"Dinosaur Killer" Asteroid Only One Part of New Quadruple-Whammy Theory
Richard A. Lovett
for National Geographic News
|October 30, 2006|
Offshore of Mexico's Yucatán Peninsula is a 110-mile-wide (180-kilometer-wide) crater dubbed Chicxulub, widely believed to be the site of an asteroid impact 65 million years ago that wiped out the dinosaurs (map of Mexico).
The mass extinction, thought to have obliterated two-thirds of the world's species in total, has been dubbed the K-T extinction because it bridges the Cretaceous and Tertiary geologic periods.
But a single asteroid impact doesn't tell the whole story, says a small but vocal group of geologists led by Princeton University paleontologist Gerta Keller in New Jersey.
Keller and her collaborators believe that the Chicxulub impact predated the K-T extinction by about 300,000 years.
The dinosaurs, they say, were killed not by a lone asteroid strike but by the quadruple whammy of global climate change, massive volcanism, and not one but two gigantic collisions. (Related: "Yucatán Asteroid Didn't Kill Dinosaurs, Study Says" [March 9, 2004].)
Peeling Back the Layers
The complex scientific detective story involves tiny glass beads, the rare element iridium, and sediments that might be deposits from gigantic tsunamis kicked up by the Chicxulub impact.
Both sides of the debate agree that the glass was created when the Chicxulub event filled the atmosphere with vaporized rock that quickly condensed and rained to Earth as tiny spherules, about a tenth of an inch (1 to 4 millimeters) in diameter.
The opposing sides also agree that the iridium came from an iridium-rich asteroid.
The problem is that the spherules and iridium occur in separate layers of earth, or strata, separated by as much as 25 feet (8 meters) of intervening sediment.
Conventional theory says that the intervening sediment was laid down quickly by a series of tsunami waves created by the Chicxulub strike.
But a detailed analysis of the sediment layers suggests that the sediment was formed between two separate asteroid strikes, one that laid down the glass and the other the iridium.
Keller presented her latest findings last week at a meeting of the Geological Society of America in Philadelphia, Pennsylvania.
Signs of Life
To begin with, Keller says, the intervening sediment contains burrows from sea-bottom creatures that wouldn't have been able to dig if the seabed was being washed by violent tsunamis.
Keller's group also discovered a second layer of spherules about 13 feet (4 meters) below the disputed 25 feet (8 meters) of rock.
This strata, she says, represents the true Chicxulub spherule layer. Fossil evidence suggests that the lower layer of spherules was deposited 300,000 years before the K-T extinction. Overlying spherules were set down later, eroded from deposits elsewhere.
She has also found plankton-like fossils in the sediments above the lower-lying spherules, indicating a return to normal conditions for about 300,000 years after Chicxulub.
This suggests that Chicxulub, rather than causing widespread devastation, had essentially no long-term ecological effect, Keller says.
Geological evidence indicates that Earth was undergoing a large number of changes during the 500,000 years preceding the K-T boundary, she adds.
These included a global climate change that had slowly cooled Earth during the previous several million years. Then massive volcanism in India produced a rapid 7 to 13 degree Fahrenheit (4 to 7 degree Celsius) warming. (Related: "Did Huge Volcanic Blasts Snuff Out Dinos?" [August 23, 2005].)
The Chicxulub asteroid hit about a hundred thousand years after that, but the warming continued for another hundred thousand years, she says, until the Earth suddenly cooled again.
Then, she says, this already-stressed biota was hit by the second asteroid impact that produced the iridium layer. That was the last straw, culling out all but the species that were already adept at adapting to rapid climate fluctuations.
Keller's coworker, Thierry Adatte of the University of Neuchatel in Switzerland, adds that multiple asteroid impacts aren't as rare as people might think.
"We have found proof of a third one 200,000 years after the K-T boundary," he said. "We have a second iridium layer."'
In addition, he says, mass extinctions such as the one at the K-T boundary appear to require multiple causes.
"You really need a bad combination, like volcanism, climate change, and sea level fluctuations to have a major impact," he said.
But Keller's theory remains highly controversial.
"The whole issue hinges on the interpretation of the 'event deposits,'" sediment scientist Jan Smit of Vrije Universiteit in the Netherlands said by email.
"I think all evidence points to a single Chicxulub impact event that has caused tsunamis.... All the different sublayers ... are the results of the complicated tsunami waves' surges and associated currents."
In studying the impact's relationship to the K-T extinction, Smit adds, geologists need to look at regions distant from the Gulf of Mexico, where the geology hasn't been so strongly influenced by the direct effects of the impact.
"If you look elsewhere," he wrote, "only one impact can be documented."
But Princeton's Keller isn't likely to be deterred.
"This is more religion than anything else," she says. "A lot of people are so wedded to [the Chicxulub] theory that it seems that no evidence can ever convince them of anything else."
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