No Evidence of Impact at Onset of Younger Dryas

An international team of scientists led by researchers at the University of Hawaii at Manoa has found no evidence supporting an extraterrestrial impact event at the onset of the Younger Dryas approximately 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 Proceedings of the National Academy of Sciences.

The paper is here:

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

1. François S. Paquay (a,1)
2. Steven Goderis (b,c)
3. Greg Ravizza (a)
4. Frank Vanhaeck (c)
5. Matthew Boyd (d)
6. Todd A. Surovell (e)
7. Vance T. Holliday (f)
8. C. Vance Haynes Jr.(f)
9. Philippe Claeys (b)

a Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, HI 96822;

b Earth System Sciences and Department of Geology, Vrije Universiteit Brussel, 1050 Elsene, Brussels, Belgium;

c Department of Analytical Chemistry, Universiteit Ghent, 9000 Ghent, Belgium;

d Department of Anthropology, Lakehead University, Thunder Bay, ON, Canada P7B 5E1;

e Department of Anthropology, University of Wyoming, Laramie, WY 82070; and

f Departments of Anthropology and Geosciences, University of Arizona, Tucson, AZ 85721

1 To whom correspondence should be addressed. E-mail: paquay@hawaii.edu

Abstract:

High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabrean fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution. First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition.

The back and forth continues. Reading the paper now.