A report out in yesterday's issue of Science, "Nanodiamonds in the Younger Dryas Boundary Sediment Layer," describes billions of nanometer-sized diamonds [right, credit D.J. Kennett et al, Science] from six locations, including Murray Spring, Arizona, that "provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval [12.9K years ago], producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America."
Three Arizona geologists were among the 8 co-authors of the study: Allen West of GeoScience Consulting in Dewey, Ariz.; Ted Bunch and Marilee Sellers, both at Northern Arizona University.
In a separate commentary, Science writer Richard Kerr talks about a paper published a year ago, that included some of the present authors, that first proposed the comet hypothesis, which speculated that "such a cataclysm could have wiped out mammoths and other large mammals, abruptly ended the Paleo-Indian Clovis culture, and triggered a millennium-long return to near-glacial cold."
Critics challenge whether the nanoparticles are diamonds, whether they originated in comets, and whether such comets would cause such environmental consequences.