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WASHINGTON (AP) -- Scientists who drilled core samples from the ocean bed
said Sunday they have found proof that a huge asteroid smashed into the
Earth 65 million years ago and probably killed off the dinosaurs.
"We've got the smoking gun," said Richard D. Norris, leader of an
international ocean drilling expedition that probed the Atlantic Ocean
floor in search of asteroid evidence. "It is proof positive of the
impact."
Norris said the expedition recovered three drill samples that have the
unmistakable signature of an asteroid impact about 65 million years ago.
The drill cores include a thin brownish section that the scientists called
the "fireball layer" because it is thought to contain bits of the asteroid
itself.
'It is proof positive' "These neat layers of sediment bracketing the
impact have never been found in the sea before," Norris said in a
telephone interview. "It is proof positive of the impact."
The 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. The team penetrated up to 300 feet beneath
the sea bed, drilling past 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, is 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," he said.
Although the dinosaur-killing impact occurred in the southern Gulf of
Mexico, Norris went to the Atlantic Ocean, near the edge of the
Continental Shelf. He said that the violence of the impact, followed by
huge waves, roiled the Gulf of Mexico so much that it is unlikely clear
core samples would be found there.
He said he theorized that waves from the impact would have washed
completely across Florida, depositing debris in the Atlantic. And that's
where he found it.
'The most significant discovery' in 20 years
Robert W. Corell, assistant director for Geosciences of the National
Science Foundation, said the core samples are the strongest evidence yet
that an asteroid impact caused the extinction.
"In my view, this is the most significant discovery in geosciences in 20
years," he said, also in a telephone interview. "This gives us the facts
of what happened to life back then. I would certainly call it the smoking
gun."
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's now widely believed that an asteroid of 6 to 12 miles in diameter
smashed into Earth at thousands of miles per hour. It instantly gouged a
crater 150 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, sulfur 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.
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