Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

Abstract

A carbon-rich black layer, dating to approximately 12.9 ka, has been previously identified at approximately 50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at approximately equal 12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at approximately 12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

Fig. 1.

Sediment profiles for seven sites. Concentrations are shown for magnetic grains, microspherules, charcoal, soot, glass-like carbon, carbon spherules, Ir, Cr, and Ni, which peak mostly in a narrow stratigraphic section spanning only a few hundred years. Ir open circles indicate values below detection, typically <0.5–1 ppb. Ir uncertainties are ±10% at 117 ppb and ±90% at 2 ppb. Cr and Ni are less than ±20%. Keys are color-coded to match the respective curves, and graph points correspond to sampling locations on the photograph. The depth is in centimeters above or below the YDB. The Blackwater Draw image is a composite of three photos. There is no photo for Gainey. A profile for the Belgian site at Lommel is shown in SI Fig. 8. The locations of all sites that were sampled are shown in SI Fig. 9.

Fig. 2.

High-titanomagnetite microspherules from Blackwater Draw, NM (120 μm) (a); Chobot, AB, Canada (150 μm) (b), Gainey, MI (90 μm) (c), and Howard Bay, NC (100 μm) (d).

Fig. 3.

Low-density carbon spherules are shown whole from the Chobot site (a), sectioned and by SEM from Bay T13 (b), and at high magnification by SEM from Bay B14 (c).

Fig. 4.

Examples of glass-like carbon from Gainey, Bay M31, and Topper.