Dating of Impact Crater Was Off by 55M Years

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(Newser)

The 19-mile-wide impact crater hiding under a glacier in Greenland isn’t just among the world’s largest. At 58 million years old, it’s also among the most ancient, forming just 8 million years after the asteroid strike on Mexico’s Yucatán Peninsula wiped out the dinosaurs, according to new research. When they first identified the impact crater using ice-penetrating radar, researchers estimated that it was between 12,000 and 3 million years old. That led some to speculate that the meteorite that caused it also triggered a 1,000-year cooling period, the Younger Dryas, that began some 12,800 years ago. Not so, according to a new study by the same research team, published in Science Advances.


Though their initial study noted apparent irregularities in ice layers older than 11,700 years, hinting at an impact around that time, the researchers now suggest those irregularities instead came with the sudden collapse of ice sheets that covered Greenland and the Canadian Arctic Archipelago during the last ice age, per Science. In 2019, researchers pulled rocks that had apparently melted from the heat of the impact from rivers at the foot of the Hiawatha Glacier in northwest Greenland, and from those rocks emerged tiny crystals of the mineral zircon, which carried the distinctive linear fracture patterns that indicate an extraterrestrial impact. Using trace amounts of radioactive uranium, researchers dated the crystals to 58 million years old, give or take 1 million years.


That was backed up by a separate method of dating, using argon gas extracted from grains of sand, that determined the crater to be between 56 million and 66 million years old, per Science Alert. “It’s probably safe to put the Younger Dryas impact hypothesis back to rest for a while,” study co-author and impact modeller Brandon Johnson of Purdue University, West Lafayette, tells Science. Indeed, a leading advocate of the Younger Dryas impact theory now acknowledges there can be no relation. Still, there are others who speculate that the new date is off by as much as 3 million years. In that case, the impact could be related to the Paleocene–Eocene Thermal Maximum, a poorly understood 100,000-year period of intense warming. (Read more crater stories.)

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