PALEONTOLOGY:
Whiff of Gas Points to Impact Mass Extinction

Richard A. Kerr


 Two hundred fifty-one million years ago, as the Permian period gave way to the Triassic, Earth experienced its greatest mass extinction ever. Ninety percent of all marine species, including the last of the trilobites, disappeared, while on land pervasive extinctions opened the way for the rise of the dinosaurs. But despite the magnitude of this "mother of all mass extinctions," its cause has remained mysterious.

 A new analysis of rock that marks the Permian-Triassic (P-T) extinction now suggests that it was caused by the hypervelocity impact of an asteroid or comet similar to the one thought to have killed off the dinosaurs 65 million years ago. The evidence that some catastrophe triggered the P-T extinction has been building for the last 5 years. Although it was once thought to have lasted for 8 million years, it now appears to have occurred in a geological heartbeat--perhaps even instantaneously. So sudden does the extinction now appear, in fact, that many paleontologists presume it had a single, abrupt cause--a mega-volcanic eruption, a catastrophic release of toxic chemicals from the ocean's depths, or an impact. But no one had been able to implicate such a catastrophe by placing it at the geologic moment of extinction.

 That's where the new work comes in. On page 1530, geochemists Luann Becker of the University of Washington, Seattle, Robert Poreda of the University of Rochester in New York, and their colleagues report that they have detected the noble gases helium and argon apparently trapped in the molecular cages of carbon "buckyballs," or fullerenes, extracted from rock laid down at the P-T extinction. Analysis of these gases shows, the researchers say, that their isotopic compositions are much more like those found in meteorites than on Earth. Thus, they conclude that a giant impactor delivered the chemicals to Earth just when the extinctions occurred.

 Some researchers find the argon and helium analyses persuasive. "It's the noble gases that make the case" for an impact, says physicist Robert Pepin of the University of Minnesota, Twin Cities, who works on noble gases in meteorites. Still, claims of finding buckyballs--closed lattices made of nothing but 60 or more carbon atoms--in natural samples such as impact debris and meteorites have been controversial. Indeed, the suggestion that they provide a marker for a P-T impact recalls the early days of the controversy over the impact at the Cretaceous-Tertiary (K-T) boundary, 65 million years ago.



Figure 1
Bird in a cage. The carbon lattices of molecular buckyballs can trap gases, some of which suggest a mass extinction by impact 251 million years ago.

CREDIT: LUANN BECKER/UNIV. OF WASHINGTON



The first clue to the K-T impact, discovered in 1979, was an abundance of the element iridium at the geologic instant of the mass extinction. Because iridium is plentiful in meteorites, the iridium-rich deposit suggested impact debris, but some researchers argued that the layer could instead have been produced by the iridium-rich exhalations of volcanoes.

 Fullerenes are also proving to be a suggestive but unconvincing impact marker. Previous work by others showed that they are present in rock at the K-T boundary, a finding confirmed by Becker and her colleagues, who also detected them in two meteorites. Together these findings suggested that fullerenes are impact markers like iridium. That prompted Becker and her colleagues to look for the compounds in rock at the P-T boundary at the classic site at Meishan, South China, and at Sasayama in southwest Japan. The researchers did in fact detect fullerenes in boundary rock, but not in similar rock a few centimeters to meters above or below the boundary. But fullerenes can have more mundane sources than meteorites. They are produced by forest fires and even by the mass spectrometers used to separate and identify them.

 In the case of the K-T mass extinction, the clincher was the discovery of shocked quartz, distinctively veined crystals made only in the extreme pressures of large, hypervelocity impacts. Shocked quartz has not been confidently identified at the P-T, but noble gases may yet serve to make the case. Because of their structure, fullerenes can trap gas atoms like birds in a cage. When Becker and her colleagues then analyzed the gases trapped in fullerenes from P-T-boundary rocks, they found that the abundance of helium-3 jumped 50-fold above what it was above or below the boundary. The ratio of helium-3 to helium-4 entrapped there was typical of that found in meteorites--not in earthly atmosphere and rock. And the ratio of argon-40 to argon-36 in boundary fullerenes is well below that of air and approaches that of meteorites. The recovery of such fullerene-encapsulated gases, says Becker, is "the best case for an extraterrestrial event coincident with the P-T extinction." And she adds, "it was likely the trigger."

 Researchers who study fullerenes aren't so sure. "The [fullerene] work of Luann Becker and colleagues has been a bit controversial," notes microscopist Peter Harris of the University of Reading, U.K. "Some people have found it hard to accept that fullerenes can survive for billions of years." Although Becker claims to have detected fullerenes in two other impact deposits and in two meteorites, he notes, only the K-T fullerenes have been found by an independent group, despite a number of searches. Still, Harris has recently reported that transmission electron microscopy reveals what look like fullerene molecules in a meteorite, so he is "fairly convinced" that the fullerenes and noble gases mark an impact at the P-T. But "I'm perhaps in a minority," he says.

 Among noble gas workers, the reception has been warmer, however. Geochemist Kenneth Farley of the California Institute of Technology in Pasadena calls the anomalously low ratio of argon-40 to argon-36 "astounding. I can't imagine how you could have any other interpretation" than an impacting meteorite that carried in the noble gases. "There appears to be an extraterrestrial component in the [P-T] boundary layer," agrees Pepin. "I think they've demonstrated that rather convincingly." Still, even noble gas workers want to see more. "This result needs to be replicated by somebody else," says Farley, "as any such measurement does."

Volume 291, Number 5508, Issue of 23 Feb 2001, pp. 1469-1470. 
Copyright © 2001 by The American Association for the Advancement of Science.