Mammoth Collision: Did asteriod kill ice age mammals?

October 2, 2007

Courtesy of  Far North Science
By Doug O'Harra

Woolly Mammoth
Credit: Wikipedia Commons

As any video-watching child knows well, the late centuries of the ice age fairly rocked with these immense beasts and their furry companions.

And then, about 13,000 years ago, most of this fabulous megafauna rapidly died out. Mammoths went extinct. Lions and tigers and horses and sloths disappeared. Scientists have long debated the causes.

Was it overhunting by Pleistocene humans with spears, wild fire and shrewd killing strategies? Or was it a sudden shift in the climate that destroyed the productive grassy steppe that supported this network of grazers and their predators?

Now, an international team of scientists led by Richard Firestone of Lawrence Berkeley National Laboratory has uncovered evidence that an immense explosion - accompanied by a nuclear-strength shockwave - triggered a catastrophic climate shift that wiped out life across America. The work follows up on earlier findings by Firestone that a supernova 41,000 years ago may have created a killer space rock and sent it tumbling into the home planet.

In other words, just like the dinosaurs before them, the mammoths may have been fried by an asteroid.

"Our theory isn't a slam dunk," said Brown University planetary geologist Peter Schultz, one of the researchers, in an online story. "We need to study a lot more sediments to get a lot more evidence. But what is sobering about this theory of ours is that this impact would be so recent. Not so long ago, something may have fallen from the sky and profoundly changed our climate and our culture."

The paper (in PDF format), published last week in the Proceedings of the National Academy of Sciences describes a startling theory that one or more comets or meteorites smashed into North America - or exploded in the atmosphere above the continent - and creating a catastrophic, life-smothering shift in the climate.

"We propose that one or more large, low-density (extraterrestrial) objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering (Younger Dryas) cooling," wrote the 25-member scientific team in the paper abstract. "The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America."

The amount and type of material - combined with the lack of an impact crater of sufficient size and date - suggests that a 2.5-mile-wide comet slammed into the mile-thick ice sheet about 12,900 years ago, thus leaving no single impact crater. An alternative explanation has the comet breaking into several pieces or exploding in the air.

The impact would have struck with about 10 million megatons of energy, the scientists said. The most poweful hydrogen bomb ever detonated released an estimated 58 megatons. Do the math. The comet that killed the mamoths was as powerful as 170,000 hydrogen bombs.

The description of what happened next portrays a hellish ecological apocalypse, complete with unimaginable winds, immense tornadoes and vast forest fires, all of it climaxing with a 1,000-year reign of ice.

The authors write:

The ... event would have created a devastating, high-temperature shock wave with extreme overpressure, followed by underpressure, resulting in intense winds traveling across North America at hundreds of kilometers per hour, accompanied by powerful, impact-generated vortices.

In addition, whether single or multiple objects collided with Earth, a hot fireball would have immersed the region near the impacts .. Thus, multiple, larger airbursts would have ignited many thousands of square kilometers. At greater distances, the reentr y of high-speed, superheated ejecta would have induced extreme wildfires (53), which would have decimated forests and grasslands, destroying the food supplies of herbivores and producing charcoal, soot, toxic fumes, and ash.

With so much ash and poison ejected into the air, sunlight would have been blocked, triggering the Younger Dryas period - when the Northern Hemisphere abruptly chilled up to 20 °F and returned to heavily glaciated conditions. Previous theories for the cause of this planetary cold snap has focused on the shutdown of the Atlantic Ocean circulation that brings warm Gulf Stream water north. (The inspiration for the schlocky disaster movie The Day After Tomorrow.)

Here's more detail from Brown University.

Found in more than 50 sites around North America, this puzzling slice of geological history is a mere three centimeters deep and filled with carbon, which lends the layer its dark color. This black mat has been found in archaeological digs in Canada and California, Arizona and South Carolina - even in a research site in Belgium.

The formation of this layer dates back 12,900 years and coincides with the abrupt cooling of the Younger Dryas period, sometimes called the "Big Freeze." v This coincidence intrigued the researchers, led by Richard Firestone of Lawrence Berkeley National Laboratory, who thought that the black mat might be related to the mass extinctions.

So the researchers studied black mat sediment samples from 10 archaeological sites dating back to the Clovis people, the first human inhabitants of the New World. Researchers conducted geochemical analysis of the samples to determine their makeup and also ran carbon dating tests to determine the age of the samples.

Directly beneath the black mat, researchers found high concentrations of magnetic grains containing iridium, charcoal, soot, carbon spherules, glass-like carbon containing nanodiamonds and fullerenes packed with extraterrestrial helium - all of which are evidence for an extraterrestrial impact and the raging wildfires that might have followed.

Schultz, professor of geological sciences at Brown and an impact specialist, said the most provocative evidence for an extraterrestrial impact was the discovery of nanodiamonds, microscopic bits of diamond formed only from the kind of intense pressure you'd get from a comet or meteorite slamming into the Earth.

"We don't have a smoking gun for our theory, but we sure have a lot of shell casings," Schultz said. "Taken together, the markers found in the samples offer intriguing evidence that North America had a major impact event about 12,900 years ago."