Micrometeorite Impacts in Beringian Mammoth Tusks and a Bison Skull

AGU Fall Meeting, 10-14 December 2007, San Francisco, CA
Paper U23A-0865

Abstract: We have discovered what appear to be micrometeorites imbedded in seven Alaskan Mammoth tusks and a Siberian bison skull. The micrometeorites apparently shattered on impact leaving 2-5 mm hemispherical debris patterns surrounded by carbonized rings. Multiple impacts are observed on only one side of the tusks and skull consistent with the micrometeorites having come from a single direction. The impact sites are strongly magnetic indicating significant iron content. We analyzed several imbedded micrometeorite fragments from both tusks and skull with Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS) and X-ray Fluorescence (XRF). These analyses confirmed the high iron content and a uniform composition highly enriched in nickel and depleted in titanium. The Fe/Ni and Fe/Ti ratios are comparable to urelite meteorites and are unlike any terrestrial sources. Prompt Gamma-ray Activation Analysis (PGAA) of a micrometeorite extracted from the bison skull indicated it contained ~0.4 mg of iron, in agreement with a micrometeorite ~1 mm in diameter. Several tusks have an average radiocarbon age of ~33 ka. This age coincides with sudden increases in global radiocarbon ~35 ka ago^a and 10Be ~32 ka ago^b, the Mono Lake geomagnetic excursion ~34 ka ago^c, and significant declines in Beringian bison, horse, brown bear, and mammoth populations and genetic diversity <36 ka ago^d. The bison skull shows evidence of new bone growth over the micrometeorite impact sites indicating the animal survived the bombardment and is dated at ~26 ka which is younger than the tusks. This age is consistent with exposure of the bison to an enriched source of radiocarbon following the impact. It appears likely that the impacts, cosmogenic isotope increases, magnetic excursion, and population declines are related events (Occam’s razor), although their precise nature remains to be determined.
^a K. Hughen, et al., Science 303, 202-207 (2004).
^b L.R. McHargue, P.E. Damon, & D.J. Donahue, Geophys. Res. Lett. 22, 659-662 (1995).
^c J.E.T. Channell, Earth Planet. Sci. Lett. 244, 379-393 (2006).
^d I. Barnes, et al, Current Biology 17, 1-4 (2007).

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Richard B. Firestone, e-mail: rbf@lbl.gov
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