O Ther C Ycles

Rampino and Stothers have gone on to argue for a 32 ± 3 million-year periodicity not only in mass extinctions and impact cratering, but in a variety of other major geologic processes: flood basalt eruptions, magnetic reversals, appearance of oxygen-poor oceans, large changes in sea level, and episodes of seafloor spreading. Later, Ram-pino and Bruce Haggerty went on to develop what they call their Shiva (Siva) hypothesis.22 If they are correct, a single cause is likely to drive most or all of the earth's large-scale processes. Dare I say it? If Rampino and colleagues are right, as shown in Figure 27, they are on the trail of a grand unified theory of earth systems!

They imagine the cycle starting with the impact of an asteroid or comet, say the one that forms the 50-million-year crater, which has a diameter of 100 km. The asteroid that produced it would penetrate at least 20 km into the earth. The nearly instantaneous evacuation of a large section of the crust would relieve the pressure on the underlying mantle, causing it to melt and giving rise to floods of basalt, which would then erupt at the surface, possibly hiding the parent crater. The shock of impact would upset the magnetic dynamo in the earth's core, causing it to reverse. The ocean floor would rift and spread; sea level would fall. The poisonous effects of the gases emitted during flood basalt eruptions, added to those of impact, would cause a mass extinction and thus neatly tie the whole package together.

This takes us far out on the slenderest branch yet. One can probably count on one's digits the number of geologists who believe

FIGURE 2 7 A grand unified theory of earth systems? [After Stothers and Rampino.23]

Rampino and Stothers are onto something. Yet recall that the K-T boundary is not the only one to have a flood basalt of nearly, if not exactly, the same age: so does the Permian-Triassic. Its age matches closely, some say identically, that of the Siberian traps. Rampino, Stothers, and most recently, French geologist Vincent Courtillot, have searched the literature for other examples of geologic boundaries with flood basalts of similar age, and have found many.24 On the other hand, just as some craters appear to have no associated extinction, neither do some flood basalt eruptions. The boundary that we know best, the K-T, formed 1 million or 2 million years after the Deccan flood basalt volcanism began. The age of the Per-mian-Triassic boundary is not as well dated as the K-T, and it remains to be seen whether the claimed correspondence between the age of the Permian-Triassic and that of the Siberian traps will hold up.

p ERIODICITY a SSESSED

Writing in 1989 and basing his remarks on his latest compilation of the extinction rate for genera, Sepkoski concluded that 9 of 11 extinction peaks lie on or close to the 26-million-year periodicity. The probability of this happening by chance is less than one in a million.25 The periodicity of cratering is far less firm, due to the much smaller sample size, but it has not been falsified. Yet in spite of the evidence for extinction periodicity, and its importance if true, interest seems to have waned. One of the reasons is that periodicity falls deep in the cracks between disciplines—it is not really the province of geologists, or paleontologists, or astronomers, or statisticians, or anyone—it is a scientific orphan in a world of limited time, scarce resources, and orthodoxy. Even the pro-impactors need not endorse it—impact could be of great importance in earth history and not be periodic.

But there is clearly another reason why periodicity has failed to continue to excite scientists: No one has been able to find Nemesis, or Planet X, or any logical reason as to why extinction, and possibly cratering, should be periodic. We know from the history of science that we should not reject what seem to be sound observations just because we cannot account for them, yet it is human nature to do so. Recently, however, an intriguing explanation has come to light that may mean that a plausible source mechanism is available.

Astronomers have been studying the effect of passage of the Oort cloud—that vast reservoir of comets out beyond the solar system—through the disk of the Galaxy, and have found previously unrecognized gravitational effects that could cause comets to strike periodically and that would repeat every 30 million to 35 million years.26'" Previously, Shoemaker was on record as thinking that cra-tering periodicity was a statistical fluke, but this new work made a believer out of him: "Impact surges are real . . . and [the comet flux is] controlled by the fluctuating galactic tides," he said. The new work "is a landmark contribution in understanding the history of bombardment of the earth."28 Add this to the opinion of paleontologist Douglas Erwin that "The periodic signal continues to shine through the turmoil, battered but resilient," and we can see that periodicity of both the fossil record and terrestrial cratering are hypotheses that are alive and well.29

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