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Single Asteroid Impact, Not Two, Killed Dinos, Study Says

Richard A. Lovett
for National Geographic News
November 30, 2006
 
A new study of seabed sediments adds support to the theory that the dinosaurs were wiped out by a single asteroid impact near Chicxulub, Mexico, 65.5 million years ago.

The impact is widely believed to have obliterated two-thirds of the world's species in an event called the K-T extinction, which bridges the Cretaceous and Tertiary geological periods (Mexico map).

A competing theory, however, holds that at least two asteroid strikes 300,000 years apart, along with global climate change and volcanism, contributed to the extinctions. (Read news story: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory [October 30, 2006].)

This theory, championed by Gerta Keller of Princeton University in New Jersey, is based on studies from Mexico and Texas in which two layers of impact-related debris are separated by up to 15 feet (4.5 meters) of intervening sediments.

Keller's group argues that the separation between the layers indicates that they come from different impacts.

But single-impact proponents argue that the intervening sediments were created by a gigantic tsunami resulting from the Chicxulub impact.

Layered Debate

In an effort to resolve the competing theories, the new study, to be published early in 2007 in the Geological Society of America Bulletin, examined core samples from the Demerara Rise, a submarine plateau near the northeast coast of South America (South America map).

The location is significant because it would have been out of range of the worst effects of a tsunami but close enough to have still received substantial fallout.

"Sedimentation at the site was very simple," says the study's lead author, Ken MacLeod of the University of Missouri, Columbia.

Prior to the impact, he said, there were two million years of normal marine deposits filled with fossils typical of the Cretaceous.

Immediately above those are "air fall" deposits from the impact and sediments from the first 10,000 years of the Tertiary.

"We have nothing to indicate another impact," MacLeod said.

But Princeton's Keller remains unconvinced. In an email, she described the new study's claims as "rather hyper-inflated."

Among other things, she said, there are signs of erosion between layers in the core sample, indicating that critical layers might be missing or rearranged. She also challenged the dates ascribed to certain rock layers.

"They do not have a complete record of the K-T extinction," she wrote.

MacLeod, however, believes the features that Keller sees as signs of erosion were actually left in the first hours after the impact, before the arrival of fallout.

The impact, he said, generated a magnitude 12 to 13 earthquake with seismic waves that stirred up the top millimeters of sediment, which later deformed slightly under the weight of the impact layer.

Rather than indicating missing time, he said, "this layer represents a moment in time."

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