Tue, Nov 14, 2006 - Page 6 News List

Meteorite not only culprit in dinosaur extinction: report


In a scenario resembling the dramatic conclusion to a TV crime drama, paleo-forensics experts have produced new evidence to show that the dinosaurs were bumped off by a different meteor than the one that has received the rap for their extinction.

The German paleontologists insist that a mysterious meteor or comet must have done the deadly deed -- long after the notorious Yucatan meteor that has hitherto been blamed.

Until now, it has been generally accepted that the Chicxulub impact off the coast of Mexico 65 million years ago wiped out the dinosaurs. Evidence of the crater left by the giant asteroid or comet has been found under the sea off the coast of Yucatan.

But a group of scientists led by Gerta Keller of Princeton and Wolfgang Stinnesbeck of the University of Karlsruhe begged to differ. They uncovered a series of geological clues which suggests the truth may be far more complicated. In short, they say that the crater in the Yucatan is too old to have killed off the dinosaurs.

Keller says marine microfossils in sediments drilled from the ocean floor show that Chicxulub hit Earth 300,000 years before the mass extinction it was supposed to have caused.

The Chicxulub impact conspired with the Deccan Flood Basalt eruptions in India, a period of prolonged and intense volcanic activity, to nudge species towards the brink, Keller said.

Vast amounts of greenhouse gas were pumped into the atmosphere by the Deccan volcanism over a period of more than a million years. By the time Chicxulub struck, land temperatures were seven to eight degrees Celsius warmer than they had been 20,000 years earlier. Weakened by these events, species were finally killed off by the second impact.

The previous impact theory drew much of its evidence from a thin layer of rock known as the "KT boundary." This layer is 65 million years old (which is around the time when the dinosaurs disappeared) and is found around the world.

For supporters of the impact theory, the KT boundary layers contained two crucial clues. It contains high concentrations of a rare element called iridium, which scientists thought could only have come from an asteroid. Right underneath the iridium was a layer of spherules, tiny balls of rock which seemed to have been condensed from rock which had been vaporized by a massive impact.

But Keller's team concentrated on a series of rock formations in Mexico where the iridium layer was separated from the spherule layer by many meters of sandstone. The team concluded that there was a gap of some 300,000 years between the deposition of the spherules (from the Chicxulub crater) and the iridium (from an asteroid). Therefore, there must have been two impacts.

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