The giant meteor that crashed off the coast of Mexico around 65 million years ago hit the Earth too early to explain the dinosaurs' demise and was too tame to even hurt a protist, according to a new study published today (Apr. 27) in the Journal of the Geological Society. Some researchers, however, maintain that the study's authors are needlessly wishing away the shooting star.

Image: NSF/Zina Deretsky

The Chicxulub crater in Mexico's Yucatan peninsula has been hailed as the smoking gun that proves the theory that an asteroid impact exterminated around two-thirds of all living species, including dinosaurs, at the end of the Mesozoic era. But "we can show now for the first time that it did not cause mass extinction," Gerta Keller, a Princeton University paleontologist and geologist, told The Scientist. "It didn't kill the dinosaurs. In fact, it didn't cause much damage that we can determine from the geological record."

Keller and her colleagues studied sandstone sediment in Northeastern Mexico, around 600 km from the impact crater, where ejectile debris from Chicxulub has been found. First, they characterized the deposits and showed that the asteroid impact predated the mass extinction at the Cretaceous-Tertiary (K-T) boundary by around 300,000 years. This was not in itself a new finding -- Keller previously found similar evidence in Texas and in a core drilling from the crater itself. But when she inspected the sediments more closely for the fossil remains of tiny protists she found what she considers the "nail in the coffin" to the asteroid-impact theory.

Keller measured the diversity of the abundance of marine zooplankton called planktic foraminifera and showed that all 52 species that she found below the ejected Chicxulub debris were still present in the fossil record after the asteroid impact. In contrast, only 13 out of 44 species remained in the geological record following the K-T mass extinction, demonstrating that the earlier asteroid impact did not polish off the protists, and, by extension, the dinosaurs, either. In a paper published in the January issue of Palaeogeography, Palaeoclimatology, Palaeoecology, Keller also reported similar effects on species extinction at a shallower, interior seaway in Texas.

"This is definitive proof that Chicxulub could not have caused the mass extinction," said Keller. "It's just not possible with all the evidence that has come in." These finding should come as no great surprise, she added. "None of the mass extinctions in the history of the Earth were caused by impacts... but a whole bunch of them are closely associated with massive volcanic eruptions."

Instead of a single devastating impact event that wiped out most of life in a geological instant, Keller said that gigantic volcanic eruptions at the Deccan Traps in India, possibly in conjunction with other subsequent meteor impacts, probably wrought the environmental destruction and climate change that eventually pushed most species over the edge.

Other researchers, however, remain unconvinced. "[Keller] is making a mountain out of a molehill," said Luann Becker, a Johns Hopkins University geochemist. "To me, it's really quibbling. When you think about geological time, even a few hundred thousand years isn't a long time and you can argue that there are some pretty big error bars on that [date estimate]." Becker argues that both the Chicxulub impact and the Deccan volcanism together facilitated the demise of life on Earth. "[Keller] seems to want to decouple the two, but unquestionably these things are linked."

Ralph Von Frese, a geophysicist at Ohio State University in Columbus, said that the Declan eruptions "could have been energized or initiated by the impact." What's more, the oceans might have buffered the effects of the environmental destruction triggered by the asteroid impact, he noted. Thus, the marine protists might not have gone extinct right after the asteroid impact, which could explain Keller's findings. "It's a very imperfect [geological] record," he said. "I don't know that [this study] settles the issue one way or another."


Related stories:

Mass extinction effect on mammals disputed
[28th March 2007]

'Biotic revenge' and the death of dinosaurs
[26th January 1987]