[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Another Dino Extinction Theory: This BOTH Volcanoes and Asteroids
Dinosaur dust-up: Princeton paleontologist produces evidence for new
theory on extinction
Princeton NJ -- 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 a mass extinction.
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 dogma.
Keller has accumulated evidence, including results released this year,
suggesting that the Chicxulub crater probably did not coincide with the
dinosaur extinction. 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.
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.
Princeton geophysicist Jason Morgan said Keller's detailed analysis of
these microorganisms gives her work real credibility. "It's not like
finding an isolated dinosaur bone," said Morgan. "You have thousands of
organisms in a single sample. You can do real statistics on them."
The time of the dinosaur extinction is known as the Cretaceous-Tertiary
boundary, or K/T for short. In addition to dinosaurs, many other forms of
life were wiped out, including all tropical and subtropical species of
foraminifera. Looking at ocean sediments from before and after the K/T
boundary "is like day and night," Keller said, because so much life
Keller began studying the K/T boundary several years after coming to
Princeton in 1984 and soon suspected that the story might not be so
straightforward. In a series of field trips to Mexico and other parts of
the world, she has accumulated several lines of evidence. 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
In other studies spread across a range of excavation sites, Keller has
found evidence that the ecological disruption caused by the Chicxulub
impact may not have been as severe as originally thought. She found normal
marine sediments lying directly on top of the fallout layer, suggesting
that there were no tsunami waves or other major disturbances.
In addition, Keller and her students conducted studies throughout Mexico,
Guatemala and Haiti (see related story below) that revealed signs of as
many as three meteorite impacts: the Chicxulub impact, evidenced by the
fallout of glass beads; the K/T impact with its iridium layer and mass
extinction; and probably a third smaller impact, evidenced by another
iridium layer about 100,000 years after 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 Keller.
Because her results are among the first to quantify the biotic effects of
volcanism, they may also help other scientists understand the likely
effects of greenhouse warming resulting from volcanism or other causes,