Posted on 11/20/2003 12:31:59 PM PST by LibWhacker
Catastrophic asteroid impacts are gaining a credible edge over violent volcanic eruptions as the greatest killers Earth has ever seen, according to two pieces of scientific detective work reported in tomorrow's issue of the journal Science.
The first cataclysm in question occurred about 250 million years ago, when according to the fossil record more than 90 percent of Earth's marine species and 70 percent of life on land perished. The event is known as the Permian-Triassic (P-T for short) mass extinction, named because it falls on the boundary between the two geological eras.
The second event, known as the Cretaceous-Tertiary (K-T) mass extinction, occurred about 65 million years ago, leading to the demise of the dinosaurs and most of the creatures and plants that lived with them.
The geologic record shows that cataclysmic volcanic eruptions occurred around the same time as two periods of mass extinctions. But recent research shows that asteroids may have had a more significant impact on prehistoric flora and fauna. The Ida asteroid, the 1991 eruption of Mt. Pinatubo in the Philippines, and a magnified meteorite fragment are shown above.
According to the geological record, bouts of extreme volcanism occurred around the same time as these mass extinctions and many scientists have suggested that the volcanic activity is directly responsible for the loss of life.
However, the discovery and analysis over the past few decades of a crater from an asteroid impact about 65 million years ago, and of meteorite fragments from an apparent asteroid impact about 250 million years ago, is leading some scientists to believe that the impact events, not volcanism, were the primary cause of the extinctions.
Two studies published in the November 21 issue of Science support these theories. One study presents further evidence for an impact event about 250 million years ago and the second study suggests that the volcanism around the K-T boundary was probably not a major contributor to the K-T mass extinction.
Asteroid Impacts
In 1991 scientists located a 112-mile-wide (180-kilometer-wide) and 3,000-foot-deep (900-meter-deep) crater on Mexico's Yucatán Peninsula that appeared to have been made by a giant comet or asteroid that slammed into the Earth about 65 million years ago. The collision has gained favor as the cause of the K-T mass extinction and is referred to as the "dinosaur killer."
Now, a team of scientists led by Asish Basu, a geochemist at the University of Rochester in New York, has found dozens of unusual mineral grains from two rock samples taken from Graphite Peak, Antarctica, that they say are pieces of a meteorite that impacted Earth 250 million years ago.
"We analyzed them and they seem to be pieces of extraterrestrial material," said Basu.
The researchers also found bits of nearly pure metallic iron in the Antarctic rock that they say is of neither terrestrial nor extraterrestrial origin. Rather, they say the particles resemble those reported by Kunio Kaiho of Tohoku University in Sendai, Japan, from the P-T boundary in Meishan, China, that formed as an impact cloud condensed.
A third, controversial, impact marker—clusters of carbon atoms called buckyballs—have also shown up at Graphite Peak, co-authors of the Science paper Luann Becker of the University of California, Santa Barbara and Robert Poredea of the University of Rochester reported earlier this year in the journal Astrobiology.
Basu and his colleagues suggest in Science that the material from Asia and Antarctica is from an impact event associated with the P-T mass extinction just as the impact crater in the Yucatán is now widely associated with K-T mass extinction.
Gregory Retallack, a geologist at the University of Oregon in Eugene who in 1995 collected one of the Antarctic samples analyzed by Basu and colleagues, also concluded that the rocks contained evidence of an asteroid impact.
"This new paper is a splendid corroboration of our earlier work," he said in an e-mail sent from Antarctica where he is currently searching for more impact beds associated with the P-T boundary.
Meanwhile, Basu and his colleagues are actively searching for an impact crater associated with the P-T mass extinction. Such a find, said Basu, would further clarify whether the impact occurred and, if so, its association with the mass extinction.
"The search is going on. So far in the published literature no one has found it, but we are working on it actively. We are working on it right now," he said.
Volcanic Isotopes
In a second Science study, geochemists analyzing the ratios of two osmium isotopes in seawater suggest that the bout of violent volcanic activity around the K-T boundary likely caused a major global warming event but was probably not a major contributor to the demise of the dinosaurs.
Greg Ravizza, a co-author of the study at the University of Hawaii in Honolulu, said the finding substantiates earlier studies based on analysis of the past climate by bringing more resolution to when the volcanism occurred in relation to the impact event.
"It's the first thing we have that substantiates the previous interpretations of the paleoclimate," he said.
The finding suggests that while the period of volcanism spanned the K-T impact event, the bulk of the volcanism occurred several hundred thousand years before the asteroid slammed into the Yucatán, causing the mass die off.
Ravizza said he would like to apply the dating technique he and his colleague Bernhard Peucker-Ehrenbrink of the Woods Hole Oceanographic Institution in Massachusetts used to the volcanism at the P-T boundary as well, but 250-million-year-old sediments on the seafloor are not well preserved.
Such work, if eventually possible, will help answer the still outstanding question of how significant a contribution the volcanism at the P-T boundary made to the mass extinction, versus the contribution of the impact.
Basu said it may even be possible that the volcanism at the P-T boundary is directly linked to the impact event. "That is the $64 million question," he said. "People are trying to figure out whether an impact could trigger volcanism. That would be a double punch."
With many folks out there these days, you probably wouldn't. My next door neighbor is scheduled for some minor brain surgery.
By definition, ANY surgery on his brain is minor.
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Note: this topic is from 11/20/2003. Thanks LibWhacker.
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