In The Field

GSA 2009: Younger Dryas impact criticised again

Posted on behalf of Rex Dalton

The fast-sinking idea that a comet struck North America 13,000 years ago, killing off the Clovis Paleo-Indians and triggering the Younger Dryas cold snap, was again undermined by geochemical tests released this week.

A US-Belgium team has reported that it can find no evidence of a comet impact at seven sites and in sediments in two ocean cores corresponding to when the object reportedly exploded over the North American ice sheet. Francois Paquay, a doctoral student at the University of Hawaii in Honolulu, reported the team’s results at the annual meeting of the Geological Society of America in Portland, Oregon.

There was no rise in iridium at the Clovis sites, no change in other platinum group element levels that would be altered by an impact, and no concurrent iridium rise in cores from the Guaymas Basin in the Gulf of California and the Caribbean Sea south of Tortuga, he said.

“There is nothing; there was no impact,” says Paquay, whose co-authors are Philip Claeys, a prominent researcher on impacts at the Free University of Brussels, and Greg Ravizza of Hawaii.

Findings from the study are at the heart of an article now under peer review at the Proceedings of the National Academy of Sciences.

Earlier this month, PNAS published a study by researchers led by Todd Surovell of the University of Wyoming at Laramie; they were unable to find the abundant levels of magnetic spherules purported to be remnants of the explosion.

Proponents of the comet theory insist their idea has validity. One author of the original theory, Richard Firestone of the Lawrence Berkeley Laboratory in California, wrote in an email: “The authors of the recent papers that found nothing have an agenda to disprove our work and to keep anyone else from publishing positive results. This and the previous paper are examples of sloppy research.”

But among those who study extraterrestrial impacts, there is little support for the theory. The GSA report is likely to only accelerate the level of criticism.

For instance, Paquay found in examining the ratios of isotopes of osmium, a platinum group element, there was no change at the studied sites. And there was no change in the examined levels of the cores, drilled years ago as part of the Integrated Ocean Drilling Program. All the study results were confirmed in two independent laboratories, Paquay added.


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