Giant crater linked to mass extinction.

Earth scientits have moved within one step of gaining a conviction in nature's grandest murder case: the mass extinction that closed the age of the dinosaurs 65 million years ago. Researchers have determined that a large crater-like structure in Mexico dates from this exact time, all but sealing the case that a mammoth meteorite or comet slammed into Earth and wreaked havoc at the boundary between Earth's Cretaceous (K) and Tertiary (T) periods.

"This lets us go on to a new phase in the whole program of research," says Walter Alvarez, a geologist at the University of California, Berkeley, who participated in the new study and was part of the team that originally proposed the idea of an impact at the K-T boundary. "It should allow us to stop arguing about whether there was or was not an impact at precisely the time of the extinctions. This essentially ties that down."

Since the late 1970s, geologists have found numerous signs of an extraterrestrial impact at the K-T boundary, but until recently they had failed to locate the most important clue: a large crater of the right age. In the last two years, investigators have focused on a buried circular feature in northern Yucatan, under the town of Chicxulub (SN: 1/25/92, p.56). With a diameter of 180 kilometers, the proposed crater is the largest known on Earth. But questionable sedimentary evidence suggested that the structure was too old.

In the Aug. 14 SCIENCE, geochronologist Carl C. Swisher III of the Institute of Human Origins in Berkeley, Calif., and his colleagues report that rocks from inside the Chicxulub circle formed exactly 65 million years ago and are the same age as impact debris found around the Caribbean/Gulf of Mexico region. To date the rocks, Swisher's group used a radiometric technique that relies on the radioactive decay of potassium-40 to argon-40 over millions of years.

Swisher and his co-workers dated rock samples collected in the 1950s when PEMEX, the Mexican national oil company, drilled into the Chicxulub structure, which lies beneath a kilometer of sedimentary rock. Because a warehouse fire years ago destroyed much of the rock collected from that drilling project, most U.S. researchers thought it would be impossible to find a sample suitable for radiometric dating. But several Mexican scientists had saved pieces of the core.

Like a central piece in a puzzle, the date for Chicxulub pulls together many disparate clues in the K-T mystery. Geologists have found several signs that the K-T crash occurred in the Caribbean/Gulf of Mexico region. In particular, researchers in that area recently found glassy tektite rocks, formed when melted rock is sprayed into the air and then quenched as it falls. When Swisher dated tektites from Haiti and the Mexican mainland, he discovered that their age was identical to that of rocks from the Chicxulub structure. Along with previous work that shows similarities in chemical composition, the new dating study strongly supports the idea that an impact at Chicxulub created the Mexican and Haitan tektites, Swisher and his colleagues say.

Before they truly claim a conviction, researchers must resolve why sedimentary records collected by PEMEX indicate the Chicxulub structure is older than K-T age, says Gerta Keller, a paleontologist at Princeton (N.J.) University. They must also confirm the widespread suspicion that the buried circle is indeed a crater. A drilling project can answer both of those questions.

Geoscientists can then start examining how the Chicxulub crash affected life around the globe. While impact supporters believe that cataclysm can explain the entire extinction story, others believe the impact was only one of many disasters that befell life at the time, wiping out many species in addition to the last remaining dinosaurs (SN: 2/1/92, p.72). The list of remaining suspects includes climate change, a drop in global sea levels, massive volcanic eruptions, and other impacts. Geoscientists are currently drilling into a small impact crater in Iowa to determine whether it also formed precisely at the K-T boundary. Other candidate craters exist in Alaska and Siberia.