Extraterrestrial impact possibly wiped out all life forms on Earth during the Ice Age, a new study by a team of international researchers, including two Northern Arizona University geologists, has revealed.

What caused the extinction of mammoths and the decline of Stone Age people about 13,000 years ago remains hotly debated. Overhunting by Paleoindians, climate change and disease lead the list of probable causes.

But now, a new study has reported evidence of a comet or low-density object barrelling toward Earth, exploding in the upper atmosphere and triggering a devastating swath that wiped out most of the large animals, their habitat and humans of that period.

"The detonation either fried them or compressed them because of the shock wave. It was a mini nuclear winter," said Ted Bunch, NAU adjunct professor of geology and former NASA researcher who specializes in impact craters.

Till date, no one has found a giant crater in the Earth that could attest to such a cataclysmic impact 13,000 years ago.

But the research team has offered evidence of a comet, two and a half to three miles in diameter, that detonated 30 to 60 miles above the earth, triggering a massive shockwave, firestorms and a subsequent drastic cooling effect across most of North America and northern Europe.

"The comet may have broken up into smaller pieces as it neared the Earth and then these pieces detonated in various places above two continents," Bunch said.

They said the evidence for multiple detonations came from a four-inch-thick "black mat" of carbon-rich material that appears as far north as Canada, Greenland and Europe to as far south as the Channel Islands off the coast of California and eastward to the Carolinas.

Two sites still exist in Arizona at Murray Springs and Lehner Ranch, both near Sierra Vista, they said.

They said evidence of mammoths and other megafauna and early human hunters, known as the Clovis culture, are found beneath the black mat but are missing entirely within or above it.

The team believes this is clinching evidence to prove that an extraterrestrial impact wiped out many of the inhabitants of the Late Pleistocene period.

The researchers said the black mat was formed by bonding of water and algal blooms and contains carbon, soot and glassy carbon - remnants of burned materials, adding that some of these remnants are extraterrestrial in nature.

The team has identified fullerenes, spherical carbon cages resembling a soccer ball, which are formed in shock events outside the Earth's atmosphere. Trapped inside the fullerenes is a concentration of helium 3 that is many times greater than what is found in the Earth's atmosphere.

Scientists say the black mat has also turned up nanodiamonds, which are formed in the interstellar medium outside the solar system, by or by a high-explosive detonation.

"Either these things came in with the impactor or they were made during impact detonation. We have no other explanation for their presence," Bunch said.

Bunch said the magnitude of the detonations would have been huge.

"A hydrogen bomb is the equivalent of about 100 to 1,000 megatons. The detonations we're talking about would be about 10 million megatons. That's larger than the simultaneous detonation of all the world's nuclear bombs past and present," said Bunch.

The team says the detonations destabilized a vast ice sheet, known as the Laurentide Ice Sheet, that covered most of what was then Canada and the northern US.

Heat from the detonation and firestorms would have melted much of the ice sheet, releasing water vapour into the atmosphere. The result was rapid cooling of about eight degrees over the next 100 years. The melting of the ice sheet and subsequent climate change would explain the water-based nature of the black mat," Bunch added.

The study appears online in the Proceedings of the National Academy of Sciences.