Web Extra Friday, March 5, 2004

Extinction debate continues

In this week's edition of the Proceedings of the National Academy of Sciences, Gerta Keller of Princeton University and colleagues published findings that the Chicxulub impact off the Yucatan coast in Mexico is not the "smoking gun" that killed the dinosaurs 65 million years ago (see Geotimes, January 2004). Based on research from a core drilled from the crater from late 2001 to early 2002, Keller suggests the impact event preceded the mass extinction event by 300,000 years. She presented similar results last April at the EGS-AGU-EUG Joint Assembly in Nice, France.

Keller's research is based on the finding of 50 centimeters of layered limestone in the crater that she says postdated the impact but predated the extinction event and accumulated over hundreds of thousands of years. She says she found foraminifers that survived the initial extraterrestrial impact, which proves that the Chicxulub impact could not have caused the ultimate demise of 75 percent of all species and 50 percent of all genera, including the dinosaurs, at the end of the Cretaceous. It is more likely, Keller and co-authors write, that a more complex series of events caused the mass extinction, such as an additional asteroid impact, extensive volcanism and greenhouse warming.

However, over the past 25 years most scientists have come to believe that Chicxulub is what caused the mass extinction. Keller is "isolated in her views and not at all representative" of what most of the scientific community believes about the Chicxulub impact and the Cretaceous-Tertiary extinction event, says Peter Ward, a paleontologist at the University of Washington.

Jan Smit of the Free University in Amsterdam has been studying foraminifers for years, and as a principal investigator on the Chicxulub Scientific Drilling Program (CSDP), he performed extensive tests on the cores. He said in an interview in November that Keller's "foraminifers" are not organic at all. They are actually dolomite rhombs (minerals) that mimic foraminiferal testwalls — not a "stunning find," he said.

Dieter Stöffler, a planetary geoscientist from Humboldt University in Berlin and another CSDP principal investigator, also said in a November interview that even if Cretaceous microfossils were found in the crater core, he would not be surprised. When the Cretaceous sediments were lambasted with the extraterrestrial projectile, he explains, much of the sediments that were blown into the air would settle back into the crater. "Keller doesn't take that into consideration," he said.

Ward agrees, saying, "a more parsimonious explanation is that the so-called 'deep water deposits' were redeposited within weeks after the impact — not 300,000 years." Although Keller is an expert in foraminifers, he says that in this case her conclusions are faulty. Additionally, he says, if there were multiple impact events, there should be impact debris deposits all over the world — "and there are not."

Scientists studying Chicxulub and the mass extinction at the Cretaceous-Tertiary boundary admit that this is an acrimonious topic, the details of which are doubtful to be resolved any time soon.

Megan Sever


"Unraveling the Chicxulub Case," Geotimes, January 2004
"Charcoal clues in dinosaur debate," Web Extra, Geotimes, January 2004

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