Princeton paleontologist produces evidence for new theory on extinction
By Steven Schultz
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.
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.
Working back in time
Unlike many children today who lap up a steady diet of dinosaur-related books, toys and television programs, Keller knew nothing of the creatures when growing up in Liechtenstein and Switzerland. She became interested in paleontology in the 1970s as a graduate student in earth sciences at Stanford University and began studying the periodic episodes of extinctions and abrupt climate changes that punctuate Earth's 4 billion years.
"I am interested in major events in Earth's history," said Keller. "How did they change life on Earth? What caused the big changes in evolution?"
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."
Keller first used fossilized foraminifera to study climate changes in the last several hundred thousand years. Then, going to work for the U.S. Geological Survey, she became interested in earlier periods and began working her way backward in time. "I'm now down to 100 million years and can't go much further," she said, noting that these microorganism records extend back only about 200 million years.
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 disappeared.
At many locations around the world, the K/T boundary is clearly visible in rock formations, which contain a thin layer of clay rich in the element iridium. Because iridium is more common in asteroids and comets than on Earth, scientists, led by father and son Luis and Walter Alvarez, proposed in l980 that an asteroid or comet must have struck Earth just at the K/T boundary. When the Chicxulub impact crater was discovered in 1990, it appeared to be a likely source for the iridium and seemed to confirm the hypothesis.
Several lines of evidence
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 extinction.
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, Keller said.
Together Keller's results give her hope that her ideas may gain greater recognition, but she remains cautious about how many people she is likely to convince. "When you have such a large group of scientists who became famous based on the idea that a single impact at Chicxulub caused the K/T mass extinction, you can't easily change their minds," she said.
Courtillot, whose views largely concur with Keller's, is optimistic that the opposition may be softening, particularly concerning the role of volcanism in the K/T extinction. "Recent years are vindicating our minority views -- at least I hope that is the case," he said.
In the meantime, Keller has further studies planned, including trips to extract sediments from Brazil, the Indian Ocean and the Middle East. She hopes these samples could broaden and clarify the story of the last days of the dinosaurs. "We want to nail it down as far away from Chicxulub as possible," she said.