Living World / Extinction

In 1991 Becker was working toward her Ph.D. at the Scripps Institution of Oceanography in La Jolla, California, when her adviser, Jeffrey Bada, showed her an article about the discovery of a new form of carbon molecule called a fullerene. Fullerenes are hollow, closed lattices shaped like nanoscale soccer balls or geodesic domes (they’re also known as buckyballs, after Buckminster Fuller, the dome’s inventor). They had first been synthesized in the laboratory in 1985, but some scientists thought they might also be made in space, in the furnaces of stars.

If fullerenes are star dust, Bada reasoned, they could be among the cosmic debris that has fallen to Earth more or less constantly since the birth of the planet. The biggest payloads, of course, would arrive via meteorites. But would they survive an impact? Becker—who was planning to be an environmental geologist—got swept up in Bada’s enthusiasm. The two decided to search for fullerenes near known impact craters. They soon found them, in 1993, at an impact site in Canada nearly 2 billion years old. The molecular cages at the so-called Sudbury site might have been forged on Earth from the intense heat and pressure of the impact or in a common forest fire. Yet in their hollow centers the fullerenes held captive helium gas with an unearthly composition that was distinctive of some meteorites and interplanetary dust.

“We were absolutely taken aback,” Becker says. “What was in these little buckyballs was an extraterrestrial signature.”

Becker next succeeded in isolating fullerene molecules directly from meteorites. Encouraged that she had found a new way to trace impact events, she joined with geochemist Robert Poreda of the University of Rochester in New York, who had helped develop the technique to find trapped fullerene gases, to look for buckyballs at the sites of mass extinctions. First they found some at the K-T boundary. Then they found some at the P-T, in rocks from the Meishan section and at another site called Sasayama in Japan. In the first of several controversial papers, Becker and her colleagues reported that the P-T fullerenes contained trapped helium and argon gases with extraterrestrial compositions. The helium content of the Sasayama fullerenes, for example, is more than 50 times higher than background levels.

“Thus, it would appear that [extraterrestrial] fullerenes were delivered to Earth at the P-T [boundary], possibly related to a cometary or asteroidal impact event,” Becker and her colleagues concluded. “Our results are consistent with recent paleontological studies that now point to a very rapid extinction event.”

Becker’s fullerene report received guarded praise. True, the notion of alien gases trapped in microscopic carbon cages for millions and even billions of years strains credulity, especially when you imagine the force of the impact that supposedly delivered them. But by the time Becker’s work appeared, impact geologists were sorely in need of alternative tracers. Their two favorites from the K-T days—iridium spikes and shocked quartz—hadn’t turned up in any incriminating abundance in the rocks associated with other mass extinctions. So, fullerenes from outer space? Why not? “They looked like a possible winner in terms of a signature of an impact,” says Rampino, a coauthor of that first report.

Two years later, Becker and geochemist Asish Basu of the University of Rochester published another paper with still more unconventional evidence for a Permian impact. Becker’s group claimed to have found dozens of actual fragments of meteorites in rocks from the P-T boundary in Antarctica. That evidence is unconventional because meteoritic remains are so easily turned to dust. If they had somehow avoided being incinerated on entry or pulverized on impact, they would have disintegrated in a year of heavy rain—long before geologic processes could fold them into native rock. Less than half a dozen meteorite fragments have been found intact in rock layers the world over.

Becker’s fragments are intact and unweathered, although they are supposed to be a quarter of a billion years old. “The meteorite fragments . . . are so well preserved that their preservation must be due to rather unusual circumstances,” the authors themselves concede. But as far as they are concerned, “the two largest mass extinctions in Earth history at the K-T and P-T boundaries were both caused by catastrophic collisions with chondritic meteoroids.”

Seven months later, Science published Becker’s report of the proposed impact site. This time Becker, Basu, and four other coauthors described a submarine hump called Bedout High that is buried in ocean sediments 100 miles off the northwest coast of Australia. Geologist John Gorter of ENI Australia was surveying for offshore oil there when he spotted the plateau on a seismic profile of the seabed in the late 1990s. Becker hadn’t learned of Gorter’s find until 2002, but when she called him he said he could also get her rock cores drilled from the top and the flank of the structure’s uplifted bull’s-eye. “I got my rear end over there and started looking at those samples,” she says.

In those seafloor samples, Becker’s team reports finding shocked and melted minerals and glass that could be produced only by the intense heat and pressure of a bolide, or meteoric, crash. The researchers dated one of the mineral grains and got a familiar number: 250 million, give or take a few million. They say a gravity model of the site, a kind of topographical map of buried geologic structures, looks much like the gravity model of Chicxulub, the K-T impact structure. Becker and company say the signs of impact at Bedout are compelling enough to warrant further scrutiny. And they are getting it.

Maybe it’s because none of her coauthors are Nobel Prize winners. Maybe it’s because she and her colleagues are the only ones who know how to find a fullerene. Maybe it’s because evidence of an extinction four times older than the K-T is that much more difficult to find and interpret. For whatever reason, Becker’s latest paper—“as spectacular and annoying to some people as the Alvarez paper in the 1980s,” she says—has fared no better than that historical example. Except there’s no sign of eventual acceptance—even from a former coauthor and fan of the impact theory.

“The dates are not unequivocally 250 million years, the shocked minerals don’t look like shocked minerals, and the gravity anomaly doesn’t look like the gravity anomaly you’d get from an impact,” Rampino observes. “There’s no evidence of a crater, let alone a crater of that time period.”

Becker’s critics have aired their grievances in caustic missives to Science. A group led by British sedimentologist Paul Wignall of the University of Leeds writes about examining rocks cored from basin sediments 600 miles south of Bedout. “At no level in the core . . . is there evidence for a layer of impact ejecta or a tsunamite,” the authors contend. Becker’s team responds that Wignall’s core hasn’t even been proved to include material from the Permian-Triassic boundary. Another group headed by geochronologist Paul Renne of the Berkeley Geochronology Center in California notes that the gravity map of Bedout bears no resemblance whatsoever to another confirmed impact site called Vredefort. Becker admits that her gravity signature is a little irregular: In the map’s caption she calls it “significantly reduced and more subdued” than Chicxulub. But she says the reference to Vredefort—which is plainly visible on the surface of the South African desert—just shows how irrational her critics have become: “Comparing a crater that’s exposed at the surface to something that’s been buried under four kilometers of debris? Give me a break. I mean, hello!”

Renne and other investigators also charge that Becker’s supposedly shocked minerals don’t have the telltale patterns of impact-induced features: narrow, parallel bands crisscrossing at various angles, like a microscopic tartan weave. Rampino says Becker’s group offers nothing nearly as persuasive as the shocked minerals found in K-T boundaries across the globe in the years following the Alvarezes’ breakthrough paper. He still remembers the day in 1983 when he saw the first slides of K-T shocked quartz at a meeting. “I went down the hall to see it, and I came back convinced,” he recalls. “Had there been a picture like that in [Becker’s] paper, these questions wouldn’t have come up at all.”

Additional questions surround Becker’s impact glass, which Renne and others believe could be volcanic. In a recent online analysis, earth scientist Andrew Glikson of Australian National University asserts that Bedout is probably just a buried, burned-out volcano. The oil prospectors who originally collected the Bedout cores also assumed that the rocks were volcanic. But everyone thought Chicxulub was a volcanic crater, too, Becker says, until tests done in the late 1980s proved otherwise.

“It’s tectonically and geologically impossible for it to be [a volcanic crater],” she claims. “At the time this thing formed, it was in the middle of a basin that was nowhere near a subduction zone—it was nowhere near the kind of geologic activity that would cause a volcano to form.”

Even the fullerene tracers that were once warmly received have come under attack, because geochemist Ken Farley of Caltech in Pasadena found no helium in P-T rocks from Meishan when he tried to replicate Becker’s work.

“Nobody can reproduce her results,” says Melosh, who maintains that Becker’s means of isolating fullerenes could also be used to synthesize them. “Possibly she’s fooling herself because she’s making the fullerenes she’s detecting.”

To each of these charges, Becker has detailed and spirited retorts. She points out that Farley, for example, did not examine the same Meishan rock samples she did and that he looked for helium in bulk rather than isolating fullerenes first and then looking for gases trapped within them. “We’ve got everybody hounding us because it’s a spectacular claim,” says Becker. “They feel threatened. Why else would they make such absurd statements?”