back to the CSDP pages or to the debate

Chicxulub Drilling debates, Is the Chicxulub crater the KT boundary killer or Not? (USA Today)

from USA Today, Wed, November 11, 2003


By Dan Vergano, USA TODAY

The killer asteroid that may have doomed the dinosaurs 65 million years ago is still making a lot of noise, at least among scientists.

The reason: renewed debate about evidence that widespread volcanic activity helped bring down the curtain on the age of dinosaurs through long-term climate change. This theory holds that any asteroid impacts were a secondary occurrence.

The question matters to more than just scientists. The demise of the dinosaur draws special interest because of the public's fascination with the vanished giant creatures. Meanwhile, the world looks for lessons from the past about the effects of global climate change or asteroids striking Earth.

The normally polite academic debates over such matters have turned as messy as a prehistoric bog. In a yearlong verbal brawl, now featured in an online debate sponsored by the Geological Society of London, Princeton researcher Gerta Keller has charged another scientist with sabotaging her efforts to prove the overriding importance of volcanoes in the demise of dinosaurs.

Clues from a crater

Mass extinction events appear in the fossil record repeatedly. At least five such widespread kill-offs of entire species have occurred over the last 550 million years.

The current dispute revolves around core samples from the Gulf of Mexico's Chicxulub (CHICK zuloob) crater, off the Yucatan Peninsula. A comet or asteroid impact there 65 million years ago is widely thought to have triggered tidal waves and a brief ice age that killed off about 50% of all species. Scientists call this point in time between the age of the dinosaurs and the current age of mammals the "K-T boundary."

It was only last year, after a decade of wrangling, that a core sample from the crater became available to scientists. In April 2002, the 3-inch-wide drilling core was given to micropaleontologist Jan Smit of Holland's Vrije Universiteit, an expert on microscopic fossils left by tiny sea creatures over hundreds of millions of years. Smit was to divvy up slices of the sample to about two dozen researchers.

Keller suggests that Smit, a supporter of the impact-extinction theory, purposely delayed getting core samples to other scientists. To present analyses at the American Geophysical Union meeting in France in April, researchers needed to send in findings by mid-January.

But Smit didn't deliver the core samples until November, leaving little time for analysis by other researchers. Keller says Smit would have been alone at the podium, presenting his results ahead of everyone else with no one to challenge him.

The Sept. 7 Nature magazine report that first aired her charges included complaints from two other researchers about the samples they received. Smit says no one complained about the delivery schedule when he first proposed it.

Keller says she hustled to do her analysis. Contradicting Smit's results, she reported that Chicxulub couldn't be the K-T boundary because microfossils from the site show now-extinct sea creatures lived for 300,000 years after the crater's formation.

Volcanoes are the true cause of the mass extinction, she suggests. As the Indian subcontinent headed for its collision with Asia 55 million years ago, the crunch that created the Himalayas, an era of intense volcanic activity occurred in the region, causing worldwide climate change leading to mass extinction. The Chicxulub impact, followed by another asteroid strike several hundred thousand years later, simply added to the mass extinction woes of the times, she says. "It's a more complex story than we've been told."

Smit calls the sabotage suggestion "ridiculous." In an e-mail to USA TODAY, he said, "I was the organizer and convener of the (meeting) symposium about the crater drilling, and I would be really stupid to delay her findings, if I were to have a successful symposium!"

The post-impact microfossil layer that Keller describes is simply the remnants of mud thrown up by the tidal waves after the strike, he says.

Each team's analysis will be published in coming months.

K-T boundary issues gained steam in 1980 when Nobel-prize-winning University of California physicist Luis Alvarez and colleagues first proposed in a Science magazine report that an asteroid impact led to a fallout cloud blanketing the Earth, blocking sunlight, which with tidal waves, global forest fires and a host of other apocalyptic events caused mass extinction.

Until then, the cause of the K-T boundary had been the subject of more speculation than evidence. In the fossil record, microscopic sea creatures appear to flourish right up to the boundary, but dinosaur species started dwindling millions of years earlier, a discrepancy that still stymies many experts. Geologists noted that oceans had been shrinking, volcanoes had been active and the climate likely changing 65 million years ago. But geologists say no one had put together all those factors into a rigorous explanation that made more sense than the asteroid-impact theory.

Less likely theories about nearby exploding stars blanketing the Earth with deadly radiation or early mammals eating all the dinosaurs' eggs also were proposed. Most scientists hoped for more evidence.

Alvarez's team analyzed a layer of iridium-laced clay in 65-million-year-old Italian soil. Iridium is rare on Earth but common in space; the researchers calculated a comet or asteroid 6 miles wide likely laid down the layer worldwide. By 1990, two years after Alvarez's death, the 186-mile-wide Chicxulub crater had emerged as the likely impact site, and the theory had gained wide acceptance among geologists and many paleontologists. They expected to see conclusive traces of iridium and microscopic fossil deaths in a Chicxulub sample.

'A double whammy'

Current thinking is that although volcanoes were very active 65 million years ago, most researchers, except for dinosaur experts, believe the impact theory explains K-T boundary extinction, says Peter Ward of the University of Washington in Seattle, a paleontologist and geologist who has looked at the extinction of sea creatures at the K-T boundary. Keller's explanation seems unlikely, he says, given that intense volcanism had been occurring for millions of years before the impact with no apparent effect on fossils or geologic records.

Keller plans to continue looking for evidence of a second asteroid impact crater in the Indian Ocean. Her research suggests that volcanoes did trigger intense climate change, showing that temperatures zoomed up and then down over the 300,000 years before the Chicxulub impact. That was followed, she believes, by another impact several hundred thousand years later that led to the mass extinction. "A double whammy knocked them out," she insists. "If we could have watched the planet from somewhere else, it surely must have been an amazing time."


Though common wisdom among many scientists - and newspaper reporters - widely favors an apocalyptic asteroid ending the age of dinosaurs, one group of scientists remains decidedly undecided: dinosaur experts.

After examining surveys of researchers at various scientific meetings over the past two decades, paleontology student Keynyn Brysse of Canada's University of Alberta concludes that although most experts agree that an asteroid struck the Earth 65 million years ago, most don't agree enough evidence exists to definitively conclude that it killed the dinosaurs.

Dinosaur research belongs to the field of vertebrate paleontology, the study of fossil remains of creatures with a backbone. Researchers from this discipline - often in isolation from other scientific fields - study dinosaur fossils as well as fossils from mammals, amphibians and archosaurs, the reptilian beast that preceded dinosaurs by tens of millions of years.

Supporters of the impact theory include geologists, who study rock layers, and invertebrate paleontologists, who study the fossils left behind by shelled sea creatures. Members of those disciplines overwhelmingly agreed in surveys that an asteroid impact caused the mass extinction 65 million years ago. But echoing past surveys, about 72% of a group of Society of Vertebrate Paleontology members surveyed by Brysse this year contended that the extinction resulted from a combination of volcanic and climate effects along with the impact. Only 20% thought the impact alone killed off the dinosaurs. The remainder were uncertain about any cause.

Dinosaur experts such as William Clemens of the University of California-Berkeley have long

argued that a withering of dinosaur species is seen in the fossil record starting about 6 million years before the impact. Clemens' fossil work in Montana shows that a similar drop in diversity did not occur in mammals, even after the impact.

Other researchers find that dinosaur remains, fossilized pollen and leaves also support this interpretation. "Prior to the extinction event, the environment was changing and, we suspect, putting non-avian dinosaurs and some other groups under ecological stress," Clemens said in an e-mail.

Younger vertebrate paleontologists might be more sympathetic to the lone-asteroid theory, Brysse says, suggesting that a shift may be occurring as older researchers retire. For now, though, the jury remains out among dinosaur experts on a theory that much of the rest of the world has come to accept.

By Dan Vergano

Copyright 2003, USA Today




Look here for the Yaxcopoil-1 core segment 793.85 to 794.60 m: the transition of the impact to post-impact crater infill,

back to top