WASHINGTON (AP) — Scientists who drilled core samples from the ocean bed said Sunday they have found “smoking gun” evidence that a huge asteroid smashed into the Earth 65 millions 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.
Norris said the expedition recovered three drill samples that have the unmistakable signature of a 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.
“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 asteroid.
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. 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 2 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, believing that waves from the impact would have washed completely across Florida.
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 happen 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 six to 12 miles in diameter smashed to Earth at thousands of miles an 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, 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.