zone," the minimum unit of geologic time that paleontologists can detect using specific assemblages of fossils. An extinction the size of the K-T has a waiting time of about 100 million years, whereas one the magnitude of the more lethal Permian-Triassic occurs at intervals of 1,000 million or even 10,000 million years. Since life has never been completely exterminated, Raup assumes that the curve must level off to form an "S," never reaching 100 percent killed, no matter how long the waiting time.
Raup next turns to Shoemaker's observations of comets and asteroids, which allow an estimate of how frequently craters of different sizes form (Table 5).20 Figure 23 and Table 5 use two completely independent sets of data, one obtained from the record in the rocks, the other derived from searching the heavens. The first relates waiting time to percent species killed; the second relates waiting time to crater size. We could write an equation that would plot out each graph, and from high school algebra we know that we could eliminate the common variable from the two equations, waiting time, and relate percent species killed directly to crater size. In Figure 24, Raup has done so. (The curve is dashed above the 150-km-crater diameter because that is as far as Shoemaker's estimate went. Since these estimates inevitably have large associated errors, the dashed upper and
FIGURE 23 Raup's kill curve, showing the average time spacing between extinction events of different intensity. The Big Five extinctions (with the possible exception of the Permian-Triassic at 96 percent), are 100-million-year events. [After Raup.19]
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