Giant Crater Linked to Mass Extinction
Monastersky, Richard, Science News
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. …