By Molly Bentley
Woolly mammoths disappeared rapidly from North America
The controversial idea that space impacts may have wiped out woolly mammoths and early human settlers in North America has received new impetus.
Nano-diamonds and other exotic impact materials have been unearthed in thin sediments, Science magazine reports.
The age of these materials coincides with the start of a millennium-long climate cooling event known as the Younger Dryas - some 13,000 years ago.
Many large animals vanish from the archaeological record at this time.
It is also the period in Earth history that sees the demise of Clovis culture - the prehistoric hunters that many regard as the first human occupants of North America.
Taken together, it all makes for a compelling story, claims the team behind the latest research.
Question of origin
The group used transmission electron microscopy (TEM) to identify tiny impact diamonds found at a range of sites - four of them Clovis archaeological digs - across North America. Diamonds form through intense pressure and heat.
"We've discovered nano-diamonds that are not normally produced through average processes on the surface of the Earth," said James Kennett, a geologist at the University of California, Santa Barbara, and author on the Science paper.
"They indicate there was an extra-terrestrial event on Earth 12,900 years ago," he told BBC News.
The nano-diamonds coincide with the onset of cooling, says the team
Scientists last year reported the discovery of five types of nano-diamonds along with impact material such as iridium and magnetic microspherules in the Younger Dryas impact layer, a thin blanket of sediment 12,900 years old.
The new analysis with TEM, they said, confirmed an abundance of diamonds in carbon spherules - melt material that forms in a fraction of a second - and the identification of lonsdalite, or hexagonal diamonds, associated with meteorite explosions.
The sheer number of diamonds - up to a million times that found in neighbouring sediment - and their presence inside spherules, refutes the speculation that the material is the normal rain of meteorite debris, says Allen West, a retired geophysicist in Arizona and a co-author.
"There is no other way that hexagonal diamonds could have ended up in a carbon spherule in this number," said Dr West.
The absence of some traditional impact material and visible craters in North America led researchers to speculate that a meteoroid or comet disintegrated before exploding in a cluster of airbursts.
Researchers argue that the airbursts could have triggered a series of dramatic climate shifts - including colder temperatures and an abrupt change in vegetation - that would have made survival difficult for large mammals and Clovis hunters.
The cause of the disappearance of Clovis culture and megafauna has long been debated.
Sceptics of the impact theory are not won over by the latest data. While scientists agree that something dramatic occurred on Earth 12,900 years ago, the theory that it was an exploding space rock has been cast by some as long on dramatic flair, short on compelling evidence.
Nicholas Pinter, a geologist at Southern Illinois University, said he had yet to see classic evidence of an asteroid impact.
The so-called discrete layers of material were not of a uniform age, he said. Microspherules, for example, rain down all the time and are present throughout the geological record.
"My graduate student found some on his mailbox," said Dr Pinter.
While Dr Kennett proposed that ordinary carbon was forged into diamonds in the intense pressure of an airburst, Dr Pinter said nano-diamonds are now being identified at other locations and times without credible evidence of any impact.
The suggestion that they could have been produced by an airburst event is "untested and highly implausible," he argued.
"Time will tell, but so far the Younger Dryas impact looks like an increasingly desperate fishing expedition for supporting evidence," said Dr Pinter.
Impact theorists maintain that the diamonds peak in abundance in the impact stratum.
The thumb width layer appears in a number of sites across North America including Murray Springs in Arizona and the Channel Islands off the coast of Southern California.
It lies beneath a black mat of biomass formed during the Younger Dryas.
Diamonds occur at the base of this black layer at Murray Springs, directly above extinct animals remains
The bottom-most film contains charcoal and soot, thought to be associated with impact fires, said University of Oregon geo-archaeologist Doug Kennett, son of James Kennett and another author on the Science paper, who has studied sedimentary vegetation and charcoal records.
No mega-fauna skeleton or Clovis artefact has been found above the impact layer or the black mat, he said.
"The black mat covers them like a blanket," said Dr Kennett.
Before they disappeared, woolly mammoths and other massive beasts such as sabre-toothed cats, giant sloth, camels, and teratorns (predatory birds with a nearly four-metre wingspan) roamed North America.
Doug Kennett doubts the theories of over-hunting, climate change and disease used to account for their extinction. There are not enough Clovis kill sites to suggest that the animals were over-hunted, for example, he said.
The animals' disappearance coincides with that of Clovis artefacts in the archaeological record 12,900 years ago. Prehistoric Clovis Indians lived broadly across North America for a few hundred years.
They were big game hunters, who introduced a sophisticated new Stone Age technology - the fluted spear point, known today as the Clovis Point.
The Paleo-Indians vanish at the onset of the post-Ice Age Younger Dryas, or Big Freeze, that snapped Earth back to near glacial conditions, where it lingered for about 1,200 years.
The causes of the woolly mammoth extinction, the collapse of Clovis culture and the onset of the cold snap have long been debated. But only the impact theory accounts for the simultaneous occurrence of all three, said Doug Kennett.
Others are wary of the link. Jeff Severinghaus, a geochemist who studies ice cores at Scripps Institution of Oceanography, is sceptical that an impact could have led to the temperature plunge.
He said records from Greenland suggested the cooling began earlier than 12,900 years ago. However, he is keeping an open mind.
"I'm still in a wait-and-see mode," added Dr Severinghaus.
Cosmic impacts are known to have profound climate consequences.
In this case, scientists propose that flash heat and pressure from an explosion destabilised the edges of the Laurentide ice sheet that covered North America, adding fresh water to the North Atlantic, and slowing the conveyor of warm water that heats Western Europe.
Impact debris kicked into the atmosphere would have cooled the Earth and led to a number of ecological disruptions, including abrupt shifts in vegetation.
Critics say that with an impact comes a crater, such as at Chicxulub in the Yucatan, Mexico, which supports the theory that dinosaurs were wiped out by an asteroid impact 65 million years ago.
But an airburst in which an impactor explodes in the Earth's atmosphere - such as that over Tunguska, Siberia, in 1908 - might not produce a crater, said Dr James Kennett.
The energy released in the Tunguska blast was at least five megatons, said Dr West. The Younger Dryas impact would have been much larger.
"Imagine 1,000 to 10,000 atomic bombs detonating within a few minutes over two continents," he said.
Had the Clovis people witnessed the event, said Dr West, they would have seen a brilliant flash followed by others in quick succession.
The sky would be a canopy of fire, and shock waves would flatten trees. Miniscule diamonds would drizzle over tens of thousands of kilometres, a third of the way around the planet.
Nasa (Ames) space scientist David Morrison says the abundance of nano-diamonds is an "interesting mystery", but he does not think they were produced by cosmic airbursts.
The Clovis culture featured elegant stone implements
A comet or asteroid that fragmented in Earth's atmosphere might have time to disperse over a few hundred kilometres, but certainly not thousands of kilometres across a continent, he said.
"I know of no mechanism that would break up a comet and distribute it over North America in the way they suggest," Dr Morrison told BBC News.
"It violates what we understand about cosmic impacts," he said.
Scepticism is necessary when building a new scientific theory. But, Dr West said, there was particular resistance to that of a Younger Dryas impact because the event occurred in modern human history and was so abrupt.
"People still like to think of geological processes happening slowly over time," he explained.
"It's unsettling that something happening in a few minutes could flip our climate and cause widespread extinctions."