More evidence for asteroid impact found

Jeff Poling

Scientists drilling for core samples in the Atlantic Ocean announced Sunday, February 16, that they have found what they believe to be strong evidence of an asteroid impact 65 million years ago.

"We've got the smoking gun," said Richard D. Norris, leader of the international ocean drilling expedition that probed the Atlantic Ocean floor in search of asteroid evidence. "It is proof positive of the impact."

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 K-T extinction.

Geologist Walter Alvarez of the University of California, Berkeley, first proposed in 1980 that the non-avian dinosaurs disappeared from the fossil record 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. Recent studies suggest that the asteroid hit at a shallow angle, causing the debris to travel in a "parabola of death" that incinerated much of North America. Temperatures around the globe plunged.

Up to 70 percent of all species, including the non-avian dinosaurs, perished.

Norris said the expedition recovered three core samples that have the unmistakable signature of an asteroid impact approximately 65 million years ago. The cores include a thin brownish layer that the scientists called the "fireball layer" because it is thought to contain remains of the actual asteroid itself.

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 end of the Creataceous.

Norris said the deepest layers contain fossil remains of many animals and came from a healthy "happy-go-lucky ocean" just before the impact layers. Next youngest is a layer with small, green glass pebbles, thought to be ocean bottom material melted by the massive energy release of the impact. Then comes a rusty brown layer which Norris said is thought to be 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, possibly ejecting some of the matter back into 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.

Following the brown layer, is a 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 [in earlier layers in the core sample] are gone," said Norris. "There are just some very minute fossils. These were the survivors in the ocean."

This layer represents about 5,000 years, said the scientist, and then the core samples show evidence of renewed life. "It is amazing how quickly the new species appeared," Norris said.

Although the 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 tsunamis (giant waves), roiled the Gulf of Mexico so much that it is unlikely to find clear core samples there. He said the theorized waves from the impact would have washed completely across Florida, depositing debris in the Atlantic.