Meteor Strike Suspected in Biggest Mass Extinction

Earth’s greatest mass extinction--an Armageddon that wiped out nearly all life on the planet 250 million years ago--may have been triggered by a massive meteor collision like the one that millions of years later helped end the reign of the dinosaurs, a team of scientists reported Friday.

The attention-grabbing findings are reviving a long-standing debate over whether single dramatic events like asteroid impacts are largely at fault for the series of catastrophic die-offs that has plagued the planet, or whether slower processes, such as volcanoes run amok, climate change and toxic shifts in sea water chemistry, should shoulder much of the blame.

For decades, scientists have puzzled over what could have caused the Permian-Triassic catastrophe 250 million years ago. The extinction is called “The Great Dying,” because it wiped out 90% of ocean species and 70% of those on land.

“This was the mother of all extinctions,” said Luann Becker, a geochemist at the University of Washington and lead author of the study published in today’s issue of the journal Science. “What makes it so remarkable is that virtually all marine life and a good portion of land life forms were eliminated in a very short period time.”

The dinosaur extinction 65 million years ago, which is better known to the public, is closely linked to a meteor; a large impact crater, unique space metals deposited by the meteor and crystals altered by the impact have all been found. But no telltale traces of a meteor had been found for the Permian extinction event and the team that produced the paper still has no direct evidence of an impact.

But in analyzing ancient rock, the scientists say, they have found unique chemical fingerprints--an “asteroid calling card”--suggesting that a space rock slammed into the Earth about 250 million years ago, the same time as the extinction.

Judging by the extent of the damage, the space rock would have been three to seven miles across, surmised Robert J. Poreda, an earth scientist at the University of Rochester and one of the paper’s coauthors. “The impact of a bolide of this size releases an amount of energy that is basically about 1 million times the largest earthquake recorded during the last century. It was like a magnitude 12.0 earthquake,” he said.

The clues consist of chemical traces, including unique carbon structures called “Buckyballs.” Buckyballs, or buckminsterfullerenes, are named after R. Buckminster Fuller because they resemble the geodesic dome that he invented. The cage-like molecules can trap gas within them as they form. The Buckyballs were found in rock that existed at the time of the extinction, Becker said.

The Buckyballs are clearly stowaways from space, the team reports, because they are filled with forms of helium and argon gases that could have been formed only in stars, far outside our solar system. They were probably delivered to Earth by a comet or an asteroid, the team suggests.

Ken Farley, a Caltech geochemist and an expert on the use of such gases to study Earth processes, said he believed the gas within the Buckyballs came from outer space. “There’s nothing on Earth that looks like that,” he said.

Although the finding suggests that some kind of asteroid impact occurred, Farley said, he warned that there was no evidence to show that the impact was large enough in itself to have caused an extinction event. The fullerenes could have come from a series of smaller meteor impacts that were not damaging.

“One needs to be very careful not to say this caused the [Permian] extinction,” he said.

Interpretations made using fullerenes have tended to be controversial. There have been questions about whether fullerenes could survive the billions of years required to make the journey to Earth from a star and how they could survive a massive impact. Fullerenes can also be created on Earth and are produced during forest fires.

Though there is widespread acceptance of the idea that an asteroid impact contributed to the extinction of the dinosaurs --and 60% of the other species alive at that time--the issue of how important the asteroid was in the extinction process is far from resolved among scientists.

“Just because there’s an impact doesn’t mean everyone drops dead,” said Nan Crystal Arens, the curator of fossil plants at the UC Berkeley Museum of Paleontology and an expert on how plants survive times of massive environmental stress. Arens agrees it is clear that there was an impact during the age of the dinosaurs that left a huge geological consequence. But she says the fossil record is far from clear on the biological cost of the impact.

A recent analysis of the impact crater from 65 million years ago suggests it was not large enough to kill off so much life. A large number of scientists agree with a theory put forward by Dewey M. McLean, a professor emeritus of geology at Virginia Polytechnic Institute, that massive volcanic activity at the time of both the dinosaur and the Permian extinctions caused the demise of species. It did so by spewing floods of lava, darkening the sky with ash and severely altering the climate, according to the theory.

During the dinosaur extinction, there were massive volcanic flows in India. During the Permian extinction, volcanoes in Siberia unleashed enough lava to cover the entire Earth with 10 feet of rock--making it the most massive volcanic event ever.

Higher-than-normal amounts of iridium, a rare metal discovered in a thin layer of clay formed at the same time as the dinosaur-era impact, could be the trace of a meteor--but could also have formed within volcanoes, said McLean, who now says that impacts and volcanoes could both contribute to extinction events.

Poreda suggested that an impact could have somehow triggered volcanism, but other geologists say there is currently no good scientific evidence--or even a mathematical model--to suggest such a linkage.

It’s possible that the impact was a final blow to a planet already weakened by extensive volcanism and a drop in sea level caused by the configuration of the continents at the time. “Other circumstances on Earth may have conspired to make lineages vulnerable,” Arens said. “Everything wasn’t rosy until that horrible day.”

The fossil record, many paleontologists argue, suggests that extinctions were spread over a longer period of time than can be accounted for by a single dramatic event, like a meteor impact. It is still unclear just how quickly the Permian extinction occurred.

One estimate suggests that the entire extinction occurred in less than 8,000 years--a geological eye blink--but that analysis remains unconvincing to many scientists, who say that there is enough evidence to show only that the extinction occurred in less than 500,000 years.

There are also traces in the geological record that show massive climate change occurred at the times of both extinctions.

“I have always been suspicious of single causes for mass extinctions; history doesn’t usually work that way,” said Doug Erwin, a Smithsonian Institution paleontologist and an expert on mass extinctions. He said the new Buckyball evidence may eventually show that the impact theory is correct, but added, “It is a bit early to declare victory and go home.”

Erwin said he suspects the cause lies with the extensive volcanism of the time and some complex set of factors involving global warming and acid rain. “Killing off perhaps 90% to 95% of all species in the ocean is actually really tough to do,” he said, “and there is no paleontological reason to think that the explanation will be a simple one.”

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Sudden Impact

Scientists have debated whether asteroids are the cause of catastrophic dieoffs. The new focus of the discussion is on whether an asteroid caused major extinction about 250 million years ago.

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Source: University of Washington.