WASHINGTON (AP) -- Ocean core samples contain "proof positive" of a massive asteroid colliding with the Earth 65 million years ago and triggering a global extinction that probably killed off the dinosaurs, scientists report.
Richard D. Norris, leader of the international ocean drilling expedition, said Sunday that three drill samples have the unmistakable signature of an asteroid impact, including a brownish clay that he called the "fireball layer" because it seems to contain bits of the asteroid itself.
"We've got the smoking gun," said Norris in a telephone interview. "These neat layers of sediment bracketing the impact have never been found in the sea before. It is proof positive of the impact."
A team of international scientists, working on the drill ship Joides Resolution, spent five weeks off the east coast of Florida collecting cores from the ocean floor in about 8,500 feet of water. They drilled up to 300 feet beneath the sea bed, collecting sediments laid down at the time of the dinosaur extinction.
Norris said the deepest layers contain fossil remains of many animals and came from a healthy "happy-go-lucky ocean" just before the impact.
Just above this is a layer with small green glass pebbles, thought to be ocean bottom material instantly melted by the massive energy release of the impact.
Next was a rusty brown layer which Norris said is thought to be from the "vaporized remains of the asteroid itself."
The heat of the impact would have been so intense, said Norris, that the stony asteroid would have instantly been reduced to vapor and thrown high into the sky, some of it perhaps even reaching outer space. It then snowed down, like a fine powder, all over the globe.
Norris said brown deposits, like that in the core sample, have been found elsewhere and they have a high content of iridium, a chemical signature of asteroids.
Just above the brown layer, are two inches of gray clay with strong evidence of a nearly dead world.
"It was not a completely dead ocean, but most of the species that are seen before (early in the core sample) are gone," said Norris. "There are just some very minute fossils. These were the survivors in the ocean."
This dead zone lasted about 5,000 years, said the scientist, and then the core samples showed evidence of renewed life. "It is amazing how quickly the new species appeared," Norris said.
Robert W. Corell, assistant director for Geosciences of the National Science Foundation, called the core samples the strongest evidence yet that an asteroid impact caused the dinosaurs' extinction.
"In my view, this is the most significant discovery in geosciences in 20 years," he said. "This gives us the facts of what happen to life back then. I would certainly call it the smoking gun."
Although the impact with the asteroid occurred in the southern Gulf of Mexico, Norris went to the Atlantic Ocean, near the edge of the continental shelf in his search to document it. He surmised that the violence of the impact, followed by huge waves, roiled the Gulf of Mexico so much that the area was unlikely to have clear sediment layers dating to the dinosaur era.
Norris theorized that waves from the impact would have washed completely across Florida, depositing debris in the Atlantic. That is where he found it.
The ship bearing the core samples returned to port on Friday and the NSF announced the findings Sunday, just hours before NBC was airing a movie about a fictional asteroid hitting the Earth and causing widespread destruction.
"The impact of the asteroid featured in tonight's NBC-TV show is peanuts compared to the real thing faced by the world 65 million years ago," said Corell.
Geologist Walter Alvarez of the University of California, Berkeley, first proposed in 1980 that the dinosaurs disappeared from fossil history suddenly because of a massive asteroid hit. At first, the theory had few supporters.
But in 1989, scientists found evidence of a huge impact crater north of Chicxulub, on Mexico's Yucatan Peninsula. Later studies found evidence of debris washed out of the Gulf by waves that went inland as far as what is now Arkansas.
It is now widely believed that an asteroid of six to 12 miles in diameter smashed to Earth at thousands of miles an hour. It instantly gouged a crater `50 to 180 miles wide.
That energy release was more powerful than if all of the nuclear weapons ever made were set off at once, said Norris. Billions of tons of soil, sulphur and rock vapor were lifted into the atmosphere, blotting out the sun. Temperatures around the globe plunged.
Up to 70 percent of all species, including the dinosaurs, perished. Among the survivors, scientists believe, were small mammals that, over millions of years, evolved into many new species, including humans.
Original file name: CNI - Asteroid Killed Dinos
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