More Than a Meteor
Likely Killed Dinosaurs 65 Million Years Ago
(18 October 2006) Growing evidence
shows that the dinosaurs and their contemporaries were not wiped out by the
famed Chicxulub meteor impact alone, according to a palaeontologist who says
multiple meteor impacts, massive volcanism in India and climate changes
culminated in the end of the Cretaceous Period.
The Chicxulub
impact may have been the lesser and earlier of a series of meteor impacts and
volcanic eruptions that pounded life on Earth for more than 500,000 years, say
Princeton University palaeontologist Gerta Keller and her collaborators Thierry
Adatte from the University of Neuchatel, Switzerland, and Zsolt Berner and
Doris Stueben from Karlsruhe University in Germany.
A final, much larger
and still unidentified impact 65.5 million years ago appears to have been the
last straw, said Keller, exterminating two-thirds of all species in one of the
largest mass extinction events in the history of life. It's that impact - not
Chicxulub - that left the famous extraterrestrial iridium layer found in rocks
world-wide that marks the impact that finally ended the Age of Reptiles, Keller
believes.
"The Chicxulub impact alone could not have caused the mass
extinction," said Keller, "because this impact predates the mass
extinction."
Keller is scheduled to present that evidence at the annual
meeting of the Geological Society of America (GSA) in Philadelphia, on Tuesday,
October 24, 2006.
"Chicxulub is one of thousands of impact craters on
Earth's surface and in its subsurface," said H. Richard Lane, program director
in the National Science Foundation (NSF) Division of Earth Sciences, which
funded the research. "The evidence found by Keller and colleagues suggests that
there is more to learn about what caused the major extinction event millions of
years ago, and the demise of the dinosaurs at the end of the
Cretaceous."
Marine sediments drilled from the Chicxulub crater itself,
as well as from a site in Texas along the Brazos River and from outcrops in
northeastern Mexico, reveal that Chicxulub hit Earth 300,000 years before the
mass extinction. Microscopic marine animals were left virtually unscathed, said
Keller.
"In all these localities we can analyse their microfossils in
the sediments directly above and below the Chicxulub impact layer, and cannot
find any significant biotic effect," said Keller. "We cannot attribute any
specific extinctions to this impact."
The story that seems to be taking
shape, according to Keller, is that Chicxulub, though violent, actually
conspired with the prolonged and gigantic volcanic eruptions of the Deccan
Flood Basalts in India, as well as with climate change, to nudge species
towards the brink. They were then pushed over with a second large meteor
impact.
The Deccan volcanism released vast amount of greenhouse gases
into the atmosphere over a period of more than a million years leading up to
the mass extinction. By the time Chicxulub struck, the oceans were already 3-4
degrees warmer, even at the bottom, Keller said.
"On land it must have
been 7-8 degrees warmer," she said. "This greenhouse warming is
well-documented. The temperature rise was rapid over about 20,000 years, and it
stayed warm for about 100,000 years, then cooled back to normal well before the
mass extinction."
Where's the crater? "I wish I knew," said
Keller.
(source: National Science Foundation)