Sharpening the case that a meteor impact wiped out the dinosaurs

About 65 million years ago, dinosaurs and other species died out in a geological eye blink. Many scientists attribute the disappearance to environmental changes precipitated by a meteor slamming into the Yucatán Peninsula of Mexico. But other factors—volcanic eruptions, for example—may have played an important role. Unraveling the principal cause of the extinction has remained problematic in part because the best that geochronological evidence could do was to place the meteor impact and mass extinction within a few hundred thousand years of each other. Indeed, some data indicate that the impact postdated the extinction. Now a multinational research group led by Paul Renne of the Berkeley Geochronology Center and the University of California, Berkeley, has used radionuclide dating with argon isotopes to establish that to within the 30 000-year uncertainty of the measurement, the two events occurred simultaneously, 66.04 million years ago. To date the meteor strike, Renne and company analyzed numerous tektites, glassy droplets created by the meteor’s energetic impact. To determine the time of the extinction, the group studied clays obtained from coal-bed strata such as the ones in Montana indicated in the figure; the strata were at or near the level where clues such as pollen changes indicate a shift from the dinosaurs’ Cretaceous period to the following Paleogene period. The million years before the mass extinction saw numerous episodes of significant global cooling. The associated stress, suggest the researchers, may have made Earth’s ecosystem particularly susceptible to the environmental havoc wrought by the meteor’s devastating strike. (P. Renne et al., Science 339, 684, 2013 .)—Steven K. Blau