Extinctions tied to impact from space.

The massive wave of extinctions on Earth 250 million years ago appears to have been triggered by an impact from space, according to an analysis of rocks from Japan and China.

The die-offs at the end of the Permian period exterminated almost 95 percent of the species in the ancient oceans, as well as many land animals, and set the stage for the rise of the dinosaurs. Scientists have proposed varying causes for the widespread extinctions, including changes in ocean chemistry and widespread volcanic activity (SN: 2/1/97, p. 74). Although recent studies hadn't identified a specific cause, they suggested the deaths occurred suddenly on the geologic time scale--possibly in as little as 8,000 years (SN: 7/15/00, p. 39).

Now, researchers say they've found a high concentration of fullerenes--soccerball-shape molecules of carbon also known as buckyballs--in sediments laid down at the so-called P-T boundary, the point in time between the Permian and Triassic periods. Analysis of the gas atoms trapped within the fullerenes confirms their extraterrestrial origin, says Luann Becker, a geochemist at the University of Washington in Seattle. Becker and her colleagues describe their findings in the Feb. 23 SCIENCE.

"There have been many explanations suggested for the P-T extinctions, but none has been compelling," says Richard A. Muller, a physicist at the University of California, Berkeley. "This is potentially a fantastic discovery, and the presence of fullerenes will be a key to interpreting what happened."

Although scientists say fire and lightning can produce fullerenes, buckyballs have been found in meteorites, too. Becker also found fullerenes in rocks at the so-called Cretaceous-Tertiary, or K-T, boundary (SN: 3/25/00, p. 196). That boundary has been associated with an asteroid impact that wiped out the dinosaurs about 65 million years ago.

In their new work, Becker and her colleagues cracked open the P-T-boundary buckyballs and studied the gas atoms trapped inside. The isotope ratios of helium and argon were much different from those found on Earth and mirrored the isotope ratios previously found in meteorites and the K-T boundary fullerenes.

"It takes an extraterrestrial environment for these fullerenes to form and to trap isotopes in these ratios," Becker told SCIENCE NEWS. "There's no question that this points to an extraterrestrial impactor."

"This is the first really strong bit of evidence that this mass extinction [at the end of the Permian] was associated with an extraterrestrial impact," says Kenneth A. Farley, a geochemist at the California Institute of Technology in Pasadena. In particular, Farley notes, the ratio of argon isotopes within the fullerenes is much different from that found anywhere on Earth. Most of the argon in the atmosphere comes from radioactive decay of the potassium in Earth's crust.

Becker's team estimates that the object that struck Earth 250 million years ago was about 9 kilometers across. This is about the size of the asteroid that scientists believe struck Earth about 65 million years ago, Becker says. Material kicked up by that iridium-rich asteroid, which slammed into relatively shallow water off Mexico's Yucatan peninsula, formed the layer that scientists say marks the death of the dinosaurs. This band of clay at the K-T boundary contains shocked quartz minerals, which typically form only in extraterrestrial impacts.

The relative lack of iridium and shocked quartz in layers marking the P-T boundary has caused many scientists to doubt that mass extinctions then were triggered by a killer space rock. But Richard K. Bambach, a paleontologist at Virginia Polytechnic Institute and State University in Blacksburg, says these differences might be explained by the composition of the object that struck Earth or by the location of the impact.

About 250 million years ago, what are today's continents were in one big landmass. That means there was one big ocean, Bambach notes. If the object slammed into the basalt-rich crust underneath the ocean, there wouldn't be much shocked quartz to be found. He says an ocean strike would also complicate the search for the point of impact, because all of the seafloor from that time has been recycled back into Earth's mantle.

"If [the object] didn't hit a block of continental crust, then you cannot and will not find an impact site," Bambach says.

The lack of iridium and shocked quartz could also indicate that the object that struck Earth 250 million years ago was not an asteroid but a comet. "This was clearly a different type of object" from the one that struck Earth 65 million years ago, Becker says. The presence of fullerenes could become a new hallmark for layers marking extraterrestrial impacts, she adds.