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Canadian comet impact fingered for triggering prehistoric climate shift

Massive impact killed megafauna and pushed humans into agriculture

Dawn's image of Vesta. Pic: NASA

Scientists have discovered new evidence that an extraplanetary body came down over Canada around 12,900 years ago, possibly triggering the death of the giant animals then roaming the North American continent, and starting a cooling spell that helped drive mankind towards agriculture and civilization.

During the Younger Dryas climatic period, which kicked off around 13,000 years ago, world temperatures fell sharply, with parts of the northern hemisphere dropping around five degrees Centigrade in a decade or less. The effects lasted for over a thousand years before temperatures warmed up again.

The Younger Dryas has been cited as a reason why North America lost the unique megafauna that roamed the continent, including huge saber-toothed cats, a giant sloth, and colossal camels. The cold period is also thought to have spurred the shift from hunter-gathering to agriculture as the means of production for humanity.

The prevailing theory as to what caused the Younger Dryas is that the climate was altered by the draining of Lake Agassiz, a vast fresh-water lake the size of the Black Sea that was dammed by glaciers in the center of the North American continent. When this vented, it sent an enormous amount of fresh water into the Atlantic and Arctic seas, which could have disrupted warming sea currents.

But an alternative theory suggests that the continent may have undergone a series of bombardments from asteroids or comets, triggering the cooling period. One 4km-wide crater from around that time, the Corossal crater, has already been spotted in Canada.

But Professor Mukul Sharma, from Dartmouth University, tells The Register that his team has found direct evidence of another massive impact.

Sharma studied core samples from six different sites along the Eastern seaboard, looking for spherules, which are crystalline rocks formed by temperatures of 2,000°C. These can only be formed naturally by volcanoes or impacts – forest or coal fires don't burn hot enough – and Shama's team found concentrations of these indicating that an impact had occurred in Quebec at around the time the Younger Dryas started.

"It definitely came down in Quebec," he said. "The spherules that we found have iridium isotope signals which are consistent with a derivation from this area, just north of the St. Lawrence river."

The composition of the spherules differs from the element concentration that would have come from the Corossal crater. Sharma says the team now needs to find another crater to account for the new data, and are looking for a suitable target, according to the paper published in the Proceedings of the National Academy of Sciences.

Sharma said that he wasn't discounting the Lake Agassiz breach theory, saying it was a "very good theory". But he postulates that the impact, or impacts, may have helped (or caused) the lake to flood out, as well as changing the local climate. ®

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