Meteor Blamed for Earth's Greatest Extinction Event

SANTA BARBARA, California, December 1, 2003 (ENS) - The Earth's greatest number of extinctions appear to have been caused by the impact of an enormous meteor some 251 million years ago, says a research team working in Antarctica.

The theory, published by the team in the November 21 issue of the journal "Science," holds that this extinction event occurred much earlier than the disappearance of the dinosaurs approximately 65 million years ago, also believed to have been caused by a large meteor impact.

The evidence is the most convincing yet for a meteoric impact at the "end-Permian," a time commonly referred to as "The Great Dying," at the boundary between the Permian and Triassic eras when life was nearly erased from the Earth, explains team leader Luann Becker, a geochemist with the Institute for Crustal Studies in the Department of Geology at the University of California, Santa Barbara.

Meteors striking the Earth (Artwork by Shigemi Numazawa courtesy Japan Planetarium Lab)

Scientists have theorized that the extinctions were caused by a greenhouse effect, volcanic eruptions in Siberia, a nearby supernova, or environmental changes wrought by the formation of one single continent.

To determine whether a meteor was the cause of this great extinction or not, Becker and her team used a newly discovered extraterrestrial tracer, fullerene - a third form of carbon besides diamond and graphite. Impact tracers are the geological markers that show evidence of large meteors hitting the Earth.

The unique ability of the fullerene molecule to trap noble gases, such as helium, neon, and argon, inside of its caged structure, enables researchers to determine the origin of the fullerenes. They trap extraterrestrial gases in space which then travel to Earth in the meteor.

The fullerene molecule is a hollow cage of a structure made of 60 or more carbon atoms. It is also called a buckyball, in honor of Buckminster Fuller, designer of the geodesic dome that resembles the molecule.

Becker's team had previously found fullerenes bearing extraterrestrial gases in rock layers associated with two known impact events - the 65 million year old Cretaceous-Tertiary impact which ended the dominon of the dinosaurs, and the 1.8 billion year old Sudbury impact crater in Ontario, Canada.

Becker and Ted Bunch, a scientist at NASA's Ames Research Center in California's Silicon Valley, in 1999 first identified naturally occurring fullerenes in a meteorite. The scientists found significant quantities of very large fullerene molecules, some containing as many as 400 carbon atoms, in samples from the 4.6-billion-year-old Allende meteorite that landed in Mexico three decades ago.

Geochemist Luann Becker holds a model of a fullerene. (Photo courtesy University of Hawaii)

"Researchers are now discovering hints of ancient impacts at sites marking history's top five mass extinctions, the worst of which eliminated 90 percent of all living species," she wrote.

Becker is now in Antarctica with a scientific team searching for more impact tracers, the geological markers that show evidence of large meteors hitting the Earth. Becker has made several research trips to Antarctica, and in July 2001 she received the National Science Foundation Antarctic Service Medal.

At the Graphite Peak in the Central Transantarctic Mountains, Antarctica, the team found several meteoritic fragments, metallic grains, in a thin claystone breccia layer. Breccia is ejected debris that resettled in a layer of sediment.

Becker's research shows that the metallic grains also appear in the same claystone layer in Meishan, southern China and resemble grains found in the same strata in Sasayama, Japan - all from the end-Permian period.

The scientists believe this to be evidence for a large impact that appears to have triggered the Great Dying, at a time when all the landmass on Earth was a single continent.

The team also found what Becker calls "shocked quartz" in this same layer at Graphite Peak. Quartz can be fractured by extreme volcanic activity, but only in one direction. Shocked quartz is fractured in several directions and is believed to be a good tracer for the impact of a meteor.

"These observations lead us to believe that continued research on such materials from additional Permian-Triassic boundary samples will finally lead to a resolution of the long sought and contentious issue of a catastrophic collision of a celestial body with the Earth at the end-Permian," Becker and her team write in "Science."

The first impact tracer linked to a severe mass extinction was an extraterrestrial concentration of iridium, an element that is rare in rocks on Earth but abundant in many meteorites The researchers are surprised that they have not found iridium at Graphite Peak.

From the discovery of iridium as a marker in 1980 came the idea that a giant impact ended the reign of the dinosaurs - and that such events may be associated with other severe mass extinctions over the past 600 million years.

The discovery was strongly debated around the world and scrutinized by geologists. The increased attention brought about the discovery of more impact tracers, including the fullerenes found in the Graphite Peak boundary layer.