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novembre 2018   01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Despite mainstream opposition, a controversial comet impact hypothesis persists


Around 13,000 years ago, Earth was emerging from its last great ice age. The vast frozen sheets that had covered much of North America, Europe and Asia for thousands of years were retreating. Giant mammals — steppe bison, woolly mammoths and saber-toothed cats — grazed or hunted across tundra and grasslands. A Paleo-Indian group of hunter-gatherers who eventually gave rise to the Clovis people had crossed a land bridge from Asia hundreds of years earlier and were now spread across North America, hunting mammoth with distinctive spears.

Then, at about 12,800 years ago, something strange happened. Earth was abruptly plunged back into a deep chill. Temperatures in parts of the Northern Hemisphere plunged to as much as 8 degrees Celsius colder than today. The cold snap lasted only about 1,200 years — a mere blip, in geologic time. Then, just as abruptly, Earth began to warm again. But many of the giant mammals were dying out. And the Clovis people had apparently vanished.

Geologists call this blip of frigid conditions the Younger Dryas, and its cause is a mystery. Most researchers suspect that a large pulse of freshwater from a melting ice sheet and glacial lakes flooded into the ocean, briefly interfering with Earth’s heat-transporting ocean currents. However, geologists have not yet found firm evidence of how and where this happened, such as traces of the path that this ancient flood traveled to reach the sea (SN: 12/29/12, p. 11).

Laurentide Ice Sheet
But for more than a decade, one group of researchers has stirred up controversy by suggesting a cosmic cause for the sudden deep freeze. About 12,800 years ago, these researchers say, a comet — or perhaps its remnants — hit or exploded over the Laurentide Ice Sheet that once covered much of North America (SN: 6/2/07, p. 339)...



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