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 analyze the marine microfossils in the sediments directly above and below the Chicxulub impact layer and cannot find any significant biotic effect," said Keller. "We cannot attribute any specific extinctions to this impact." No one has ever published this critical survival story before, she said. Keller's research was funded by the National Science Foundation.
The story that seems to be taking shape is that Chicxulub, though violent, actually conspired with the prolonged and gigantic eruptions of the Deccan Flood Basalts in India, as well as with climate change, to nudge species towards the brink. They were then shoved over with a second large impact.
The Deccan volcanism did the nudging by releasing vast amount of greenhouse gases into the atmosphere over a period of more than a million years leading up tothe mass extinction. By the time Chicxulub struck, the oceans were already 3-4 degrees warmer, even at the bottom, she says.
"On land it must have been 7-8 degrees warmer," says Keller. "This greenhouse warming is well documented. The temperature rise was rapid, over about 20,000 years, and it stayed warm for about100,000 years, then cooled back to normal well before the mass extinction."
Marine species at the time suffered from the heat. Most adapted to the stress conditions by dwarfing, growing less than half their normal size and reproducing rapidly with many offspring to increase the chances for survival. The Chicxulub impact coincided with this time. By the time climate cooled back to normal, most tropical species were on the brink of extinction. Then the second large impact hit and pushed them over the brink ?many straight to extinction.
As for how the dinosaurs were affected, that's a bit harder to say specifically, since dinosaurs did not leave a lot of fossils behind to tell the tale.
"Dinosaur fossils are few and far between," Keller said. "People love the dinosaurs but we can only really study wha t happened to them by looking at microfossils because these little critters are everywhere at all times. In just a pinch of sediment we can tell you the age, the prevailing climate, the environment in which it was deposited and what happened. It's remarkable."
What the microfossils are saying is that Chicxulub probably aided the demise of the dinosaurs, but so did Deccan trap volcanism's greenhouse warming effect and finally a second huge impact that finished them off. So where's the crater?
"I wish I knew," said Keller. "There is some evidence that it may have hit in India, where a crater of about 500 kilometers in diameter is estimated and named Shiva by paleontologist Sankar Chatterjee from the Museum of Texas Tech University in Lubbock. The evidence for it, however, is not very compelling at this time."