Friday, December 01, 2006

One Impact or Two?

Impact and extinction in remarkably complete Cretaceous-Tertiary boundary sections from Demerara Rise, tropical western North Atlantic. 2007. K. G. MacLeod et al. Bulletin of the Geological Society of America, in press.
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.
From National Geographic News:

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. 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 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.

The new study examined core samples from the Demerara Rise, a submarine plateau near the northeast coast of South America. 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.

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.