May 25, 2007
ACAPULCO, Mexico -- There's a new extraterrestrial suspect in the mysterious, highly debated disappearance of the woolly mammoth some 12,900 years ago. A team of two dozen scientists say the culprit was likely a comet that exploded in the atmosphere above North America.
The explosions sent a heat and shock wave across the continent, pelted the ground with a layer of telltale debris, ignited massive wildfires and triggered a major cooling of the climate, said nuclear analytic chemist Richard Firestone of Lawrence Berkeley Laboratory, one of the scientists who presented the controversial new theory Thursday at a conference of the American Geophysical Union in Acapulco.
At least 15 species, mostly large mammals including mammoths, mastadons, giant ground sloths, camels and horses, were wiped out about the same time. Firestone and his colleagues think some may have been killed by the explosions, and the rest died off after fires burned the vegetation they depended on.
"It seems awfully coincidental that the mammoths died at exactly the same moment where we find this impact layer," said Allen West, a member of the team from GeoScience consulting in Dewey, Ariz.
The scientists are bracing themselves for fiery reaction to their theory.
The extinctions were already a hotly debated event with scientists split between two theories. The leading theory is that man hunted the animals into extinction soon after arriving in North America, but some scientists think climate upheaval as the earth warmed up from the last ice age was the killer. Others think it was more of a one-two punch with climate change (http://www.redorbit.com/news/space/946673/comet_may_have_doomed_mammoths_lawrence_berkeley_lab_scientist_involved/index.html?source=r_space#) weakening the animal populations and hunters delivering the final blow.
But the impact theory has the advantage that it would explain why the Clovis hunting culture disappeared along with the animals, said archeologist Douglas Kennett of the University of Oregon (http://www.redorbit.com/news/space/946673/comet_may_have_doomed_mammoths_lawrence_berkeley_lab_scientist_involved/index.html?source=r_space#).
For 300 years, Clovis people hunted mammoths and other animals with distinctive spear heads with fluted ends known as Clovis points.
The team visited archaeological sites containing Clovis points across North America and collected samples of the sediment that covered the last signs of Clovis people and the last mammoth bones.
They found many distinctive signs of an extraterrestrial impact, including rounded magnetic grains that formed when pieces of the comet were thrown back into space after the explosion and then melted into round grains as they re-entered the atmosphere.
They also found high levels of iridium and titanium, a special form of carbon molecules known as fullerenes that had helium atoms trapped inside, and microscopic diamonds called nanodiamonds. They also found strange rounded carbon grains and glass-like carbon that looks like hardened black taffy. Lots of soot and charcoal are evidence of the ensuing wildfires.
"We really have found the mother lode of impact material," Firestone said.
Above this layer of strange sediment there are no signs of mammoths or many of the other large mammal species of the day, or of the distinctive Clovis points. At many of the sites there is a gap in evidence of humans for several hundred years before new types of spear heads show up.
Just like the mammoths, many of the Clovis people could have been killed by the comet. Those who lived would have struggled to survive once many of the animals they hunted died. The remaining people may have delivered the final blow to the last of the large mammals. These people made it through, and slowly the human population rebounded.
Post-Clovis people were more culturally diverse, as reflected by an array of different spear heads, Kennett said. "This may suggest population fragmentation."
The survivors could have been separated into smaller groups across the continent that developed differing cultural traditions and spear head designs.
The explosion may also be the key to an enigmatic episode of cooler climate, known as the Younger Dryas, that began about the same time and lasted about 1,000 years. The earth had been steadily warming up after the last ice age due to cyclical changes in the planet's orbit that brought it slightly closer to the sun. Then suddenly, the climate started cooling for no obvious reason at a time when it should have still been warming up.
Previously, the leading theory for the cause of the cooling was that as the ice sheet that covered most of North America retracted to the north, the meltwater that had been flowing to the south switched to the east as eastern river drainages were uncovered. This surge of fresh water into the Atlantic caused a change in the ocean circulation that shut down a northward flow of warm water that had acted as a heater for the northern hemisphere.
"Earth was warming and then suddenly went back into cooling for 1,000 years," Firestone said.
The evidence for a change in ocean circulation fits with the comet theory as well, said paleoceanographer James Kennett of UC Santa Barbara. He argues that the massive explosion above the North American ice sheet would have caused a huge amount of melting and destabilization around the edges of the ice that would have sent a torrent of fresh water into the Atlantic Ocean that could have affected circulation.
The new theory will undoubtedly come under fire from several directions, including the scientists that study the North American extinctions as well as from those who study extraterrestrial impacts.
Princeton University paleontologist Gerta Keller studies impacts and is skeptical that the explosion 12,900 years ago was big enough to have had such a big effect on climate and animals.
Keller has found evidence that much larger impacts did not cause any major extinctions. According to her, even the asteroid that left a massive crater on the Yucatan Peninsula known as Chicxulub and is thought to have killed off the dinosaurs (http://www.redorbit.com/news/space/946673/comet_may_have_doomed_mammoths_lawrence_berkeley_lab_scientist_involved/index.html?source=r_space#) did not actually cause their extinction.
"If Chicxulub didn't cause extinctions, then how could something this piddling cause these extinctions, and how could it have changed climate for 1,400 years?" she asked.