- 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.
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- The discovery was made after drilling boreholes into
the Chicxulub crater in Mexico.
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- 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.
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- 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.
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- The evidence makes a case for the famous crater having
been formed about 300,000 years before the mass die-off.
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- "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.
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- "[The smoking gun] can't be even a 100 years older
than the K-T boundary. There is room for inquiry here."
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- 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.
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- Layers of rock tell a different story
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- Layers of rocks from the Yaxcopoil 1 borehole are stacked
like old newspapers; they are older as you go down.
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- 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.
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- 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.
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- 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.
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- 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.
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- So what caused the K-T mass extinction? It was probably
another asteroid impact combined with intense volcanic activity.
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- "It might have been a one-two punch," said
Lucas.
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- In fact, he said, many dinosaur researchers suspect that
dinos were on the decline before the final mass extinction.
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- Chicxulub might have played a role in "softening"
the dinos, he said, after which they may have never quite recovered.
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- The second, still undiscovered, impactor might have been
the terminal blow.
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- ©2004 ABC http://www.abc.net.au/science/news/stories/s1245404.htm
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