New Extinction Clues Point to Space
May 11 2001 @ 09:31
New evidence shows that an extinction event in which more than half of all Earth species died 200 million years ago happened quickly, possibly as a result of an impact from outer space.
WASHINGTON (AP) - The extinction, at the boundary of the Triassic and Jurassic periods of geologic history, is similar in its suddenness to two extinction events that have been linked to space rocks' impacts on the Earth.Researchers analyzing deposits from a rock formation on a remote beach front in Canada found evidence of a sharp shift in organic carbon levels at precisely the point in time that the Triassic-Jurassic extinction occurred.
This is the first time scientists have found a clear carbon signature for what is called the TJ event, said Peter D. Ward, a researcher at the University of Washington.
``We just lucked out in finding a place on Earth where that signal is preserved,'' said Ward, lead author of a study appearing in the journal Science on Friday. ``Our study shows that this extinction was very, very, rapid.''
Prior studies had suggested that the extinction occurred slowly over about five million years. But Ward said that work was not based on a clear rock signature. He said the new work shows that the extinction actually happened in about 10,000 years, a brief moment in geologic time.
This suggests that a sudden change in the Earth's conditions, such as the kind resulting from a space impact, could have caused the extinction.
Similar evidence has been found for the Cretaceous-Tertiary extinction event 65 million years ago that killed off the dinosaurs, and for the much earlier Permian extinction 250 million years ago that killed 90 percent of all species.
The new study, however, lacks the distinctive chemistry that marked the dinosaur-killing event. Studies have found that the space rock that killed the dinosaurs left worldwide deposits of iridium, a chemical common in asteroids but rare on Earth.
Ward said his team is awaiting results of an iridium study of samples from the Canadian site.
``This looks exactly like the (dinosaur extinction) boundary, but we have not found any iridium yet,'' he said.
The teams plan to return to the site, on the Queen Charlotte Islands in British Columbia, this summer and used drills to take core samples of rock formations. These, plus earlier samples, will be checked for iridium.
If the researchers find evidence that a space rock impact caused the TJ extinction, it will mean that three of the five major extinctions in the 4.5-billion-year history of the Earth are linked to the impact of asteroids or comets.
Earlier studies have shown that the Permian extinction, the most severe of all, was linked to a type of helium isotope within a carbon molecule that is thought to come from asteroids.
Both the earlier extinctions are marked in geologic formations by the sudden disappearance of species that had been common on the Earth.
Ward said the TJ extinction wiped out large plant-eating animals, called mammal-like reptiles, that roamed the land, and whole groups of shell fish and one-celled animals that were common in the sea.
Many types of dinosaur and some small mammals, ratlike animals that are thought to be the ancestors of all mammals, including humans, survived the TJ event. After that, dinosaurs evolved further and dominated the Earth for more than 100 million years - until another asteroid hit the planet.
Ward said that no impact crater on Earth has been shown to have a proven link to the TJ extinction event, although the Manicougan Crater in Quebec is considered a candidate. That crater was caused by a space impact, but it has been dated at 214 million years, well before the TJ event.
He said it is likely that the crater's dated age is wrong, and efforts are under way to see if more accurate age can be determined.
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