[meteorite-list] Hawaiians Say No To Dryas Impact

[JoshuaTreeMuseum]

Aloha Listerians:

It seems they're having trouble replicating the original data.




http://www.hawaii.edu/news/article.php?aId=3280

Absence of evidence for a meteorite impact event 13,000 years ago
as a trigger for the Younger Dryas abrupt cooling and the Megafauna 
extinction
University of Hawai?i at Manoa

Contact:
Tara L Hicks Johnson, (808) 956-3151
Outreach Spec, School of Ocean and Earth Sciences and Technology
Francois Paquay, 808-673-3137
Graduate Student, Geology and Geophysics, SOEST

Posted: Dec. 7, 2009

An international team of scientists led by researchers at the University of 
Hawai?i at Manoa have found no evidence supporting an extraterrestrial 
impact event at the onset of the Younger Dryas ~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 Hawai?i 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."

This project was supported by the Geological Society of America and the 
National Science Foundation. Sediment samples were provided by the 
Integrated Ocean Drilling Program.

The other authors from this paper are Greg Ravizza (also from UHM), Steven 
Goderis and Philippe Claeys from Vrije Universiteit Brussel, Frank Vanhaeck 
from the Universiteit Ghent, Matthew Boyd from Lakehead University, Todd A. 
Surovell from the University of Wyoming at Laramie, and Vance T. Holliday 
and C. Vance Haynes, Jr. from the University of Arizona at Tucson.

This research will be presented at the American Geophysical Union Fall 2009 
Meeting in San Francisco. Wednesday December 16th,  2:52 p.m. - 3:04 p.m. 
room 2006 Moscone West

Session Title: PP33B. "Younger Dryas Boundary: Extraterrestrial Impact or 
Not? II"

 Absence of geochemical evidence for an impact event at the 
Bølling-Allerød/Younger Dryas transition. François S. Paquay, Greg Ravizza 
(University of Hawai?i at Manoa), Steven Goderis,  Philippe Claeys (Vrije 
Universiteit Brussel), Steven Goderis, Frank Vanhaeck (Universiteit Ghent), 
Matthew Boyd (Lakehead University), Todd A. Surovell (University of Wyoming 
at Laramie), Vance T. Holliday,  C. Vance Haynes, Jr. (University of Arizona 
at Tucson)

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



For more information, visit: 
http://www.soest.hawaii.edu/soest_web/soest.news



Phil Whitmer