The report centers on spherules, droplets of solidified molten rock ejected by the impact of a meteor or comet.

An asteroid impact in Quebec some 12,900 years ago has been linked for the first time to an intense climate shift, according to a new study led by Dartmouth researchers.  The asteroid impact slaughtered the majority of the planet’s large mammals, forcing early humans to diversify into hunter-gatherer behaviors, rather than relying on hunting only big game for subsistence.  The findings appear this week in the Early Edition of the journal Proceedings of the National Academy of Sciences.

The impact occurred at the beginning of the Younger Dryas period, marking a sudden global change to a colder, dryer climate with extensive effects on humans and animals.  In North America, the big animals, such as mastodons, camels, giant ground sloths, and saber-toothed cats, died out.  Their predators, known to archaeologists as the Clovis people, turned their focus to a hunter-gatherer subsistence diet of roots, berries and smaller game.

“The Younger Dryas cooling impacted human history in a profound manner,” said Dartmouth Professor and study co-author Mukul Sharma. “Environmental stresses may also have caused Natufians in the Near East to settle down for the first time and pursue agriculture.”

According to Dartmouth researchers, there has long been controversy over the cause of these environmental stresses, though scientists are in agreement that these changes did indeed occur.  The archetypal view of the Younger Dryas cooling interlude has been that an ice dam in the North American ice sheet ruptured, releasing a massive quantity of freshwater into the Atlantic Ocean.  The sudden influx is thought to have shut down the ocean currents that move tropical water northward, resulting in the cold, dry climate of the Younger Dryas.

Sharma and his research team have discovered irrefutable evidence linking a celestial impact with this environmental alteration.  The report centers on spherules, droplets of solidified molten rock ejected by the impact of a meteor or comet.  The spherules at the center of the study were extracted from Younger Dryas boundary layers at locations in Pennsylvania and New Jersey, the layers having been deposited at the beginning of the period.  The geochemistry and mineralogy profiles of the spherules are identical to samples found in southern Quebec, where Sharma and his colleagues contend the impact took place.

“We have for the first time narrowed down the region where a Younger Dryas impact did take place, even though we have not yet found its crater,” said Sharma.  “It may well have taken multiple concurrent impacts to bring about the extensive environmental changes of the Younger Dryas.  However, to date no impact craters have been found and our research will help track one of them down.”

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