About 12,900 years ago, as Earth was warming up after the latest ice age, the globe abruptly entered an unusually cold 1,300-year spell, the Younger Dryas. The cause of the cooling, however, remained unclear. Now, Richard B. Firestone, a nuclear chemist at Lawrence Berkeley National Laboratory, and seventeen colleagues are bolstering the controversial theory that a cataclysmic event is to blame.
Using samples from eighteen sites on three continents, the researchers examined sediment sequences from the base of the Younger Dryas deposits, marking the onset of the cold period. Those lower-boundary sediments are a carbon-rich layer of earth, known as the “black mat.” All samples displayed abundant iron- and silica-rich microspherules, structures that are markers of a cosmic impact.
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More important, melted glass matching the microspherules geochemically was found in samples from Pennsylvania and South Carolina and from Abu Hureyra, an archaeological site in Syria. For that material to form, sand-melting temperatures in the neighborhood of 3,600 degrees Fahrenheit are needed. “There are no forest fires or normal occurrences that create these kinds of temperatures,” says Firestone, who led the team that first raised the hypothesis of a cosmic bombardment by fragments of an asteroid or comet. In further support of the catastrophic event, the dated geological samples were compared with samples from a known impact site, the Meteor Crater in Arizona, and from the 1945 Trinity atomic bomb test site in New Mexico. Those two high-energy events produced geochemical products that were similar to those found in the Dryas samples.
Supporters of a Younger Dryas extraterrestrial impact explanation still have some work to do before the theory becomes widely accepted, Firestone notes. Perhaps the greatest challenge is to identify impact craters of suitable age left by space rocks raining ruin on multiple continents. Let the hunt begin! (PNAS)