Impact of the Ice Age | Science

One fine day in July, two years ago, Kurt Kjær was in a helicopter flying over northwestern Greenland, a stretch of white and glittering ice. Soon, his target appeared: the Hiawatha Glacier, a layer of slow ice more than one kilometer thick. It advances on the Arctic Ocean not in a straight wall, but in a semicircle clearly visible, as it came out of a basin. Kjær, a geologist at the Natural History Museum of Denmark in Copenhagen, suspected the glacier of hiding an explosive secret. The helicopter landed near the flooding river that drains the glacier, sweeping the rocks. Kjær had 18 hours to find the mineral crystals that would confirm his suspicions.

What he reported at home validated the case for a great discovery. Hiawatha hides an impact crater 31 km wide, big enough to swallow Washington, DC, Kjær and 21 co-authors who report this week in an article by Progress of science. The crater was left when a 1.5 km ferrous asteroid struck the Earth, probably over the last 100,000 years.

Although not as cataclysmic as the impact of the Chicxulub, which killed a dinosaur, which dug a crater 200 km wide in Mexico about 66 million years ago, the Hiawatha impactor could also have left an imprint on the history of the planet. The moment is yet to be debated, but some researchers in the discovery team believe that the asteroid struck a crucial moment: about 13,000 years ago, as the world was melting away from the last ice age. It would mean that it crashed on Earth when mammoths and other megafauna were declining and people were spreading all over North America.

The impact would have been a show for anyone within 500 kilometers. A white fireball four times bigger and three times brighter than the sun would have scratched it in the sky. If the object hit a layer of ice, it would have penetrated the bedrock, vaporizing the water and the stone in a flash. The resulting explosion summed the energy of 700 1 megaton nuclear bombs and even an observer hundreds of miles away would have suffered a shock wave, a monstrous thunderclap and force winds hurricane. Later, rocky debris could have rained down on North America and Europe, and the released steam, a greenhouse gas, could have warmed Greenland locally, melting even more ice.

The news of the discovery of impact has awakened an old debate among scientists who study the ancient climate. A massive impact on the pack ice would have caused meltwater to flow into the Atlantic Ocean, potentially disrupting the ocean current transport belt and causing a drop in temperatures, particularly in the northern hemisphere. "What would it mean for species or life at the time? It's a huge open question, "says Jennifer Marlon, paleoclimatologist at Yale University.

Ten years ago, a small group of scientists proposed a similar scenario. They were trying to explain a cooling event, more than 1000 years old, called the Younger Dryas, which began 12,800 years ago, as the last ice age ended. Their controversial solution was to summon an extraterrestrial agent: the impact of one or more comets. The researchers proposed that in addition to changing the plumbing of the North Atlantic, the impact also caused wildfires on two continents, resulting in the extinction of large mammals and the disappearance of the Clovis hunter mammoth people of North America. The research group collected suggestive but inconclusive evidence and few other scientists were convinced. But the idea drew the imagination of the public despite an obvious limitation: no one could find an impact crater.

Proponents of an impact on the young Dryas now feel justified. "I was unequivocally predicting that this crater would be the same age as the Younger Dryas," says James Kennett, marine geologist at the University of California at Santa Barbara, one of the original boosters of this idea.

But Jay Melosh, an impact crater expert at Purdue University in West Lafayette, Indiana, doubts the strike is so recent. Statistically, the impacts of Hiawatha's size only occur every few million years, he explains, so the likelihood of this happening 13,000 years ago is low. No matter who's right, the discovery will give the theorists the impact of Younger Dryas – and turn the Hiawatha impactor into another type of projectile. "It's a hot potato," says Melosh Science. "You know you're going to start a fire storm?"

IT STARTED WITH a hole. In 2015, Kjær and a colleague were studying a new map of contours hidden under the ice of Greenland. Based on variations in depth profiles and ice surface flow, the map offers a rough suggestion of bedrock topography, including the trace of a hole under Hiawatha.

Kjær is remembered from a huge iron meteorite in the courtyard of his museum, near the place where he parked his bicycle. called Agpalilik, Inuit for "the Man", the 20-ton rock is a fragment of an even larger meteorite, the Cape York, found in pieces on northwestern Greenland by Western explorers but long used by Inuit as a source iron for tools. Kjær wondered if the meteorite could be a remnant of an impactor that had dug the circular element under Hiawatha. But he was still not convinced that it was an impact crater. He needed to see more clearly with radar, which can penetrate the ice and reflect on the bedrock.

The Kjær team began working with Joseph MacGregor, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who uncovered radar data from the archives. MacGregor found that NASA aircraft often flew over the site before probing the Arctic pack ice and that the instruments were sometimes lit in test mode at the exit. "It was pretty glorious," says MacGregor.

The radar images showed more clearly what looked like the edge of a crater, but they were still too vague in the middle. Many elements on the Earth's surface, such as volcanic calderas, may pose as circles. But only impact craters contain peaks and rings of central peaks, which form in the center of a new crater when – like splashing a rock into a pond – molten rocks bounce off just after a collision . To search for these features, the researchers needed a dedicated radar mission.

Coincidentally, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, had just acquired a new generation radar penetrating the ice to fit on the wings and body of their Basler aircraft, an improved DC-3 double helix, powerful in Arctic science. But they also needed funding and a base close to Hiawatha.

Kjær is busy with money. Traditional funding agencies would be too slow or prone to divulge their idea, he thought. He therefore petitioned the Carlsberg Foundation in Copenhagen, which uses profits from global beer sales to finance science. MacGregor, meanwhile, called on NASA colleagues to persuade the US military to let them work at Thule Air Base, a Cold War outpost in North Greenland, where German members team were seeking permission to work for 20 years. "I had retired and some very serious German scientists sent emojis with a happy face," MacGregor says.

Three flights in May 2016 added 1600 kilometers of fresh data from dozens of transits across the ice – and evidence that Kjær, MacGregor and their team were on the right track. The radar revealed five prominent bumps in the center of the crater, indicating a central peak rising to about 50 meters high. And as a sign of recent impact, the bottom of the crater is unusually shredded. If the asteroid had hit more than 100,000 years earlier, while the area was free of ice, erosion due to melting more inland ice would have scrubbed the crater, said MacGregor. Radar signals also showed that the deep ice layers were entangled – another sign of recent impact. According to MacGregor, the strangely disturbed patterns suggest that "the ice cap is not balanced with the presence of this impact crater."

But the team wanted direct evidence to overcome the skepticism it knew how to salute in a claim for a massive crater, which seemed to defy any likelihood of significant impacts. And that is why Kjær was found, on that beautiful day of July 2016, sampling rocks with frenzy throughout the crescent land surrounding Hiawatha's face. His most crucial stop was in the middle of the semicircle, near the river, where he collected sediments that seemed to come from inside the glacier. It was hectic, he says – "one of those days when you just check your samples, fall on the bed and do not get up for a while."

In this attack, the Kjær team closed its file. While examining the sand, Adam Garde, geologist at the Geological Survey of Denmark and Greenland in Copenhagen, discovered glass grains forged at temperatures higher than those that a volcanic eruption could generate. More importantly, he discovered shocked quartz crystals. The crystals contained a distinctive band pattern that can only be formed under the intense pressures of extraterrestrial impacts or nuclear weapons. Quartz does the trick, says Melosh. "It looks good. All the evidence is pretty convincing. "

The team must now know exactly when the collision occurred and how it affected the planet.

YOUNG DRYAS, named after a small white-and-yellow arctic flower that bloomed during the cold snap, has long fascinated scientists. Until man-made global warming occurs, this period has been one of the strongest recent fluctuations in temperature on Earth. As the last ice age faded, about 12,800 years ago, temperatures in parts of the Northern Hemisphere plummeted to 8 ° C until the ice age was read. They stayed that way for more than 1000 years, turning the forest into tundra.

The trigger could have been a disturbance in the conveyor belt of ocean currents, including the Gulf Stream, which transports heat northward from the tropics. In a 1989 article NatureKennett and Wallace Broecker, a climatologist at Columbia University's Lamont-Doherty Earth Observatory, explained how meltwater from recessed ice sheets could have stopped the conveyor. When the tropical hot water moves northward to the surface, it cools down while evaporation makes it more salty. Both factors increase the density of water until it dives into the abyss, contributing to the conveyor's behavior. The addition of a less dense freshwater pulse could hit the brakes. Paleoclimate researchers have largely supported the idea, although evidence of such a flood has been lacking until recently.

Then, in 2007, Kennett suggested a new trigger. He teamed up with scientists led by Richard Firestone, a physicist at Lawrence Berkeley National Laboratory in California, who proposed a comet strike at key point. Explosive on the pack ice covering North America, the comet or comets would have projected dust blocking the light in the sky, thus refreshing the region. Further south, inflamed projectiles would have set the forest on fire, producing soot that deepened the darkness and cooling. The impact could also have destabilized the ice and released meltwater that would have disrupted traffic in the Atlantic.

The team suggested that climate chaos could explain why the Clovis colonies emptied and the megafauna disappeared soon after. But the evidence was meager. Firestone and colleagues have identified thin sediment layers on dozens of archaeological sites in North America. These sediments appeared to contain geochemical traces of extraterrestrial impact, such as an iridium peak, the exotic element that helped cement the cause of a Chicxulub impact. The layers also provided tiny glass and iron beads – possible meteorite debris – as well as heavy soot and charcoal charges, indicating fires.

The team was immediately criticized. The decline of mammoths, giant sloths and other species had begun well before the Younger Dryas. In addition, there has been no sign of human mortality in North America, archaeologists said. The nomads members of Clovis would not stay long in any site. Vance Holliday, an archaeologist at the University of Arizona at Tucson, explains Vance Holliday, an archaeologist at the University of Arizona (Tucson). The assumption of impact was trying to solve problems that did not need to be solved.

Geochemical evidence has also begun to erode. Outside scientists were unable to detect the peak of iridium in the group samples. The pearls were real, but they were abundant throughout many geological periods, and the soot and charcoal did not seem to have grown during the Younger Dryas. "They listed all those things that are not enough," says Stein Jacobsen, a geochemist at Harvard University who studies craters.

Yet, the hypothesis of the impact is never completely dead. His supporters continued to study the putative debris layer on other sites in Europe and the Middle East. They also reported finding microscopic diamonds at various sites that could have been formed only as a result of an impact. (External researchers ask about diamond claims.)

Now, with the discovery of Hiawatha Crater, "I think we have the smoking gun," says Wendy Wolbach, a geochemist at DePaul University in Chicago, Illinois, who previously worked on fires at that time.

The team estimated that the impact would have melted 1,500 gigatons of ice, about as much ice as Antarctica has lost due to global warming over the last decade. The local greenhouse effect due to the released steam and the residual heat in the crater would have added more melt. Much of this freshwater could have been found in the nearby Labrador Sea, a main site fueling the shattering circulation of the Atlantic Ocean. "This could potentially disrupt traffic," says Sophia Hines, marine paleoclimatologist at Lamont-Doherty.

Far from the previous controversy, Kjær will not approve this scenario. "I do not put myself in front of this bandwagon," he says. But in the rough drafts of the document, he admits that the team has explicitly mentioned a possible link between the impact of Hiawatha and the Younger Dryas.

PROOF BEGINS with ice cream In radar images, remnants of distant volcanic eruptions highlight some of the boundaries between seasonal layers in the form of light reflections. These glossy layers can be associated with the same layers of gravel in dated and cataloged ice cores from other parts of Greenland. Using this technique, the Kjær team discovered that most ice in Hiawatha was perfectly stratified over the last 11,700 years. But in the old disturbed ice, the brilliant reflections disappear. In tracing the deep layers, the team matched the clutter with debris-rich surface ice on the 1200-year-old edge of Hiawatha. "It's quite logical that the flow of ice has been significantly disrupted at the earliest in Dryas," said MacGregor.

Other sources of evidence also suggest that Hiawatha could be the impact of Younger Dryas. In 2013, Jacobsen examined a core of ice from central Greenland, 1,000 kilometers away. He expected to end Younger Younger's impact theory by showing that 12,800 years ago, metal levels that asteroid impacts tend to spread have not increased. Instead, he found a platinum peak similar to that measured in samples taken from the crater site. "It suggests a connection with the Younger Dryas over there," says Jacobsen.

For Broecker, the coincidences add up. He had initially been intrigued by the Firestone newspaper, but had quickly joined the ranks of the opponents. The lawyers of the young Dryas have insisted too much on the subject: the fires, the extinction of the megafauna, the abandonment of Clovis sites. "They made a bad shine." But the platinum peak found by Jacobsen, followed by the discovery of Hiawatha, made him believe again. "It must be the same thing," he says.

Yet no one can be sure of the timing. The disturbed layers reflected nothing more than the normal stresses at the bottom of the icecap. "We all know too well that old ice can be lost by melting or shearing at the base," says Jeff Severinghaus, paleoclimatologist at the Scripps Institution of Oceanography in San Diego, California. Richard Alley, a glaciologist at Pennsylvania State University at University Park, estimates that the impact is much older than 100,000 years ago and that an underglacial lake can explain the strange textures near the base of the ice . "The flow of ice on growing and declining lakes, which interacts with rough topography, may have produced quite complex structures," says Alley.

A recent impact should also have left its mark in the half dozen deep ice cores drilled on other sites in Greenland, which document 100,000 years of the history of the current ice sheet. Yet none presents the thin layer of rubble that a strike the size of a Hiawatha should have been launched. "You should really see something," says Severinghaus.

Brandon Johnson, a global scientist at Brown University, is not so sure. After seeing a draft of the study, Johnson, who models impacts on ice moons such as Europa and Enceladus, used his code to recreate an asteroid impact on a thick layer of ice. An impact digs a crater with a central peak like the one observed in Hiawatha, he discovered, but the ice suppresses the spread of rock debris. "The first results are that it goes a lot less far," says Johnson.

EVEN IF THE ASTEROID Struck at the right moment, it might not have triggered all the catastrophes imagined by the partisans of the impact of Younger Dryas. "It's too small and too far to kill Pleistocene mammals in the continental United States," says Melosh. And it's hard to see how a strike could ignite flames in such a cold and barren region. "I can not imagine how such an impact could have caused massive fires in North America," said Marlon.

It may not even have triggered the Younger Dryas. According to Lloyd Keigwin, paleoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts, cores of ocean sediment show no trace of freshwater from Greenland in the Labrador Sea. The best recent evidence, he adds, suggests rather a flood in the Arctic Ocean through western Canada.

An external trigger may be useless in any case, says Alley. During the last ice age, the North Atlantic experienced a further 25 cooling periods, probably caused by disruptions in the upset circulation of the Atlantic. None of these spells, known as the Dansgaard-Oeschger (DO) events, were as severe as the Younger Dryas, but their frequency suggests that an internal cycle also played a role in the Younger Dryas. Even Broecker agrees that the impact has not been the ultimate cause of cooling. If the DO events represent steep transitions between two regular states of the ocean, it could "assert that the ocean was approaching instability and that, in one way or another, from another, this event had overturned him ".

Despite everything, the full story of Hiawatha will remain at her age. Even an exposed impact crate can be a challenge for dating, which requires capturing the moment the impact has altered the existing rocks, and not the age of the origin. Impactor or its target. The Kjær team tried. They fired lasers at the glass spheres to release argon for dating, but the samples were too contaminated. The researchers are looking at a blue crystal of mineral apatite looking for traces left by the decay of uranium, but it's a long time. The team also found traces of carbon in other samples, which could someday give a date, says Kjær. But the ultimate answer may need to drill through the ice to the bottom of the crater, up to the rock that melted during the impact, which reset its radioactive clock. With sufficiently large samples, researchers should be able to determine the age of Hiawatha.

Given the remote location, a drill expedition into the top hole of the world would be expensive. But it is at stake to understand the recent history of climate and what a giant impact can have on the planet. "Someone has to do an exercise there," says Keigwin. "That's all we can say about it."