The Chicxulub impact may, in fact, have been the lesser and earlier of a series of meteors and volcanic eruptions that pounded life on Earth for more than 500,000 years, say Princeton University paleontologist Gerta Keller and her collaborators Thierry Adatte from the University of Neuchatel, Switzerland, and Zsolt Berner and Doris Stueben from Karlsruhe University in Germany. A final, much larger and still unidentified impact 65.5 million years ago appears to have been the last straw, exterminating two thirds of all species in one of the largest mass extinction events in the history of life. It's that impact ?not Chicxulub ?which left the famous extraterrestrial iridium layer found in rocks worldwide that marks the impact that finally ended the Age of Reptiles.
"The Chicxulub impact could not have caused the mass extinction," says Princeton University paleontologist Gerta Keller, "because this impact predates the mass extinction and apparently didn't cause any extinctions."
Keller is scheduled to present that evidence at the Annual Meeting of the Geological Society of America in Philadelphia, 22-25 October. The results of her research, which was funded by the National Science Foundation, will be discussed in two technical sessions and a public lecture sponsored by the Philadelphia Geological Survey.
Marine sediments drilled from the Chicxulub crater itself, as well as from a site in Texas along the Brazos River, and from outcrops in northeastern Mexico reveal that Chicxulub hit Earth 300,000 years before the mass extinction. Small marine animal microfossils were left virtually unscathed, says Keller.
"In all these localities we can