The Asteroid Was Innocent,
Says Dino Study

By Larry O'Hanlon
Discovery News
The great meteorite impact that may have wiped out the dinosaurs happened hundreds of thousands of years too early to have been the one, startling new evidence indicates.
The discovery was made after drilling boreholes into the Chicxulub crater in Mexico.
It comes at a time when some geologists are convinced that the crater was the "smoking gun" of what killed 70% of living species at the Cretaceous-Tertiary (or the K-T) boundary, 65 million years ago.
Several lines of geological evidence from Chicxulub were presented to a packed meeting room by Princeton University geologist Professor Gerta Keller at this month's annual meeting of the Geological Society of America.
The evidence makes a case for the famous crater having been formed about 300,000 years before the mass die-off.
"What Gerta Keller is showing us is that there is reason to doubt," said Dr Spencer Lucas, curator of palaeontology and geology at the New Mexico Museum of Natural History and Science.
"[The smoking gun] can't be even a 100 years older than the K-T boundary. There is room for inquiry here."
Keller and her colleagues' evidence comes from Yaxcopoil 1, a borehole that was expected to provide final, irrefutable confirmation of Chicxulub's role in the K-T boundary mass extinction. It didn't.
Layers of rock tell a different story
Layers of rocks from the Yaxcopoil 1 borehole are stacked like old newspapers; they are older as you go down.
The layers tell of the Chicxulub impact with the broken "breccia" rocks. On top of the impact breccia is about 60 centimetres of gently-laid-down, thinly layered seafloor mud built up over 300,000 years, Keller said.
Those 60 centimetres of ho-hum, post-impact mud have the fossils, carbon isotopes and magnetic signal of the late Cretaceous, before the mass die-off, she said.
It's not until 300,000 years later, and about 60 centimetres higher, that a sharp change in carbon isotopes and changes in microfossils signal the massive K-T extinction event, Keller said.
Also missing from the Yaxcopoil 1 borehole rocks is any significant iridium signal, the extraterrestrial element that first gave scientists a clue that an asteroid might have caused the K-T extinctions.
So what caused the K-T mass extinction? It was probably another asteroid impact combined with intense volcanic activity.
"It might have been a one-two punch," said Lucas.
In fact, he said, many dinosaur researchers suspect that dinos were on the decline before the final mass extinction.
Chicxulub might have played a role in "softening" the dinos, he said, after which they may have never quite recovered.
The second, still undiscovered, impactor might have been the terminal blow.
©2004 ABC



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