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An ancient flood seems to have stalled the circulation of the oceans, plunging the Northern Hemisphere into a millennium of near-glacial conditions. Thirteen thousand years ago, an ice age was ending, the Earth was warming, the oceans were rising. Then something strange happened – the Northern Hemisphere suddenly became much colder, and stayed that way for more than a thousand years. For some time, scientists have been debating how this major climatic event – called the “Younger Dryas” – happened. The question has grown more urgent: Its answer may involve the kind of fast-moving climate event that could occur again. This week, a scientific team made a new claim to having found that answer. On the basis of measurements taken off the northern coasts of Alaska and Canada in the Beaufort Sea, the scientists say they detected the signature of a huge glacial flood event that occurred around the same time. This flood, they posit, would have flowed from the Arctic into the Atlantic Ocean and shut down the crucial circulation known as the “Atlantic meridional overturning circulation” (or AMOC) – plunging Europe and much of North America back into cold conditions. “Even though we were in an overall warming period, this freshwater, exported from the Arctic, slowed down the vigor, efficiency of the meridional overturning, and potentially caused the cooling observed strongly in Europe,” said Neal Driscoll, one of the study’s authors and a professor at the Scripps Institution of Oceanography. The work, published in Nature Geoscience, was led by Lloyd Keigwin of the Woods Hole Oceanographic Institution along with researchers at that institution, Scripps and Oregon State University. The result remains contested, though, with other researchers still arguing for different theories of what caused the Younger Dryas – including a very differently routed flood event that would have entered the ocean thousands of miles away. Nonetheless, the story is relevant because today, we’re watching another – or rather, a further – deglaciation, as humans cause a warming of the planet. There is also evidence that the Atlantic circulation is weakening again, although scientists certainly do not think a total shut-off is imminent, and are still debating the causes of what is being observed. Either way, the new research underscores that as the Earth warms and its ice melts, major changes can happen in the oceans. And could happen again. The researchers behind the current study, working on board the U.S. Coast Guard Cutter Healy, analyzed sediments of deep ocean mud, which contain the shells of long-dead marine organisms called foraminifera. In those shells, the scientists detected a long-sought-after anomaly recorded in the language of oxygen atoms. The shells contained a disproportionate volume of oxygen−16, a lighter form (or isotope) of the element that is found in high levels in glaciers. That is because oxygen−16, containing two fewer neutrons and therefore lighter than oxygen−18, evaporates more easily from the ocean but does not rain out again as readily. As a result, it often falls as snow at high latitudes and is stored in large bodies of ice. “This is the smoking gun for fingerprinting glacial lake outbursts,” Driscoll said. And that means the findings may also represent the trigger for the Younger Dryas. The thinking is that as the ice age ended and the enormous Laurentide ice sheet atop North America began to retreat, the resulting meltwater fed a bevy of large lakes atop the depressed surface of the continent. That included the massive glacial Lake Agassiz, which stretched from the Great Lakes northwestward across much of Canada. The approximate maximum extents of major glacial lakes that formed from the retreat of the western Laurentide Ice Sheet. (Shannon Klotsko, Scripps Institution of Oceanography, University of California at San Diego) Prior research had shown that for a while, much of the resulting freshwater drained down the Mississippi River and into the Gulf of Mexico. But at some point, as the ice sheet continued to shrink, the flow of water appears to have been suddenly rerouted to the north or to the east, where it could do more potential damage to the ocean circulation in the Atlantic. There has long been scientific debate about where all the meltwater actually entered the ocean, though – with some contending that it would have occurred through the St. Lawrence River, which flows past today’s Montreal and Quebec City and thus out into the Atlantic. The new research holds that, instead, the floodwater exited through the Mackenzie River, which stretches across today’s Northwest Territories, emptying straight into the Arctic Ocean. It would certainly have been an enormous flow of fresh water. “I would say somewhere between the Mississippi and the Amazon,” Keigwin said. That could have interfered with the Atlantic circulation, which is crucial because it carries warm water northward, and so heats higher latitudes. Eventually, the waters of the circulation become very cold as they travel northward, but because they are also quite salty, they sink because of their high density and travel back south again. Freshening is therefore the Achilles’ heel of the circulation. And the new study argues that although the glacial water would have entered the seas very far away near the present Alaska-Canada border, it would have then circulated around the Arctic, eventually traveling south past Greenland and entering the key regions that are crucial to the overturning circulation, which tend to be off Greenland’s southern coasts. Not everyone is convinced, though – including some researchers who have previously published results suggesting that the outburst flood or flow was instead to the east, through the St. Lawrence River. “They have produced a nice signal of the release of freshwater into the Arctic Ocean, but the conclusions are based on an uncertain chronology which, when trying to tie together events so closely, requires some independent confirmation,” Peter Clark, an Oregon State University geoscientist who has published evidence supporting the St. Lawrence River theory, said in an email. Anders Carlson, Clark’s co-author and colleague at Oregon State University, sent a geological study finding that, as he put

Ice Age Floods Smithsonian Article Devastating Ice Age Floods That Occurred in the Pacific Northwest Fascinate ScientistsThe Scablands were formed by tremendous and rapid change, and may have something to teach us about geological processes on Marsby Riley Black – Science Correspondent – April 19, 2022 for Smithsonian Magazine The Earth seems to change slowly. Continents shift by about half an inch in a year. Sea levels rise by less than a quarter of an inch in the same amount of time. Mountains are constantly being eroded but, to us, seem to stand today just as they did yesterday and the day before. Our planet’s geological history often seems like one of slow, grinding change. But that’s hardly the whole story. Sometimes geological change comes startlingly, violently fast, leaving scars on the Earth’s surface. The Channeled Scablands of the Pacific Northwest, a landscape full of flat-topped plateaus that rise between steep-walled canyons, are among the vastly-altered landscapes that have caused researchers to rethink what they previously presumed. The geologic wounds are dramatic evidence that quick and catastrophic changes have played a significant role in shaping our planet. The Scablands, principally located in southeastern Washington state, bear the signs of an incredible Ice Age event. Between 14,000 and 18,200 years ago, huge glacial lakes on the boundaries of ice sheets burst from their natural dams and rushed over the landscape, scouring the hills and dropping massive stones as they went. Entire hills were washed away as the floodwaters dumped gravel, boulders and sediment in new places, almost like shaking a great geological Etch-a-Sketch. But this is a relatively new understanding, only broadly accepted since the 1970s. It took decades for geologists to construct even an outline of what the Scablands represent, a realization that proved to be a turning point for science. For if intense floods could carve such features once in Earth’s history, surely they could have changed landscapes at other times and in other places—even those as far away as the Martian surface. Geologists only began to pick up on the story of the Scablands a century ago. In the 1920s, naturalist J Harlen Bretz wrote several descriptive papers on the strange basins and odd channels of the area. Those channels had been created by moving water, but the way water had once flowed through the area seemed to make no sense. “The channels run uphill and downhill, they unite and they divide, they head on the back-slopes and cut through the summit,” Bretz wrote, “they could not be more erratically and impossibly designed.” The only reasonable conclusion, Bretz proposed, was that the Scablands were created by massive and short-lived floods. Bretz’s colleagues were not ready for such a conclusion. Ever since geology came into its own as a science in the 19th century, much of the field has been influenced by the concept of uniformitarianism—that the present is the key to the past. In a broad sense, that’s an excellent rule. The Earth is still changing, and many of those alterations—from erosion to volcanic eruptions—also occurred in the past. But some additional stipulations to the older formulation of the idea were unnecessarily taken as truth. One of them was that the Earth is changing at a slow, gradual rate and that quick, catastrophic change was impossible. Bretz’s idea for how the Scablands formed flew in the face of what many geologists accepted. Channels were carved over long periods of time by rivers, other geologists thought, not by sudden floods. To Bretz, the evidence was unmistakable. Among other things, the Scablands contained layers of gravel hundreds of feet high. Slow-moving streams couldn’t have left such vast accumulations. Pieces of gravel are larger and heavier than particles of sand or silt, requiring faster-moving water to pick up the gravel and transport it. Gravel deposits as tall as skyscrapers must have required an incredible amount of fast-flowing water. The pattern was consistent with the underlying geology of the area, as well. The rock beneath the massive flood deposits was relatively friable volcanic rock, easily broken and carved. The fragility of these rock layers allowed the floods to gouge out channels and canyons in a way that harder rocks would have been more resistant to. Yet the fact that Bretz couldn’t identify the source of the floodwaters caused many to dismiss his idea, and it wasn’t until evidence of similar events—such as Ice Age flood beds found in Montana—that other experts began to reconsider what Bretz had proposed. Eventually, by the 1970s, dismissive geologists changed their tune. Something catastrophic truly did transpire to create the Scablands. Not that the behavior or history of such floods are completely understood. “There are loads of outstanding questions and many people are thinking carefully about the Scablands,” says University of Washington geology PhD candidate Kelsay Stanton. Even though experts are confident that vast glacial lakes provided the water for the floods, the precise volumes of the repeated floods are unknown, and the timing of the dozens of outbursts has yet to be determined in detail. “The glacial outburst flooding of the Pacific Northwest is hardly a closed subject,” Stanton says. Part of what’s allowing geologists to keep going back to the Scablands is that the tools available to scientists have changed a great deal since Bretz’s time. “There are lots of geochronology and computer modeling methods now that weren’t available when Bretz and other early researchers were mapping the area,” says University of Massachusetts Amherst geology PhD candidate Karin Lehnigk. These repeated floods affected the ocean, she notes, with the influx of freshwater reducing the saltiness of the northern Pacific for years and altering the way colder, saltier water in deep ocean layers circulated. The floods affected more than the land they ran over, and have acted as models for how our modern glaciers might alter ocean circulation as they melt due to global warming. New, broad scale methodologies can offer some of these insights and these refined techniques have proved to be critical, as no one has ever