Paper PP41A-02: Extraterrestrial Markers Found at Clovis Sites Across North America
* West, A (allen7633@aol.com), GeoScience Consulting, P.O.Box 1636, Dewey, AZ 86327, United States
Firestone, R B, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94710, United States
Kennett, J P, Univ. of California, Santa Barbara, Dept. of Earth Sciences, Santa Barbara, CA 93106, United States
Becker, L , Univ. of California, Santa Barbara, Institute of Crustal Studies, Santa Barbara, CA 93106, United States
A carbon-rich black layer commonly referred to as a black mat, with a basal age of approximately 12.9 ka, has
been identified at over 50 sites across North America1. The age of the base of the black mat coincides with
the abrupt onset of Younger Dryas cooling and megafaunal extinctions in North America. In situ bones of extinct
mammals, including mammoths, mastodons, ground sloths, horses, camels, many smaller mammals and
birds, and Clovis tool assemblages occur below the black mat but not within or above it. In this paper, we provide
evidence for an ejecta layer at the base of the black mat from an extraterrestrial impact event 12.9 ka ago. We
have investigated nine terminal Clovis-age sites in North America and a comparable site in Lommel, Belgium
that are all marked by a thin, discrete layer containing varying peak abundances of (1) magnetic
grains/microspherules containing iridium concentrations up to 117 ppb, (2) charcoal, (3) soot, (4) vesicular
carbon spherules, (5) glass-like carbon, and (6) fullerenes enriched in 3He. This layer also extends
throughout the rims of at least fifteen Carolina Bays, unique, elliptical, oriented lakes and wetlands scattered
across the Atlantic Coastal Plain whose major axes point towards the Great Lakes and Canada. Microspherules,
highly enriched in titanium, were found only in or near the YD boundary (YDB) layer with greatest deposition rates
(35 per cm2) occurring near the Great Lakes. Magnetic grains also peak in the YDB with maximum deposition
near the Great Lakes (30 mg/cm2). Magnetic grains near the Great Lakes are enriched in magnetite (4
mg/cm2) and silicates (23 mg/cm2) but contain less ilmenite/rutile (1 mg/cm2) than distant sites
where ilmentite/rutile deposition ranges up to 18 mg/cm2. Analysis of the ilmenite/rutile-rich magnetic grains
and microspherules indicates that they contain considerable water, up to 28 at.% hydrogen, and have
TIO2/FeO, TIO2/Zr, Al2O3/FeO+MgO, CaO/Al2O3, REE/chondrite, K/Th, FeO/MnO ratios
and SIO2, Na2O, K2O, Cr2O3, Ni, Co, Ir, Th, U, and other trace element abundances that are
inconsistent with all terrestrial and extraterrestrial sources except for Lunar Procellarum KREEP terrain (PKT).
We propose that the YDB layer is the ejecta layer from an airburst over the Laurentide Ice Sheet that deposited
local, low-speed terrestrial material near the airburst site and KREEP-like, high-speed projectile material farther
away, leaving little or no permanent crater. The associated blast wave and thermal pulse would have contributed
to the megafaunal extinctions and destabilized the Laurentide Ice Sheet, loading the atmosphere with dust, soot,
NOx, and water vapor and triggered the YD cooling.
1 Haynes, C. V., Jr. in Murray Springs: a Clovis site with multiple activity areas in the San Pedro Valley, Arizona.
C. V. Haynes, Jr. and Bruce B. Huckell, eds. Tucson: Univ. of Arizona Press, in press, 2007.