The "mother of mass extinctions," the Permian-Triassic, is a critical case. It marks such a vast change in the history of life that, like the K-T, it not only separates two geologic periods, but two great eras— the older Paleozoic and the younger Mesozoic (see Figure 2, page 8). But the Permian-Triassic extinction is much the larger—96 percent of species are estimated to have expired then, compared with 70
percent at the K-T. About 67 percent of reptile and amphibian families disappeared, opening the way for the rise of the dinosaurs; 33 percent of all insects, which usually survive almost anything, disappeared. Prior to the extinction, most marine organisms made their living anchored to the seafloor; those that came after tended to crawl on or to float above the bottom. This led Richard Monastersky of Science News to joke that we owe our modern seafood menu of "lobster bisque, fried calamari, seared tuna, and even sea urchin sushi"5 to the evolutionary path laid open for the ancestors of these delectable creatures (well, most of them anyway) by the Permian-Triassic mass extinction.
If high iridium levels, shocked quartz, or spherules were to be found in Permian-Triassic boundary layers, the case for impact would be greatly strengthened. So far, each has been reported, but in no instance have the reports been confirmed to the satisfaction of even the pro-impactors. Iridium at the Permian-Triassic boundary, for example, appears to be a factor of 10 lower than at the K-T. At the 1996 meeting of the Geological Society of America, Greg Retallack showed photomicrographs of quartz from the Permian-Triassic in Antarctica that he claimed exhibited planar deformation features.6 Specialists such as Glenn Izett and Bruce Bohor, however, were unconvinced.7 Finally, there is no good candidate crater, as Araguinha (see Table 4) appears to be too small.
Douglas Erwin of the National Museum of Natural History of the Smithsonian, in his definitive account of this boundary, The Great Paleozoic Crisis,' lists theories that have been proposed to explain the Permian-Triassic mass extinction. His list is not as long as Jepsen's (page ix), but numbers 14 and includes such familiar K-T suspects as global cooling and flood basalt eruptions. The latter idea received a recent boost in a paper by Paul Renne of the Berkeley Geochronology Center and his colleagues,9 who used the argonargon method to date volcanic rocks of Permian-Triassic age from southern China at 250.0 ± 0.2 million years, exactly the same age his group obtained for the Siberian basalts. They propose that volcanic sulfur emitted during the Siberian eruptions caused a strong pulse of acid rain, as Retallack has argued happened at the K-T. The acid rain, together with increased concentrations of various vol-canogenic poisons, caused the great Permian-Triassic mass extinction. But some geochronologists doubt that Renne's age results are quite as precise as he claims. As we saw in the case of the Deccan intertrappeans, the exact timing of events, though crucial to the argument, is exceedingly hard to pin down. Erwin has written that much of the Siberian traps eruption occurred later, in the Triassic, in which case it was too late to cause the extinction.10
Another group recently suggested that the culprit in the Permian-Triassic mass extinction was carbonated water, a familiar and seemingly innocuous liquid." These researchers propose that in the late Permian, large amounts of carbon dioxide accumulated at the bottom of the sea. Cold seawater near the surface began to sink and displaced the abyssal gas-rich layer, which rose and released its dissolved carbon dioxide. The carbonated water thus produced caused shelled marine organisms to die off. The higher levels of carbon dioxide in the atmosphere then induced a true greenhouse effect, resulting in additional extinctions. Marrying several theories together, the scientists acknowledge that either meteorite impact or the eruption of the Siberian Traps might have triggered the turnover of the oceans and the release of carbon dioxide.
While the carbonated water theory may seem far-fetched, scientists think that something frighteningly similar took place on a smaller scale at Lake Nyos in Cameroon in 1986. One day, from this extraordinarily beautiful blue volcanic crater lake, there burst a cloud of invisible gas so deadly that it instantly killed over 1,000 people and all the cattle and other animals in the vicinity. So few people survived that it has been difficult to find eyewitnesses; the few who have been interviewed say that a fountain of water hundreds of feet high sprang without warning from the center of the lake. The culprit at Lake Nyos is believed to be carbon dioxide that accumulated at the base of the lake and then, for unknown reasons, suddenly erupted.
To account for the great Permian-Triassic mass extinction, Erwin prefers what he calls a "Murder on the Orient Express" theory. In the Agatha Christie story, a man is found murdered on the famous train—his body has not 1 but 12 knife wounds. Christie's masterful sleuth, Hercule Poirot, using only his "little grey cells," deduces that each of 11 passengers, plus the porter, stabbed the man once to avenge a terrible crime committed years earlier. Similarly, Erwin theorizes that the Permian-Triassic extinction was due not to a single cause but to several acting at once.
Early on in this book, I took the position that the principal reason the solution to the mystery of dinosaur extinction was so long in coming is that it had a single cause, impact, that was unknown to geologists until the 1970s. An event that happens once in 65 million years can hardly be the first explanation that comes to mind. But Erwin shows us that a second kind of singularity might exist. He proposes, not a single cause that is exceedingly rare and therefore unfamiliar, but a coincidence of several familiar causes. It is their coincidence at a single moment in geologic time that is rare, perhaps even unique. If Erwin is right, then the greatest mass extinction of them all was not caused by impact. On the other hand, a single confirmed grain of shocked quartz at the Permian-Triassic boundary, which Retallack says he has found, would corroborate the claim that impact had occurred then.
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