Illustration by Mondolithic Studio
KILLER COLLISION
Geologist Luann Becker argues that 250 million
years ago an asteroid six miles wide crashed
into the seafloor off the coast of Australia. She
thinks the fallout from the impact provoked
the greatest of the five known mass extinctions.
One of the boldest assertions ever published in the scientific literature started with a single modest observation. In the late 1970s, geologist Walter Alvarez of the University of California at Berkeley and his father Luis, a Nobel Prize–winning physicist, found an unusual chemical signal in an ancient layer of Italian clay. The clay was enriched in iridium, a rare metal that comes mostly from meteorites, interplanetary dust, and other cosmic debris. The iridium spike appeared in sediments 65 million years old, at the so-called K-T boundary between the Cretaceous and the Tertiary periods. It coincided with the demise of the dinosaurs.
Contamination from local sources or a glitch in the iridium-counting machine could have explained the finding. But the Alvarezes found an even bigger spike in another Cretaceous-Tertiary deposit in Denmark. Their interpretation, published in 1980, was heretical.
The clay at the K-T boundary was high in iridium, they said, because it was made of the ash and dust from a six-mile-wide asteroid that had crashed into Earth with the energy of
100 million megatons of TNT. The impact instantly killed every living thing within hundreds of miles. The animals that weren’t incinerated or gassed by fumes froze or starved to death soon after, when dust kicked up by the impact blotted out the sun for more than a year, killing plant life around the globe. Dinosaurs were only the most conspicuous casualty of an epic disaster that eradicated half of all the species on Earth.
“Their idea was met by instant ridicule and derision by most geologists and paleontologists,” recalls paleontologist Michael Benton of the University of Bristol in a recent book. It took another decade of evidence gathering, including the documentation of an impact crater off the Yucatán Peninsula, for the impact theory to win acceptance, he notes. Now “ ‘Extraterrestrial Cause for the Cretaceous-Tertiary Extinction’ is considered . . . one of the most influential publications in earth sciences in the twentieth century,” Benton writes in When Life Nearly Died: The Greatest Mass Extinction of All Time.
The book, however, is not about the Cretaceous-Tertiary impact. And the death of the dinosaurs was not the greatest mass extinction of all time. That superlative belongs to a more severe crisis at the P-T boundary, between the Permian and Triassic periods. Fossil records show that about 250 million years ago, 90 percent of the species on Earth were snuffed out in an abrupt event that spanned the globe. The extinction occurred a couple hundred million years before the dinosaurs died out, so its causes, like its sediments, are buried more deeply. No one has even come close to proving what happened.
But in the past five years, one scientist has dared to implicate a familiar culprit: an asteroid or comet comparable in size and speed to the K-T perpetrator. Geologist Luann Becker of the University of California at Santa Barbara has published a series of papers describing rocks from China, Japan, and Antarctica that have subtle and sometimes unorthodox signs of an impact, including extraterrestrial gases trapped in microscopic carbon cages and minerals deformed by shock waves. Last year, her research team delivered the coup de grâce: evidence of an impact crater off the northwest coast of Australia, hidden beneath two miles of sediment on the ocean floor.
Like the Alvarezes’ theory, Becker’s Permian extinction work has been greeted with hostility. It prompted a vitriolic exchange in the journal Science and a showdown at last December’s annual meeting of the American Geophysical Union. NASA has launched an investigation to explore Becker’s claims, and some of Becker’s peers are second-guessing any findings that fit her interpretation—even if the findings are their own. In January, for example, geologist Peter Ward of the University of Washington in Seattle revised his earlier thesis that the extinction had occurred suddenly, documenting new fossil successions that suggest a more prolonged die-off. Geologist Greg Retallack of the University of Oregon in Eugene is retracting evidence of impact-shocked minerals at the P-T boundary he reported in the late 1990s.
Compared with those researchers, Becker is young and relatively inexperienced, but she cannot be dismissed as a fringe figure. Her academic credentials are impeccable, and she publishes in the country’s most prestigious science journal with experts from top-flight universities as coauthors. Although highly qualified scientists often disagree, some insiders are baffled by the heat of this particular debate. Retallack, for one, still believes that an impact scenario is credible. “I don’t know why people are trashing Luann,” he says.
There could be two reasons: She’s wrong, or she’s right. If she’s wrong, say her detractors, her crusade is drawing focus away from investigators looking at other, more likely scenarios, such as the eruption of hundreds of volcanoes in prehistoric Siberia. “All this putative impact stuff is muddying the waters,” grumbles geophysicist Jay Melosh of the University of Arizona at Tucson.
If she’s right, then a newcomer who wound up studying the Permian extinction and its “putative impact” has bested paleontological stalwarts who have devoted decades to solving the puzzle of mass extinctions. “I’ve been all over the world looking for shocked minerals at the P-T boundary, and I haven’t found any at all,” says geologist Michael Rampino of New York University.
It’s the objection of a seasoned scientist, but it could just as easily be the complaint of a runner-up. Becker could be driving a discovery as profound as any in earth science, or she could be courting career-dashing disgrace. Depending on whom you ask, the cause of the greatest extinction of all time has been either finally identified or hopelessly obscured.
To understand the fuss over Becker’s claims, it helps to know that ideas about the Permian extinction have long been subject to scholarly caprice. Two centuries ago, the very concept of extinction itself was considered scandalous. The great thinkers of the early 1800s only grudgingly acknowledged that the fossils of mastodons, mammoths, and giant ground sloths unearthed in the previous century had no living counterpart remaining on Earth. Then they portrayed extinction as a gradual event. The preeminent British geologist Charles Lyell maintained that iguanodons, ichthyosaurs, and pterodactyls might stage a comeback if hospitable habitats and climates returned. Lyell also came out against any notion of sudden, indiscriminate cataclysm in the history of life. He branded such catastrophism muzzy-headed voodoo science.
The geologists who defined the fossil hallmarks of the Permian in the 1840s must have feared Lyell’s opprobrium, for they failed to mention the signs of mass extinction at the end of that period. It seems unlikely that they simply overlooked it. The Permian extinction obliterated ecosystems as complex as any on Earth today. On land, 10-foot-long saber-toothed reptiles succumbed, and grazing, root-grubbing, and insect-eating lizards vanished, along with the plants and bugs they ate. In the ocean, reefs teeming with life were reduced to bare skeletons. The Permian even finished off the lowly trilobite—perhaps the one celebrity species of the predinosaur era.
Even when geologists finally acknowledged these disappearances in the fossil record, they decided that, while thorough, the Permian die-off had been prolonged. The best estimates had it taking about 10 million years, which doesn’t seem terribly cataclysmic. A lot can slowly go wrong in 10 million years. The climate can grow too hot or too cold; sea levels can rise or fall; the amount of oxygen in the ocean or the atmosphere can change. Most plants and animals are exquisitely sensitive to such shifts, and many might not be able to adapt. But they would die out one by one over millennia, at such a stately pace that a hypothetical human would hardly notice.
Thus Lyell’s gradualism continued to prevail, and catastrophic change stayed taboo for most of the 20th century. That’s one reason the Alvarezes’ K-T impact theory seemed so radical, even in 1980. It invoked the sort of deus ex machina that Lyell had disparaged, and it conjured improbable images of instantaneous apocalypse.
But for once, an idea that plays well in the tabloids also turned out to be true. Emboldened by that example, geologists began to revisit other scenes of carnage in rock beds around the globe. In addition to the Cretaceous and Permian die-offs, they had identified three other episodes of mass extinction in the past 500 million years (a die-off is considered a mass extinction when 50 percent or more of all species are extirpated from the fossil record). Some specialists could not help but hope that a single uncomplicated cause might explain all five of Earth’s great extinctions. “A few years ago, we thought maybe they’re all impacts,” says Rampino. The pendulum Charles Lyell had pulled far to one side swung back just as far to the other. For a few years, it stayed there. Catastrophe became all the rage.
With the paradigm shifting, geologists admitted they could not prove that the Permian extinction had been gradual after all. Evidence to the contrary began to surface. In the early 1990s, geologists examined a rock section in China that bore the critical fossils of the P-T boundary interleaved with ashy volcanic layers suitable for isotopic dating. Called the Meishan section, this felicitous stratigraphy—along with advances in radiometric methods—allowed researchers to time the extinction better than ever before. In 1998 a Chinese and American group headed by geochronologist Sam Bowring of MIT nailed the date of the Permian extinction at 251 million years ago. A carbon signature in the Meishan section suggested the catastrophe had lasted at most 165,000 years. In other words, it had happened two orders of magnitude faster than the 10-million-year textbook estimate.
Armed with the new time line, fossil experts began weighing in. In a 2000 survey of 333 marine species in the Meishan section, paleobiologist Doug Erwin of the National Museum of Natural History in Washington showed that the extinction happened abruptly in the oceans. That same year Peter Ward documented a sudden die-off of vegetation on land in present-day South Africa. Lines of evidence were converging, and figures kept ratcheting downward. The duration of the Permian extinction went from hundreds of thousands of years to tens of thousands and, finally, to just thousands. Although they couldn’t resolve time in terms of days, weeks, or months, many experts came to believe that the whole doomed Permian assemblage—flora, fauna, and foraminifera—might have bought it overnight. The abrupt demise made an impact scenario seem even more plausible. Then Luann Becker came along.
