for National Geographic News
Sixty-five million years ago, a city-size asteroid slammed into what is now Mexico's Yucatán Peninsula at a site known as Chicxulub (pronounced CHEEK-shoo-loob). The impact left a crater 110 to 170 miles (180 to 280 kilometers) wide and spewed massive volumes of ash into Earth's atmosphere.
Did that cataclysmic event trigger the extinction of the dinosaurs and 70 percent of the world's other species? For over a decade, most scientists said yes.
But authors of a controversial new study published in the Proceedings of the National Academy of Sciences (online edition) contend that the asteroid behind the Chicxulub crater impacted Earth 300,000 years earlier than previously thought. They say a second, as yet unidentified asteroid impact must have caused the mass extinction popularly attributed to the Chicxulub asteroid.
Princeton University professor of geosciences Gerta Keller led the study, which analyzed new core samples drilled at Chicxulub. The drilling was "done with the express purpose to solve the ongoing controversy of what killed the dinosaurs and prove once and for all that this is the impact that caused the mass extinction," Keller said.
However, Keller said close examination of layers in the core samples shows that the prevailing theory that the Chicxulub asteroid killed the dinosaurs "seems to be wrong."
"The Chicxulub impact hit Yucatán about 300,000 years before the mass extinction. Another impact occurred at the time of the mass extinction," she said.
While asteroid impacts played a role, Keller says several hundred thousand years of "massive volcanic eruptions" contributed to climatic changes that precipitated the mass extinctions that marked the end of the Cretaceous period.
Tracking the Smoking Gun
The single-asteroid-impact extinction theory first appeared in 1980. At the time father and son scientists Luis and Walter Alvarez produced a daring hypothesis: An asteroid or meteorite may have struck the Earth, triggering mass extinctions that drew the curtain on Cretaceous period.
The pair based their theory on research which found that the K-T (Cretaceous-Tertiary) boundarya geologic layer present in rock formations around the worldexhibits a thin layer of clay rich in iridium. Rare on the Earth's surface, the element is more commonly found in extraterrestrial sources like asteroids, or deep inside the Earth's core.
That iridium-rich layer the Alvarezes described was later found in multiple K-T localities around the world. The race was on to find the smoking gun: the crater impact site itself.
Alan Hildebrand, an associate professor at the University of Calgary, Canada, took the lead in 1990, discovering the impact crater in Yucatán. The crater appeared to be a probable source for the iridium and seemed to be the impact site of the asteroid that destroyed the dinosaurs. Using boreholes and geophysical evidence, scientists found the crater buried under sediments dating to the Tertiary period, the era between 65 and 1.8 million years ago, which followed the Cretaceous.
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