Yucatan Asteroid Didn't Kill Dinosaurs, Study Says

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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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