2009 Portland GSA Annual Meeting (18-21 October 2009)
Paper No. 234-3
Presentation Time: 8:45 AM-9:00 AM


PAQUAY, Francois1, GODERIS, Steven2, RAVIZZA, Greg1, and CLAEYS, Philippe3, (1) Geology and Geophysics, SOEST, University of Hawaii, 1680 East-West Rd, Honolulu, HI 96816, paquay@hawaii.edu, (2) Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, Brussel, B-1050, Belgium, (3) Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050, Belgium

There is currently a controversial hypothesis that an extraterrestrial (ET) impact caused the abrupt Bølling-Allerød/Younger Dryas climatic transition, the megafaunal extinction and the cultural shift of early Native Americans. This hypothesis reports evidence of the alleged impact event based on the findings of markers in black mat layers assumed to be directly derived from the bolide or resulting from it. Among these impact markers, elevated iridium concentrations (> several ng/g) measured in magnetic grains and bulk sediments are indicative that the bolide vaporized upon impact, or fully exploded into the atmosphere. In contradiction with earlier findings we show that two independent laboratories cannot corroborate the Ir evidence. For our purpose, we have measured Ir, other platinum group elements (PGE) (Os, Ru, Rh, Pt, Pd), gold concentration and 187Os/188Os ratios in bulk sediments from seven same BA/YD sections previously studied. Our results from the black mat layers show PGE’s concentrations and 187Os/188Os ratios that are typical of a crustal signature. In addition, a high-resolution study of 187Os/188Os ratios in bulk sediments from two continental margin sites (DSDP 480, Guaymas Basin; ODP 1002C, Cariaco Basin) failed to detect any ET-PGE enrichment anomaly. Our extensive data and high sample weight precludes any nugget effect that could have masked extraterrestrial components. In conclusion, if no ET PGE’s are present across the BA/YD, a chondritic bolide is precluded. Similarly if no impact crater has been found and accurately dated, differentiated achondritic, unknown carrier of nanodiamonds, are also precluded. This lead us to speculate that nanodiamonds, found within carbon spherules, are formed in a yet understood process, during intense wildfires but do not constitute an impact marker. Our study discredits the YD impact hypothesis, leaving room for the “homegrown” perturbation as the trigger for the BA/YD transition, as originally proposed.

2009 Portland GSA Annual Meeting (18-21 October 2009)
General Information for this Meeting
Session No. 234
Impact Cratering from the Microscopic to the Planetary Scale II
Oregon Convention Center: A106
8:00 AM-12:00 PM, Wednesday, 21 October 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 595

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