News from PRINCETON UNIVERSITY
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For immediate release: September 25, 2003
Contact: Steven Schultz, (609) 258-5729, s s c h u l t z @ p r i n c e
t o n . e d u
Princeton paleontologist produces
evidence for new theory on dinosaur
PRINCETON, N.J. -- As a paleontologist, Gerta Keller has studied many
aspects of the history of life on Earth. But the question capturing her
attention lately is one so basic it has passed the lips of generations
of 6-year-olds: What killed the dinosaurs?
The answers she has been uncovering for the last decade have stirred an
adult-sized debate that puts Keller at odds with many scientists who
study the question. Keller, a professor in Princeton's Department of
Geosciences, is among a minority of scientists who believe that the
story of the dinosaurs' demise is much more complicated than the
familiar and dominant theory that a single asteroid hit Earth 65
million years ago and caused the mass extinction known as the
Cretacious-Tertiary, or K/T, boundary.
Keller and a growing number of colleagues around the world are turning
up evidence that, rather than a single event, an intensive period of
volcanic eruptions as well as a series of asteroid impacts are likely
to have stressed the world ecosystem to the breaking point. Although an
asteroid or comet probably struck Earth at the time of the dinosaur
extinction, it most likely was, as Keller says, "the straw that broke
the camel's back" and not the sole cause.
Perhaps more controversially, Keller and colleagues contend that the
"straw" -- that final impact -- is probably not what most scientists
believe it is. For more than a decade, the prevailing theory has
centered on a massive impact crater in Mexico. In 1990, scientists
proposed that the Chicxulub crater, as it became known, was the remnant
of the fateful dinosaur-killing event and that theory has since become
Keller has accumulated evidence, including results released this year,
suggesting that the Chicxulub crater probably did not coincide with the
K/T boundary. Instead, the impact that caused the Chicxulub crater was
likely smaller than originally believed and probably occurred 300,000
years before the mass extinction. The final dinosaur-killer probably
struck Earth somewhere else and remains undiscovered, said Keller.
These views have not made Keller a popular figure at meteorite impact
meetings. "For a long time she's been in a very uncomfortable
minority," said Vincent Courtillot, a geological physicist at
Université Paris 7. The view that there was anything more than a single
impact at work in the mass extinction of 65 million years ago "has been
battered meeting after meeting by a majority of very renowned
scientists," said Courtillot.
The implications of Keller's ideas extend beyond the downfall of
ankylosaurus and company. Reviving an emphasis on volcanism, which was
the leading hypothesis before the asteroid theory, could influence the
way scientists think about the Earth's many episodes of greenhouse
warming, which mostly have been caused by periods of volcanic
eruptions. In addition, if the majority of scientists eventually reduce
their estimates of the damage done by a single asteroid, that shift in
thinking could influence the current-day debate on how much attention
should be given to tracking and diverting Earth-bound asteroids and
comets in the future.
Keller does not work with big fossils such as dinosaur bones commonly
associated with paleontology. Instead, her expertise is in one-celled
organisms, called foraminifera, which pervade the oceans and evolved
rapidly through geologic periods. Some species exist for only a couple
hundred thousand years before others replace them, so the fossil
remains of short-lived species constitute a timeline by which
surrounding geologic features can be dated.
In a series of field trips to Mexico and other parts of the world,
Keller has accumulated several lines of evidence to support her view of
the K/T extinction. She has found, for example, populations of pre-K/T
foraminifera that lived on top of the impact fallout from Chicxulub.
(The fallout is visible as a layer of glassy beads of molten rock that
rained down after the impact.) These fossils indicate that this impact
came about 300,000 years before the mass extinction.
The latest evidence came last year from an expedition by an
international team of scientists who drilled 1,511 meters into the
Chicxulub crater looking for definitive evidence of its size and age.
Although interpretations of the drilling samples vary, Keller contends
that the results contradict nearly every established assumption about
Chicxulub and confirm that the Cretaceous period persisted for 300,000
years after the impact. In addition, the Chicxulub crater appears to be
much smaller than originally thought -- less than 120 kilometers in
diameter compared with the original estimates of 180 to 300 kilometers.
Keller and colleagues are now studying the effects of powerful volcanic
eruptions that began more than 500,000 years before the K/T boundary
and caused a period of global warming. At sites in the Indian Ocean,
Madagascar, Israel and Egypt, they are finding evidence that volcanism
caused biotic stress almost as severe as the K/T mass extinction
itself. These results suggest that asteroid impacts and volcanism may
be hard to distinguish based on their effects on plant and animal life
and that the K/T mass extinction could be the result of both, said
Note: A longer version of this news
release appeared in the Princeton Weekly