Findings Bolster Meteorite Theory : Geology: Researchers say they have discovered first surface evidence marking the precise location of the impact that may have triggered the extinction of dinosaurs. The area is on the Yucatan Peninsula.

NASA researchers said Thursday they have found the first surface evidence marking the precise location of the meteorite impact that many scientists think triggered the extinction of dinosaurs more than 60 million years ago.

Researchers from the National Aeronautics and Space Administration’s Ames Research Center in Mountain View and the Jet Propulsion Laboratory in Pasadena have used space imagery to identify a mammoth half-circle of sinkholes on the Yucatan Peninsula of Mexico that appear to outline the largest known impact crater on Earth--one that is more than 125 miles in diameter. A variety of previous evidence had suggested that the impact might have occurred in the Caribbean region.

The discovery provides the strongest evidence that intense dust and smoke injected into the atmosphere after a large impact might have blocked off sunlight and created climatic changes that led to the extinction of the largest creatures that ever roamed the Earth, as well as at least half of the other species that lived on Earth at the time.

The theory that the impact from a comet or meteorite did in the dinosaurs has been controversial since it was promoted in 1980 by a research team from UC Berkeley headed by the late physicist Luis Alvarez and his geologist son Walter.

Not all scientists accept the theory. Many attribute the species’ demise to volcanic eruptions. Those who do accept the theory believe that identification of a crater of the right size and age will finally put an end to the dispute.

“It’s a very interesting and obviously important contribution to this controversy because people have spent the last 10 years looking for the ‘smoking gun,’ ” the crater that could have marked the site of the meteorite impact that killed the dinosaurs, said planetary scientist William V. Boynton of the University of Arizona. “But there’s still going to be a lot more coming out on it.”

Geologist Bruce Bohor of the U.S. Geological Survey in Boulder, Colo., cautioned that the pattern of sinkholes could have been caused by geological events that had nothing to do with a meteorite. He argued that more evidence will be required before it can definitely be considered an impact site and noted that “unfortunately, most of that evidence would be buried pretty deeply.”

Researchers had previously found a variety of evidence throughout the Caribbean suggesting the impact of a meteorite in the appropriate time frame. Among the evidence are fractured quartz crystals that are known to be produced only by a meteorite impact, massive disruptions in sediments dating from 65 million years ago, and magnetic and gravitational anomalies that are associated with meteorites.

But no one had seen evidence on the Earth’s surface marking the impact site of the meteorite.

The unusual pattern of sinkholes in the northern section of Mexico’s Yucatan province was discovered by Charles Duller of NASA-Ames while he was searching satellite imagery for water sources used by ancient Mayan cities.

He mapped hundreds of water-filled sinkholes that form an almost perfect semicircle, which he believes marks the crater’s buried rim. The remainder of the rim is underwater, he said. The sinkholes, which average 300 to 500 feet in diameter, are found in clusters at some places along the rim and spaced as much as a mile apart at others. The ring is centered near the town of Chicxulub, for which the buried crater is named.

Duller and his colleagues, Adriana Ocampo of JPL and Kevin Pope, who is now at Geo Eco Arc Research in La Canada Flintridge, considered many other geological explanations before concluding that the formation was caused by a buried impact crater. That conclusion was presented in a letter published in the British journal Nature on Thursday.

“The apparent age, location, and size of the proposed Yucatan impact make it one of the best candidates for the global catastrophic event,” Pope said. The meteorite that caused the crater is perhaps six miles in diameter, he added, and “would have had a devastating impact on the climate, animals and plant life of the Earth.”

The site of the crater contains large quantities of limestone.

“As the buried crater rim settles over millions of years, the rock on top slumps and cracks,” Pope said. “Underground water flows through the cracks on its way to the ocean. As the water is forced around the unfractured rock in the center, the flow dissolves the limestone, causing cave-ins that create the sinkholes.”