By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
20 November 2003
A longstanding mystery over what caused five great mass extinctions, including one that destroyed the dinosaurs, has grown with the release of two studies today in the journal Science.
In one study, researchers make the bold claim that an asteroid is responsible for the death of most life on Earth in a catastrophic extinction 251 million years ago. Other scientists are not ready to accept the claim.
Many experts have become convinced over the past two decades that the dinosaurs were exterminated 65 million years ago by an asteroid impact. Some findings suggest other mass extinctions, such as the one 251 million years ago, might also have been caused by rocks from space.
But the evidence is scant. Volcanic activity remains a suspect in the extinction cases, and a growing scientific minority is skeptical of the whole death-by-space-rock scenario.
The new study uncovered 40 extraterrestrial mineral fragments in the Antarctic, indicating the asteroid impact 251 million years ago. The timing coincides with the well-documented Permian-Triassic mass extinction, the worst of five major events scientists have identified through fossil records. Some 90 percent of all species disappeared, by some accounts.
Scientists generally agree that the newfound tiny grains, called chondritic meteorite fragments, are indeed from space. But agreement stops there.
Too good to be true?
Study leader Asish Basu, a geochemist at the University of Rochester, and his colleagues are puzzled by their own discovery but have arrived at a conclusion nonetheless.
"It appears to us that the two largest mass extinctions in Earth history [65 million and 251 million years ago] were both caused by catastrophic collisions with chondritic meteoroids," the researchers write.
The pristine state of the fragments, however, does not make sense to other researchers. They should have long ago become indistinguishable soil, conventional wisdom holds. The fragments were collected from a layer dated to the Permian-Triassic boundary in time. They were embedded in rock 4-8 inches (10-20 centimeters) beneath the surface.
In a related analysis in Science by the science writer Richard Kerr, other scientists say they are stunned that the fragments survived for a quarter-billion years.
"I get the gut feeling it's wrong," said geochemist Birger Schmitz of the University of Goteborg in Sweden.
"It's astonishing, it's incredible, it's unbelievable," said Jeffrey Grossman of the U.S. Geological Survey. All those adjectives apply, Grossman later told SPACE.com, if the findings prove to be accurate. "Like all experiments it's going to have to be replicated," he said. And that replication is relatively simple. Another group of researchers can go to the same site in Antarctica, bring back their own samples, and analyze them.
Basu stands by the results. He insists the fragments were properly analyzed and that contamination in the sample was ruled out.
"We discovered them," Basu said in a telephone interview today. "Therefore they are there. Time will tell why they are there."
Basu added that the purpose of his team's scientific paper was not to explain how the grains held up over time. "The grains are there. Nobody can challenge that," he said. "We have to figure out how they survived."
Basu's team is back in Antarctica looking for more of the fragments. He said further research could solve the mystery.
Meanwhile, other researchers have been working to understand what role volcanoes might play in mass extinctions.
Deadly climate-altering gases spewed by volcanic eruptions could be the main culprit behind mass death, some figure. Others suppose volcanoes play just a supporting role. There is also the question of whether asteroid impacts trigger the volcanic activity and so are the root of all this evil either way.
Another new study in the journal suggests volcanoes might not be as deadly as some believe. And, if correct, it rules out the possibility that the dino-killing asteroid triggered intense volcanic activity known to have occurred in the era.
Researchers agree that at some time near the dinosaur extinction event 65 million years ago, a vast outpouring of volcanic material created a feature in India called the Deccan Traps, a bed of lava that covers an area about the size of Oregon and Washington states combined. But the timing has not been pinned down.
The Deccan volcanism occurred about 500,000 years before the end of the dinosaurs, according to the new research, by Greg Ravizza of the University of Hawaii and Bernhard Peucker-Ehrenbrink from the Woods Hole Oceanographic Institution.
The volcanoes loaded the air with carbon dioxide, fueling global warming, these scientists presume. Death of some species would have weakened the biological chain supporting dinosaurs.
Volcanic activity might have made life difficult for dinosaurs, it seems, but an asteroid impact remains the prime suspect in their demise.