At this point, it is reasonable to wonder how an extraterrestrial bolide the size of Mount Everest could plow into the planet without leaving an unambiguous trace. The answer is, in part, that 250 million years of heat and pressure can deal its own damage to rocks. The ocean floor, for example, recycles on a tectonic conveyor belt every 200 million years or so, erasing all signs of disturbance. Bedout sits just offshore on a continental shelf; otherwise its features—whether volcanic or extraterrestrial—would be history. Still, even that relatively stable continental crust erodes, subsides, uplifts, and deforms over the millennia, obscuring its original mien.
So scientists are left to reconstruct epochal events from nearly inscrutable remains. Retallack and others, for example, have found an iridium blip at the P-T boundary, but it’s one-tenth as large as the iridium spike reported by the Alvarezes and others at the K-T. That could imply a modest-size meteorite, not big enough to cause a worldwide extinction. But some meteorites contain very little iridium, and comets, which are mostly ice, don’t have any. If an impactor landed in the deep ocean, it wouldn’t create much shocked quartz either, because the ocean floor has less quartz in it than continental crust. If a king-size comet landed in the deep ocean, it would be like stabbing a man with an icicle: a murder with a weapon that vanishes.
There are other suspects. One is the Siberian Traps, a million-year-long volcanic eruption that flooded five time zones of Russia with basalt lava more than a mile deep. Over the past decade, ever more sophisticated dating of the ancient basalt has shown that the lava bed could be about the same age as the extinction, and recent studies have revealed that it covered twice as much area as previously supposed. “Knowing that this province was probably twice as big as we thought has a visceral effect,” says Renne. “We’re staring one of the significant coincidences [of the P-T boundary] right in the face.”
A million years of eruption might release massive clouds of sulfurous gases and carbon dioxide. “It probably wouldn’t have been a lot of fun to breathe,” says Renne. The oxidation of coal beds beneath the magma could release methane as well. The sulfur could produce torrents of acid rain, the carbon dioxide and methane could lead to rapid greenhouse warming, and life on Earth just might not have been worth living for a while.
The Siberian Traps hypothesis has been a favorite among Permian experts when times get tough with the impact theory. There has even been speculation that an impact caused the eruptions. But no one has described a convincing mechanism for an impact-induced eruption, especially not one lasting a million years. And some geologists now question whether the eruptions could have been disastrous enough to account for a global extinction. Basaltic eruptions are mild, like those in Hawaii, not spectacular like the pyrotechnic Mount Saint Helens.
“There are no big explosions,” says Melosh. “It’s very bad if you happen to be right under the lava.” As for exterminating life elsewhere on the planet, “I don’t think it’s enough.”
The one element of the Permian mystery that is certain is that something did indeed claim the lives of 9 out of every 10 species. The fossil signature of the Permian remains the only obvious signal of what went down 250 million years ago, but it, too, resists deciphering. Ward’s latest findings are a case in point: Though his 2000 report on South African plant fossils showed signs of an abrupt extermination at the P-T boundary, his new analysis of animal fossils suggests that a gradual extinction preceded that ultimate burst of fatalities.
“Different organisms have different reactions to different stresses, so if you knew the sequence of mortality you could get a handle on the sequence of events,” says Renne. Unfortunately, current dating methods aren’t precise enough to determine the exact order in which species disappear from the fossil record.
It does seem certain that the extinction was followed by an extraordinarily long recuperation, called a survival interval, of at least 4 million years. During that time, the fossil record shows that a handful of plants and creatures held on for dear life: humble things such as clams, “well adapted to living in lousy environments,” says Erwin. According to Benton, it would be fully 100 million years before the planet recovered the same level of biodiversity it had hosted before the end-Permian crises.
What then follows is oddly reminiscent of Lyell’s dubious notions on the resurrection of extinct forms. Paleontologists have documented a number of plants and animals that disappear at the end of the Permian, stay gone for millions of years, and re-emerge in the middle Triassic. They call these Lazarus species.
It’s difficult to build a convincing theory of mass extinction around such data. “I think there was definitely an impact,” says Retallack, “but I don’t think it caused the extinction necessarily.” Instead, Retallack imagines that the impact released methane stored in the seafloor when it struck. The methane essentially suffocated life. “The actual mode of death would’ve been coughing up a blood-specked frothy sputum,” says Retallack.
Rampino detects a note of desperation in such scenarios. “The search is widening from the standard cast of characters,” he says. “They’re pulling suspects off the street. It would be so much easier if we could just find a big crater in the ground, or a big, smoking volcano.”
But Renne, for one, would not mourn the loss of the impact theory or any other pat explanation for biocatastrophe. “Why should every major extinction have the same cause?” he says. “It would just be too tidy.”
There is nothing tidy about the p-t impact theory as it stands today. “Everybody is waiting for an ending to the story,” says Becker.
Well, not everybody. Some mass-extinction veterans, weary from 15 years of fruitless rock-wrangling at the Permian boundary, are throwing in the trowel.
“I really enjoyed the P-T field in the 1990s,” says Retallack. “Then it was fun. Now it’s gotten to be name-calling and acrimony. I don’t relish the kind of debates that will go on from here on out. I don’t intend to pursue further studies of impact tracers.” He says he would rather study Paleozoic paleosols—really, really old dirt.
Doug Erwin has a book in press that will bid his adieu to the Permian problem. When he wrote his first book on the extinction in 1993, few of his colleagues were interested. Then “everybody who was busy worrying about the K-T extinction got bored and decided to come down to the P-T,” he says. Now it’s just too crowded to be any fun. Rampino agrees: “You have to take a number to get to study any boundary. It’s like going to the grocery store.”
Those players still in the game can look forward to more fireworks later this year, when results from a NASA-funded effort to verify Becker’s findings are due to be released. Last fall, the NASA program sent Erwin, Becker, and Frank Kyte, a geologist at UCLA, to the Meishan section in China, where they could decide, on site and in person, which rocks to analyze and how to divvy them up. The rocks were distributed among several U.S. laboratories for independent testing.
“The expedition to do some definitive sampling is just what we need,” says Melosh.
And so the crisis in end-Permian science will continue a while longer, claiming careers and ravaging reputations. It remains to be seen whether Luann Becker and her putative impact will be survivors, casualties, or one of those mysteriously resurgent Lazarus species.
