| Summary: | We have uncovered a thin layer of magnetic grains and microspherules, carbon spherules, and glass-like carbon at nine sites across North America, a site in Belgium, and throughout the rims of 16 Carolina Bays. It is consistent with the ejecta layer from an impact event and has been dated to 12.9 ka BP coinciding with the onset of Younger Dryas (YD) cooling and widespread megafaunal extinctions in North America. At many locations the impact layer is directly below a black mat marking the sudden disappearance of the megafauna and Clovis people. The distribution pattern of the Younger Dryas boundary (YDB) ejecta layer is consistent with an impact near the Great Lakes that deposited terrestrial-like ejecta near the impact site and unusual, titanium-rich projectile-like ejecta further away. High water content associated with the ejecta, up to 28 at. % hydrogen (H), suggests the impact occurred over the Laurentide Ice Sheet. YDB microspherules and magnetic grains are highly enriched in TiO 2 . Magnetic grains from several sites are enriched in iridium (Ir), up to 117 ppb. The TiO 2 /FeO, K/Th, TiO 2 /Zr, Al 2 O 3 /FeO+MgO, CaO/Al 2 O 3 , REE/ chondrite, FeO/MnO ratios and SiO 2 , Na 2 O, K 2 O, Cr 2 O 3 , Ni, Co, U, Th and other trace element abundances are inconsistent with all terrestrial and extraterrestrial (ET) sources except for KREEP, a lunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), and other incompatible elements including U and Th. Normal Fe, Ti, and 238 U/ 235 U isotopic abundances were found in the magnetic grains, but 234 U was enriched over equilibrium values by 50 % in Murray Springs and by 130 % in Belgium. 40 K abundance is enriched by up to 100 % in YDB sediments and Clovis chert artifacts. Highly vesicular carbon spherules containing nanodiamonds, glass-like carbon, charcoal and soot found in large quantities in the YDB layer are consistent with an impact followed by intense burning. Four holes in the Great Lakes, some deeper than Death Valley, are proposed as ...
|
|---|