Author: Carolyn Gramling / Source: Science News for Students
Around 13,000 years ago, a chilly Earth was thawing. It was the end of the last great ice age.
Vast frozen sheets had covered much of North America, Europe and Asia for thousands of years. Now they were retreating. Giant mammals like woolly mammoths and saber-toothed cats roamed the tundra and grasslands. An ancient people called the Clovis roamed North America, hunting mammoth with distinctive spears.Then, about 12,800 years ago, something strange happened. Earth abruptly plunged back into a deep chill. Temperatures in parts of the Northern Hemisphere fell to as much as 8 degrees Celsius (almost 14.5 degrees Fahrenheit) colder than today. Then, just as abruptly, Earth once again began to warm.
The cold snap lasted only about 1,200 years. That’s a mere blip in the history of our planet. But afterward, many of the giant mammals were dying out. And signs of the Clovis people were now gone.
Geologists call this cold snap the Younger Dryas. Its cause is a mystery. Most researchers think a large pulse of freshwater from melting ice flooded into the ocean. This might have interfered briefly with ocean currents that carry heat around the world. However, geologists have not yet found firm evidence of how and where this happened. For example, they haven’t found traces of the path that this ancient flood took to the sea.
But for more than a decade, one group of researchers has made waves by suggesting a more controversial story. They blame a space rock for the sudden deep freeze. About 12,800 years ago, these researchers say, a comet — or pieces of a comet — hit or exploded over the Laurentide Ice Sheet. This ice sheet once covered much of North America.
Pieces of the comet most likely exploded in Earth’s atmosphere, the researchers say. This kind of aboveground explosion is called an airburst. And this triggered wildfires across North America. Those fires gave off enough soot and other compounds to block out the sun and cool the planet.
Most scientists think a similar explosion happened in 1908 over Siberia. That event produced as much energy as 1,000 Hiroshima atomic bombs. The researchers behind the impact hypothesis think the Younger Dryas started with an even larger explosion. That would neatly solve several prehistoric puzzles, they note. For instance, it could explain the extinction of large animals as well as the disappearance of the Clovis people.
Many scientists question the space rock theory
For more than a decade, researchers have argued in scientific journals about this hypothesis. Experts from a wide range of scientific fields think the impact hypothesis is wrong. They say there’s little to no solid evidence for many of its key points.
“Over and over and over, there are these things that are claimed to be [indirect evidence of] an impact,” says Vance Holliday. He’s an archaeologist and geologist at the University of Arizona in Tucson. But these bits of so-called evidence are “all debatable,” he says — “every single one.”
Allen West disagrees. He’s long been a lead supporter of the impact hypothesis. West is a retired geophysicist who owned GeoScience Consulting in Dewey, Ariz. He admits that many types of scientists have attacked the hypothesis. “We have different battles with different disciplines,” he says.
He compares these battles to the fights in the 1980s over whether an asteroid struck Earth 66 million years ago, killing off all dinosaurs except the birds.
“There were just vicious, nasty attacks for nearly a decade on that,” West says. “People said it just couldn’t have happened.” But then the crater turned up. Today, the asteroid-based extinction is widely accepted. Hard-to-refute evidence, like that telltale crater, he says, is “probably what it would take with us, too.”
Indeed, no craters have been found from the Younger Dryas. And the landscape of North America — the likely site of such an impact — has been pretty thoroughly checked out. Without a crater or other direct evidence of an impact, West and his colleagues have turned to indirect evidence. They’ve published a steady stream of findings that they say are signs of an impact some 12,800 years ago.
The latest came out in March. West and more than two dozen other researchers published a pair of papers in the Journal of Geology. These papers included data from ice cores and sediment cores — tube-shaped samples drilled from land or under the sea. The cores held signatures of giant wildfires that may have happened during widespread burning about 12,800 years ago, West notes.
Some opponents of the impact hypothesis, however, just feel fed up by the new papers. For 10 years, “they keep building on their past record, ignoring the critiques,” Holliday says. “It just drives me crazy.”
Birth of a hypothesis
The world first heard about the Younger Dryas impact hypothesis in 2007.
Four researchers, including West, sat in front of reporters at the American Geophysical Union’s spring meeting. It was held in Acapulco, Mexico. The researchers had taken a close look at about two dozen sites across North America. Underground, the sites all showed a “boundary layer” — a stripe in the sediment dating to the start of the Younger Dryas. Half a dozen of the sites also had thin layers called “black mats” right above the boundary layer. Several of those half-dozen sites also showed signs that the Clovis people lived there.
The black layers were made of soft soil with a lot of nutrients. They apparently marked when the Clovis people stopped living at these sites. For example, a…
The post An ancient cold snap causes heated debate appeared first on FeedBox.