Earth's most severe mass extinction - an event 250 million years ago that wiped out 90 percent of all marine species and 70 percent of land vertebrates - was triggered by a collision with a comet or asteroid, according to new findings by a team led by a University of Washington scientist.
The collision wasn't directly responsible for the extinction but rather triggered a series of events, such as massive volcanism and changes in ocean oxygen, sea level and climate. Those in turn led to species extinction on a wholesale level, said Luann Becker, UW acting assistant professor of Earth and Space Sciences.
"If the species cannot adjust, they perish. It's a survival-of-the-fittest sort of thing," Becker said. "To knock out 90 percent of organisms, you've got to attack them on more than one front."
Becker; Robert Poreda and Andrew Hunt from the University of Rochester, N.Y.; Ted Bunch of the National Aeronautics and Space Administration's Ames Research Center at Moffett Field, Calif.; and Michael Rampino of New York University and the Goddard Institute of Space Sciences, present their findings in the Feb. 23 edition of the journal Science.Funding for the research came from NASA's Astrobiology and Cosmochemistry programs and the National Science Foundation.
The scientists do not know the site of the impact 250 million years ago, when all Earth's land formed a supercontinent called Pangea. However, the space body left a calling card - a much higher level of complex carbon molecules called buckminsterfullerenes, or Buckyballs, with the noble (or chemically nonreactive) gases helium and argon trapped inside their cage structures. Fullerenes, which contain at least 60 carbon atoms and have a structure resembling a soccer ball or a geodesic dome, are named for Buckminster Fuller, who invented the geodesic dome.
The researchers know these particular Buckyballs are extraterrestrial because the noble gases trapped inside have an unusual ratio of isotopes. For instance, terrestrial helium is mostly helium-4 and contains only a small amount of helium-3, while extraterrestrial helium - the kind found in these fullerenes - is mostly helium-3.
"These things form in carbon stars. That's what's exciting about finding fullerenes as a tracer," Becker said. The extreme temperatures and gas pressures in carbon stars are perhaps the only way extraterrestrial noble gases could be forced inside a fullerene, she said. These gas-laden fullerenes were formed outside the Solar System, and their concentration at the Permian-Triassic boundary means they were delivered by a comet or asteroid.
The researchers estimate the comet or asteroid was 6 to 12 kilometers across, or about the size of the asteroid that left the huge Chicxulub crater near what is now the town of Progresso on Mexico's Yucatan Peninsula 65 million years ago. That impact is believed responsible for the extinction of the dinosaurs. The scientists determined the size based on two factors - if the body were smaller than 6 kilometers the effects wouldn't be seen globally, as they appear to have been; if it were larger than 12 kilometers there would have to be more gas-laden fullerenes distributed globally.
The telltale fullerenes containing helium and argon were extracted from sites in Japan, China and Hungary, where the sedimentary layer at the boundary between the Permian and Triassic periods had been exposed. The evidence was not as strong from the Hungary site, possibly because the sample came from slightly above or below the boundary layer, but the China and Japan samples bear strong evidence, Becker said. Fullerenes are found at very low concentrations above and below the boundary layer, but they are found in unusually high concentrations at the time of the extinction.
Scientists have long known of the mass extinction 250 million years ago, since many fossils below the boundary - such as trilobites, which once numbered more than 15,000 species - diminish sharply close to the boundary and are not found above it. There also is strong evidence suggesting the extinction happened very rapidly, in as few as 8,000 to 100,000 years, which the latest research supports. "That's a microsecond in geologic time," Becker said.Previously it was thought that any asteroid or comet collision would leave strong evidence of the element iridium, the signal found in the sedimentary layer from the time of the dinosaur extinction. Iridium was found at the Permian-Triassic boundary, but not nearly at the concentration found in sediments from the time of the dinosaur extinction. Becker believes that difference might be because the two space bodies that slammed into Earth had different compositions.
While the findings illustrate that impact with large space bodies can be detrimental to life on Earth, Becker noted that there also is evidence they might have been key to life starting here in the first place. Some scientists believe the first life-forming chemicals were deposited on Earth in collisions with comets or asteroids, and some believe comets carried virtually all of the water that exists on the planet today.
Becker also noted that her group's work was made more difficult because there are few 250 million-year-old rocks left on Earth. Most rocks of that age have been recycled through the planet's tectonic processes.
"It took us two years to do this research, to try to narrow it down enough so that we could see this fullerene signature," she said.
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