Thursday, March 3, 2011

A Theory Set in Stone: An Asteroid Killed the Dinosaurs, After All
A single asteroid impact near the Yucatan remains the best explanation for the massive Cretaceous-Paleogene extinction, scientists conclude in a new, deep review
By Katherine Harmon  | Thursday, March 4, 2010 | 93

DEEP IMPACT: Evidence from around the globe seems to point to a massive asteroid impact as the cause of the major extinction event on earth 65.5 million years ago. That rock was likely about 10 kilometers across and created a crater more than 100 kilometers wide.

Although any T. Rex–enthralled kid will tell you that a gigantic asteroid wiped the dinosaurs off the planet, scientists have always regarded this impact theory as a hypothesis subject to revision based on further evidence gathered from around the globe. Other possible causes, such as volcanism and smaller, multiple asteroid strikes, never actually went away, and over the years researchers raised important points that did not fully jibe with a history-changing celestial impact near the Yucatan peninsula one awful day some 65.5 million years ago.

A group of 41 researchers have pored over the evidence and decided that—in accordance with the original postulate put forth 30 years ago by a team led by father and son researchers Luis and Walter Alvarez—it was, indeed, a massive asteroid that slammed into Earth, creating Chicxulub Crater on Mexico's Gulf Coast, that killed off many of the species on the planet, including the non-avian dinosaurs.

The review, published online March 4 in Science, evaluated the whole picture, according to Kirk Johnson of the Research and Collections Division at the Denver Museum of Nature and Science and co-author of the paper. And that meant assessing the other theories that have been put forth about what spelled death for the dinosaurs.

Fiery failures
The researchers dismiss the theory that the volcanism that produced the great Deccan Trap formation in western India at the end of the Cretaceous period might have produced enough sulfur and carbon dioxide to initiate a massive shift in climate. They note that pinpointing the times when the heavy volcanism occurred is sketchy, and it likely kicked off some 400,000 years before the extinction event. In fact, as Johnson noted in a March 3 conference call with reporters, the emissions from these volcanoes likely warmed the planet slightly, actually making life easier for many animals and encouraging diversification and dispersion over wider geographical areas.

Some scientists have pointed to multiple layers of impact residue as evidence that there was more than one asteroid involved in generating the extinction. This theory did not seem to measure up, either. Johnson says they see "no evidence for multiple impacts," and sites that had turned up these various layers were so close to Chicxulub itself that the chaotic event likely churned the layers into different locations in the sediment.

An assertion that the impact occurred hundreds of thousands of years before the extinctions also failed to hold water with the researchers. Evidence of Cretaceous period shells on top of the impact crater are likely not a sign that the animals persisted after the impact, but rather that they got "washed into the hole," Johnson noted.

Global ground zero
The researchers assessed reports from some 350 sites all over the globe that had evidence of the impact—whether it was a dusting of iridium (an element much more common in extraterrestrial objects) or bits of shocked quartz—and could be traced back to the Chicxulub location. In some areas near the crater, the layer was 80 meters thick, pointing to one single devastating day for life on the planet.

"That's the single best explanation for the extinction of so many groups," says Neil Landman, a curator at the American Museum of Natural History in New York City and was not involved in the review, about the single impact theory.

"We've examined sites around the world," he notes of his study of ammonoids, which are shelled cephalopods that went extinct after the Cretaceous. And from the work he and his colleagues have done, he says, the evidence for the Chicxulub asteroid impact is the most consistent. "I'm very comfortable with this explanation."

A massive blow
Based on the size of material from rocky shrapnel and the crater diameter, researchers have estimated the dino-demolishing object to be some 10 kilometers across. And when it struck—at about 20 kilometers per second—it created an instant crater about 100 kilometers wide and 25 to 30 kilometers deep "almost piercing the crust of the Earth," Johnson noted. The final crater that formed after the initial impact was about 180 kilometers across and two kilometers deep, which is still close to the depth of the Grand Canyon, Johnson pointed out.

The impact spewed rock so high, some of it likely was shot into orbit, whereas other pieces entered the upper atmosphere, reheating as they fell back to the ground. The jolt would have spurred massive earthquakes—some surpassing magnitude 11—tsunamis and landslides. While examining ammonoid fossils in southeastern Missouri, Landman says, he found a shallow water site that was "just immediately covered over by a jumble of stuff," he says. "I think what we're seeing is a tsunami," which might have reached as far from the Yucatan impact site as southern Illinois.

Perhaps most devastating, however, the crash would have caused acid rain and darkness, as particulate matter blocked sunlight, prohibiting photosynthesis in both land and water ecosystems, effectively shutting down large swaths of the food chain. Directly after the extinction event, ferns (which reproduce from spores) proliferated and species that depended on detritus seemed to survive.

From Landman's study of ammonoids, he points out that even for groups that eventually went extinct after the collision (producing the so-called K–T boundary in the fossil record), the asteroid's impact did not mean sudden eradication. "There seems to be some suggestion of some survival for awhile after the event," he says. Fossils found above the iridium layer show that ammonoids might have survived "for tens to possibly hundreds of years afterward" perhaps because "things in the marine realm were a little more insulated," he explains. 

Although these estimates might seem rough for such a dramatic event, revealing details on the resolution of years and months "was unimaginable" in decades past, he says. "It's one of the best studied intervals of the geologic record," he notes. And all of this attention has led to increasing nuance in the timeline.

"This is not geologic time—this is instant time," Johnson said, acknowledging that it is a very tricky task to pin down a single event from 65.5 million years ago. But, judging from the chemical, geochemical and geochronological evidence, he said, "The Chicxulub Crater really is the culprit."

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