Absence of evidence for a meteorite impact event 13,000 years ago

The Younger Dryas is an abrupt cooling event in Earth's history. It coincided with the extinction of many large mammals including the woolly mammoth, the saber toothed jaguar and many sloths. This cooling period is generally considered to be the result of the complex global climate system, possibly spurred on by a reduction or slowdown of the thermohaline circulation in North America. This paradigm was challenged two years ago by a group of researchers that reported finding high iridium concentrations in terrestrial sediments dated during this time period, which led them to theorise that an impact event was instead the instigator of this climate shift.

A team led by François Paquay, a Doctoral graduate student in the Department of Geology and Geophysics at the University of Hawaii at Manoa (UHM) decided to also investigate this theory, to add more evidence to what they considered a conceptually appealing theory. However, not only were they unable to replicate the results found by the other researchers, but additional lines of evidence failed to support an impact theory for the onset of the Younger Dryas. Their results will be published in the December 7th early online edition of the prestigious journal the Proceedings of the National Academy of Sciences.

The idea that an impact event may have been the instigator for this cooling period was appealing because of several alleged impact markers, especially the high iridium concentrations that the previous team reported. However, it is difficult for proponents of this theory to explain why no impact crater of this age is known. "There is a black mat layer across North America which is correlated to the Younger Dryas climatic shift seen in Greenland ice cores dated at 13 thousand years ago by radio carbon," explains Paquay. "Initially I thought this type of layer could be associated with an impact event because concentration in the proxies of widespread wildfires are sky high. That plus very high levels of iridium (which is one indicator used to indicate extraterrestrial impact events). So the theory was conceptually appealing, but because of the missing impact site, the idea of one or multiple airburst arose."

To corroborate the theory, Paquay and his colleagues decided to take a three-pronged approach. The first was to replicate the original researchers data, the second step was to look for other tracers, specifically osmium isotopes, of extraterrestrial matter in those rocks, and the third step was to look for these concentrations in other settings. "Because there are so many aspects to the impact theory, we decided to just focus on geochemical evidence that was associated with it, like the concentration of iridium and other platinum group elements, and the osmium isotopes," says Paquay. "We also decided to look in very high resolution sediment cores across North America, and yet we could find nothing in our data to support their theory."

The team includes American, Belgian and Canadian researchers. Analysis of the sediments was done both at UHM and in Belgium, using the same sediments from the same interval and indepedently did the analysis work and got similar results. Both the marine and terrestrial sediment records do not indicate that an impact event was the trigger for the transition into the Younger Dryas cold period. "The marine and terrestrial record both complement each other to support this finding," concludes Paquay. "That's what makes the beauty of this study."

More information: This research will be presented at the American Geophysical Union Fall 2009 Meeting in San Francisco.

• PNAS Early Edition, December 7, 2009 www.pnas.org_cgi_doi_10.1073_pnas.0908874106

Source: University of Hawaii at Manoa