Controversial impact crater under Greenland’s ice is surprisingly ancient | Science

In 2018, an international team of scientists announced a startling discovery: Buried beneath the thick ice of the Hiawatha Glacier in northwest Greenland is an impact crater 31 kilometers wide—not as big as the crater from the dinosaur-killing impact 66 million years ago, but perhaps still big enough to mess with the climate. Scientists were especially excited by hints in the crater and the surrounding ice that the Hiawatha strike was recent—perhaps within the past 100,000 years, when humans might have been around to witness it.

But now, using dates gleaned from tiny mineral crystals in rocks shocked by the impact, the same team says the strike is much, much older. The researchers say it occurred 58 million years ago, a warm time when vast forests covered Greenland—and humanity was not yet even a glimmer in evolution’s eye. Kurt Kjær, a geologist at the Natural History Museum of Denmark and a co-author of the new study, says the new date is at odds with the team’s initial impression, gleaned from ice-penetrating radar. “But this is the way science works and should work,” he says.

The date is a blow to a group of scientists that for more than a decade has advanced a controversial hypothesis that the Younger Dryas, a drastic, 1000-year cooling about 12,800 years ago, was triggered when a comet struck Earth. They had seized on the first Hiawatha paper as a smoking gun: The crater seemed about the right age, and it was in the right place—near a region of the North Atlantic Ocean that heavily influences Northern Hemisphere climate. Now, says Brandon Johnson, a co-author and impact modeler at Purdue University, West Lafayette, “It’s probably safe to put the Younger Dryas impact hypothesis back to rest for a while.”

James Kennett, a marine geologist at the University of California, Santa Barbara, and a leading Younger Dryas impact advocate, says the older date for the crater is a surprise, but Kjær’s team “makes a very compelling case … I don’t think it’s related to the Younger Dryas now.” That leaves his group where it was before the discovery of Hiawatha: arguing the Younger Dryas trigger was an airburst rather than a body slamming into the ground. Kennett says the team will continue to advance its case with evidence from more than 40 sites worldwide that contain glassy spherules or platinum-rich sediments, which the group believes are indicative of an impact. “It’s all alive and well and very active.”

Kjær’s team originally thought dating the impact would be impossible without drilling through 1 kilometer or so of ice to sample rocks in the center of the crater. The radar data, however, yielded clues to what seemed to be a young age: reflections indicating ice layers older than 11,700 years are deformed, hinting at an impact around that time.

But in 2019, the team got a chance to date the impact directly. Returning to the rivers that spill out from the foot of the glacier and deposit sediment from beneath the ice, they found fist-size rocks that had experienced melting, ostensibly from the heat of the impact. Slices of those rocks went to the lab of Gavin Kenny, a geochronologist at the Swedish Museum of Natural History, who sifted out crystals of the mineral zircon smaller than grains of sand.

Some of zircon crystals were “shocked”—inscribed with linear fracture patterns that are the hallmark of an impact. Trace amounts of radioactive uranium are present in the zircon, and its decay into lead provides an accurate way to date the samples. The impact kicked out the lead impurities in the shocked zircons, effectively resetting the uranium clock. In a study published today in Science Advances, the researchers report that in 28 of these shocked zircon crystals, the decay clock points to an age of 58 million years ago, with an uncertainty of about 1 million years. Nearly 50 grains of sand collected from the same watershed, analyzed using the decay of radioactive potassium to argon, yielded about the same age.



Graphic about Hiawatha impact

Given the agreement between the two dating systems, “It seems fairly rigorous to me,” says Sandra Kamo, a geochronologist at the University of Toronto.

Now, the team is wondering whether the distorted ice it initially took as signs of a recent impact resulted instead from the sudden collapse of ice that bridged the ice sheets covering Greenland and Canada’s Arctic archipelago during the last ice age. “Eventually they disconnected—presumably with some dynamic consequences,” says Joseph MacGregor, a co-author and glaciologist at NASA’s Goddard Space Flight Center.

Similarly, the sharp, well-preserved crater rim seen on radar might be a sign not of youth, but of slow erosion beneath Greenland’s ice, Johnson adds. In that case the large, incised valleys detected beneath the ice elsewhere in Greenland could be much older than previously assumed, MacGregor says.

The 1.5-kilometer-wide asteroid that produced Hiawatha would have been regionally devastating, but there is no sign that the dust cloud and fires that might have followed the impact disrupted the global climate 58 million years ago. The strike would have come 3 million years before the Paleocene-Eocene Thermal Maximum (PETM), a 100,000-year temperature spike that some have used as an analogy for human-induced climate change.

But Sidney Hemming, a geochemist at Columbia University, says the age data are complex enough that the uncertainty might be as much as 5 million years, opening the possibility that the impact and the PETM are connected. “I’d be hard pressed to be that confident that it’s not that,” she says. She points out that glass spherules tied to the PETM, presumably forged and thrown up in an impact, have been found off the coast of New Jersey. For now, the Hiawatha team is combing through geological records for signs of disturbances 58 million years ago, Kjær says. “We’re commencing that journey now.”

Other impact mysteries in Greenland remain to be solved. Soon after the original Hiawatha paper came out, MacGregor identified a possible second impact crater nearby, larger and more eroded than Hiawatha. (It remains unconfirmed.) And the region is also famed as the home of fragments from a massive iron meteorite that weigh in total some 58 tons. “It is a hot spot for impact up there,” Kjær says.

The study is also a good reminder that, despite all the interest in catastrophic asteroid impacts, none has yet been clearly shown to have caused a global environmental change—other than the dino killer 66 million years ago at Chicxulub, on Mexico’s Yucatán Peninsula. “I love impacts more than your average scientist,” Johnson says. “But when you have some piece of data that is difficult to describe or understand, impacts are usually not the answer.”