At Last, the Smoking Gun?

If a comet did in the dinosaurs, where is the giant crater left by its impact? The answer may lie on the coast of the Yucatan Peninsula.

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As the quest for the telltale crater intensified in the middle 1980s, William Boynton, a professor of planetary science, and graduate student Alan Hildebrand, both of the University of Arizona, wondered if the boundary clay might also help reveal the site of the impact. Measuring the content of rare earth elements in samples of the clay, they determined that it contained both the basaltic rock found in the ocean floor and a lesser amount of continental rock. Their conclusion: the comet had hit on the edge of an ocean basin.

So great an impact in water must have produced monstrous seismic waves, perhaps as great as 5 km (3 miles) high, that raced across the waters, tearing up the bottom sediments and sweeping rocky debris inland. Searching through scientific literature, they uncovered reports of chaotic mixes of large rocks at the 65-million-year boundary level in Texas, Mexico, Cuba and northern South America, but none anywhere else. This suggests, says Hildebrand, "that the comet hit somewhere between North and South America."

Scientists also reasoned that the thickest layers of ejecta -- rocks that fell back to earth after the impact -- would be found closer to the Crater. Investigating one suspected ejecta layer in Haiti early in 1990, Hildebrand and another Arizona colleague, David Kring, found tektites, teardrop-shape pieces of glass formed when molten rock is splashed high into the atmosphere and solidifies on its way back down. To the Arizona scientists, the tektites suggested that the impact had occurred no more than 1,000 km (622 miles) away.

A few months later, Hildebrand learned of a report made a dozen years earlier by Glen Penfield, a geophysicist who had surveyed the Yucatan Peninsula for Pemex, the Mexican national oil company. Studying both magnetic and gravity measurements, Penfield and his Pemex supervisor, Antonio Camargo, had discerned a huge circular basin buried under the peninsula and suspected it might be an impact crater. Their report was largely ignored.

Seeking out Penfield, Hildebrand teamed up with him in a search for samples of material brought up in old oil-drilling operations in the vicinity of the basin. Analyzing a few core samples, Kring discovered compelling evidence that the basin is an impact crater. Most convincing are crystals of quartz with striations that could only have been caused by powerful shock waves stemming from a great impact, as opposed to, say, from volcanic action. Finally, the dating of nearby fossil evidence has narrowed the crater's age to within 5 million years of the Great Extinction.

Unexpected confirmation of the crater site has come from a team of scientists led by Charles Duller at NASA's Ames Research Center. While examining satellite photographs of the Yucatan in the mid-1980s, the NASA scientists were intrigued by a strange semicircle of sinkhole lakes on the northern tip of the peninsula. The Chicxulub discovery could provide an explanation. Reporting in Nature magazine, the NASA team proposes that the lake pattern developed as the buried crater rim gradually collapsed, producing depressions in the overlying limestone that were filled in by groundwater.

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