Richard B. Firestone
Lawrence Berkeley National Laboratory
Thirteen thousand years ago more than 35 species of megafauna including mammoths, mastodons, ground sloths, horses, camels, and many smaller mammals and birds suddenly disappeared. Simultaneously the Earth, which was warming from the last ice age, was suddenly plunged into a 1300-year period of cooling known as the Younger Dryas. At the onset of the Younger Dryas a black layer formed over much of North America and Europe. This layer is sometimes found draped directly over the bones of extinct megafauna and the artifacts of Clovis culture. No evidence of megafauna or Clovis people is found within or above the black layer. We have investigated sediments from the Younger Dryas boundary layer (YDB) at 10 well-dated Clovis-age sites from California to Belgium and 16 Carolina Bays that are believed to have been formed at that time. Within the YDB and throughout the Bay rims we find an impact ejecta layer enriched in iridium, metallic microspherules, carbon spherules, glass-like carbon, soot, and charcoal. The carbon spherules and glass-like carbon contain abundant nanodiamonds and fullerenes with extraterrestrial 3He/4He ratios. The distribution and composition of the ejecta layer and the orientation of the Carolina Bays suggest that the impact was an airburst over the Laurentide Ice Sheet near the Great Lakes. Analysis of the ejecta layer indicates that the impactor was unusually enriched in titanium, very different from known terrestrial or meteoritic sources, but remarkably similar Lunar Procellarum KREEP.