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"CHICXULUB IMPACT CRATER NOT RELATED TO K/T MASS EXTINCTION, REVISITED"
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(7) CHICXULUB IMPACT CRATER AND THE K/T MASS EXTINCTION, REVISITED
Jan Smit <smit@geo.vu.nl> "Chicxulub crater not related to the K/T mass-extinction??" (CCNet, 26 September 2003; http://abob.libs.uga.edu/bobk/ccc/cc092603.html)
That would be a major claim indeed, if it were true. Familiar terms are used like "mounting evidence", "growing number of scientists", where the "evidence" and "scientists" are overwhelmingly from her own (Gerta Kellers) research group itself. However, the "evidence" that purports to show that the Chicxulub crater is 300.000 years older than the K/T boundary, is exclusively based on resedimented coarse grained deposits in the vicinity of the crater, i.e. the Gulf of Mexico, that have been influenced by tsunami waves, mass-wasting and slumping and earthquakes triggered by the impact itself. Therefore, in all these places there is a sedimentological explanation for the doubling, tripling of layers with ejecta and iridium, creating the illusion of multiple impacts. The mechanisms mentioned above make the deposits very complicated indeed, including the sediments inside the crater. Among the cited evidence for a Chicxulub crater about 300kyr older than the KT boundary, is the occurrence of a so-called normal sedimentary layer with Cretaceous foraminifers above the ejecta of the Chicxulub crater. If that were correct, there would be indeed strong evidence that the crater is older than K/T. However, these "normal" sediments contain either already Tertiary fossils, and are therefore Tertiary in age, or are not "normal" sediments but part of the coarse grained deposits related to the impact itself, because they display grainsize grading. A new element in the discussion are the results of the new Yaxcopoil-1 drilling inside the Chicxulub crater, where Gerta Keller claims that also there is evidence that the Cretaceous period persisted for 300kyr after the Chicxulub impact. In this core the Chicxulub ejecta occur from 894-794.70m. The ejecta are overlain by a succession of 51cm of crossbedded dolomitic sands (794.70-794.19m), an 8cm thick hardground (794.19-794.12m), 2cm of clay (794.12-794.10m), and finally, the post impact infill of fine-grained sediments, that contain undisputed Tertiary foraminifers (794.10-404m). The cited evidence comes from the core segment just above the ejecta, that consists of cross-bedded and parallel-bedded sands (794.70-794.19m). Samples of the same interval were split in two. One part was distributed to Gerta Keller, and the other part to Jose Arz and colleages from Zaragoza, Spain. I myself obtained samples adjacent to these samples that were analysed in Amsterdam. Neither the Zaragoza group, experienced micropaleontologists, nor I were able to find any determinable foraminiferal remains in any of these samples. Instead, we found in thin sections exclusively rhomb-like idiomorphic dolomite overgrowths of the sand grains. The rhombs resemble in size and thickness somewhat the testwalls of foraminifers. The "Bombshell results" presented by Gerta Keller at the EGS-AGU meeting in Nice are based on such dolomite overgrowths. The results of the three groups working on these samples will be published in the MAPS special volume on the Yaxcopoil-1 drilling. Discussions to be continued. But even if the foraminiferal fossils were missed by the Zaragoza and Amsterdam groups, they would not permit any conclusion about the age of the crater. Cross- and parallel beds tell any sedimentologist that such sediments are deposited by currents or waves, and that all grains in those beds, including foraminiferal shells, are transported from another source. The grains could be washed in from inside the crater, from the rim or the direct surroundings of the crater, and may therefore be much older or younger than the crater itself. In other words, it is impossible to tell whether the foraminifera in that core-segments are contemporaneous, older or considerably younger than the crater. What we do know, however, is that the base of the Tertiary, including the iridium rich clay, is missing in the drill core, because the part of the magnetochron that represents this period, Chron 29R, is only 3-11cm thick in the Tertiary, where the same interval is 1.5m thick in the Gubbio (Italy) and over 5.1m in the Caravaca (Spain) sections. Where we should be looking for multiple impacts at or near the K/T boundary, is far away from the impact and the resulting energetic events that influence the sedimentary records there. It is therefore laudable that Gerta Keller plans to test the record of the latest Cretaceous for evidence of volcanism, impacts and change of biota in the Indian Ocean. However, if we look critically at the most complete records known today (i.e. El Kef in Tunisia, Agost, Zumaya and Caravaca in Spain, and the Apennine sections in Italy), there is not the slightest undisputed evidence for multiple impacts. So what and where is the best evidence that ties the Chicxulub impact to the K/T boundary extinctions? In my opinion those are the impact layers laid down in quiet coalswamps in the US and Canadian western interior. Those layers are found over a wide area ranging from Alberta; Hell Creek, Montana; Dogie Creek, Wyoming; and Raton Basin, Colorado-New Mexico. Glenn Izett, Bruce Bohor and colleagues have shown that in those areas there is a single claylayer, composed of two parts. The lower sublayer is filled with spherules that are identical to the glassy spherules from around the Gulf of Mexico. Ar/Ar age dating, chemical and isotopic composition of the glass show that, barring a miracle, those spherules are derived from the Chicxulub impact. The upper sublayer is invariably enriched in iridium (according to Gerta Keller the fingerprint of the "unknown" K/T boundary impact) and shocked minerals. Additionally, the upper sublayer contains shocked zircon crystals that tie the sublayer to the Chicxulub target (panafrican) rocks. If Gerta Keller would be correct, than the lower and upper sublayers would be separated in time by about 300kyr. In reality, the two sublayers are not even separated by a single season of falling leaves, in all the localities mentioned above, and both comtain evidence linking them to Chicxulub. I therefore still think the evidence overwhelmingly shows that the K/T boundary impact and the Chicxulub impact are one and the same. Dr. J. Smit Department of Sedimentology Faculty of Earth and Life Sciences Vrije Universiteit de Boelelaan 1085 1081HV Amsterdam the Netherlands tel: +3120-4447384 fax: +3120-6462457 e-mail: smit@geo.vu.nl http://www.geo.vu.nl/~smit |
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CCNet 77/2003 - 26 September 2003 "CHICXULUB IMPACT CRATER NOT RELATED TO K/T MASS EXTINCTION" -------------------------------------------------------- [Gerta] Keller has accumulated evidence 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.... 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. --Steven Schultz, Princeton University, 25 Sept. 2003 |
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(2) WHAT KILLED THE DINOSAURS? GERTA KELLY REKINDLES K/T CONTROVERSY
Princeton University Weekly Bulletin, 25 Sept. 2003 http://www.princeton.edu/pr/pwb/03/0922/
Dinosaur dust-up: 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 timeUnlike 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.
Softening opposition 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. |
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(3) I'VE GOT A BONE TO PICK WITH YOU, SAY FEUDING DINOSAUR EXPERTS The Observer, 7 Sept. 2003 http://observer.guardian.co.uk/international/story/0,6903,1037002,00.html Robin McKie, science editor The world's biggest bang wiped out the dinosaurs in a cataclysm that swathed our planet in choking dust - or at least that is what many palaeontologists claim. Others say dinosaurs died out gradually as Earth's climate and geology changed. It sounds a typical academic dispute - but last week it erupted into open warfare. Allegations have been made of deceit and unethical behaviour. One scientist is even alleged to have held back inconvenient evidence. 'This affair has become an object lesson on how partisan and unethical the whole dinosaur controversy has become,' said Dr Norman MacLeod, keeper of palaeontology at London's Natural History Museum. 'Young scientists are now refusing to get involved in this field because no matter what they say it will offend someone and dam age their careers. It's like the nature-nurture debate. No matter what you say, someone will hate you for it.' The furore focuses on a massive drilling project set up to study the Chicxulub crater in Yucatán. Buried under half a mile of rock, the crater was created 65 million years ago when Earth was hit by a meteorite 10 miles in diameter. The blast would have blotted out the sun for decades, or even centuries, many researchers claim. Given that around this time the dinosaurs became extinct, many scientists made a direct link. Denied sunlight and food, most of the world's animals would have starved, and choked, to death. But others disagree. Volcanoes, global warming or sea level changes were responsible, they say - pointing to evidence that most dinosaurs became extinct before the explosion and to the fact that many large animals such as alligators survived this alleged catastrophe. Things weren't that bad, they say. In a bid to resolve the dispute, a £2 million project was launched in Yucatán two years ago. Researchers drilled a pipe into the Earth's crust to bring back samples of the meteor and crater wall. By studying what happened just before and just after the meteorite impact, scientists would glean critical insights, it was argued. For example, it would show if all life was extinguished in the millennia that followed the impact. In 2002 the first samples were brought up. To the disgust of Mexican geologists, and to many scientists who doubted the Big Blast theory, these were entrusted to Jan Smit, a geologist at the Free University of Amsterdam and a leading supporter of the meteorite hypothesis. Promising to cut up the samples and distribute them to project scientists, Smit left with the precious Chicxulub remains. A year later, many scientists were still seeking the promised samples. 'We were dismayed,' geochemist Erika Elswick of Indiana University in Bloomington states in the current issue of Nature . 'There was no explanation given, no apology.' Eventually some samples were sent out, but most were too small for experiments. Dismay turned to fury. Researcher Gerta Keller, of Princeton University, pressed Smit and at last got a good set of samples. At the European Union of Geosciences conference in Nice, she presented her results, which were a bombshell. Her research, Keller claimed, clearly showed that marine plankton, far from being killed off by debris blotting out the sun, thrived for hundreds of thousands of years after the crater was created. The meteor that struck at Chicxulub was not responsible for mass extinctions, she concluded. Nor is Keller reticent in her interpretation of Smit's behaviour. 'He tried to postpone our results so that he could remain unchallenged at that meeting,' she states in Nature . Smit dismisses the allegation as 'ridiculous'. He blames the delays on his busy schedule and poor communications by those running the project. He also claims Keller misidentified some fossils in her samples. The row is far from over. Project scientists are preparing papers containing results of studies of the samples they obtained from Smit and these will be published in a special issue of Meteoritics and Planetary Science next year. Few doubt it will resolve the issue. As MacLeod says: 'It's no longer about science. It's about reputations.'
Copyright 2003, The Observer |
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