Yucatan Asteroid Didn't Kill Dinosaurs, Study Says

Angela Botzer
for National Geographic News
March 9, 2004
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) boundary—a geologic layer present in rock formations around the world—exhibits 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.

Fossil Evidence

But Keller and her colleagues say their research proves otherwise.

Keller has studied the Chicxulub site and other impact-crater sites around the world for the past decade. She believes that the asteroid impact behind Chicxulub coincided with a "time of massive volcanism, which led to greenhouse warming."

Keller says those three events—the Chicxulub asteroid impact, volcanism, and climate change—"led to high biotic stress and caused the decline of many tropical species populations," but not mass extinctions. That die-off didn't occur until later. However, Keller does believe that the initial confluence of volcanic activity, global warming, and the Chicxulub asteroid impact ultimately contributed to the mass extinction.

Key to Keller's assertions is a 20-inch-thick (50-centimeter-thick) layer of limestone found between the K-T boundary and the impact breccia, or molten lava and rocky debris, laid down when the Chicxulub asteroid collided with Earth.

Keller and her colleagues believe that the thickness of the limestone layer—a type of sedimentary rock characteristically formed under large bodies of water like oceans, seas, and lakes—indicates that it accumulated in the crater over some 300,000 years after the impact. As proof, Keller points to fossils of microscopic organisms called foraminifera and fossil burrows present in the limestone layer.

According to Keller, those fossils indicate the sediment was deposited after the asteroid impact but before the period of mass extinction that marked the end of the Cretaceous.

Many other scientists disagree with that interpretation, however. They say the layer of fossil-rich limestone was deposited quickly as backwash and infill caused by a huge tsunami that followed the Chicxulub asteroid's impact with Earth. The layer, they say, did not take 300,000 years to accumulate.

In her defense, Keller says the quick-accumulation theory is unsupported by evidence that would have been found during her analysis of core samples gathered at Chicxulub and 45 localities in northeast Mexico.

But Alan Hildebrand, a proponent of the quick-accumulation theory, says the burrows were "made by organisms digging after the fireball layer was deposited."

Thomas R. Holtz, Jr., a vertebrate paleontologist at the University of Maryland in College Park, supports the view that the limestone was quickly laid down as crater infill. He said he is not surprised that Cretaceous fossils were found in the limestone layer.

"If an asteroid clobbered the Eastern seaboard of the U.S. today, I would expect that most of the infilling would be Chevys and Hondas and shopping malls and houses and cows and McDonald's burger wrappers," Holtz said. "Only a tiny bit might be mastodons and Clovis points and Miocene whales." In other words, the crater would quickly fill with objects common on Earth at the time of impact.

So where do researchers in the Keller camp look next for the possible K-T crater? Keller says she's unsure, although "some scientists have suggested it could be a structure called Shiva, in India. We have no convincing evidence so far that this is the case."

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