Comet Wiped Out Early North American Culture, Animals, Study Says
for National Geographic News
|May 23, 2007|
A comet exploded over North America about 13,000 years ago, causing a long bout of climate cooling, according to a controversial new theory presented today.
The extraterrestrial impact may help explain massive mammal die-offs and the demise of one of the earliest American cultures.
It would also be the first known extraterrestrial impact to affect modern humans.
Evidence for the impact comes from a thin layer of sediment found throughout North America, said James Kennett, a geologist at the University of California in Santa Barbara.
"There are materials with particular chemistries in that layer that collectively provide very strong evidence that the layer was produced by this extraterrestrial impact," he said in a telephone interview.
Kennett said the layer contains tiny spheres of carbon and metals, bits of diamonds, and extraterrestrial concentrations of helium 3 and the element iridium.
The layer dates to 12,900 years ago, he added.
At about the same time, according to the researchers, Earth's climate cooled; mammals like mammoths, mastodons, and saber-toothed cats went extinct; and one of the first American cultures disappeared.
Kennett and colleagues outlined evidence of the impact and its repercussions at a meeting of the American Geophysical Union this week in Acapulco, Mexico.
The timing of the impact, according to Kennett, coincides with an era of climate cooling known as the Younger Dryas.
"We are suggesting for the first time that this very abrupt and large cooling that occurred at 12.9 thousand years ago was triggered by the extraterrestrial impact," he said.
The impact would have destabilized an ice sheet over North America, which allowed large bodies of fresh water to drain into the ocean and alter circulation patterns, he explained.
The change in currents included a temporary halt to one that brings warm, tropical waters to the North Atlantic.
Scientists have long debated whether the cooling event or human hunters are responsible for the extinction of the so-called North American megafauna.
At least 17 species are thought to have gone extinct around the time of the proposed impact, according to the researchers.
(Read related story: "Humans to Blame for Ice Age Extinctions, Study Says" [August 10, 2005].)
"We find it rather unbelievable that human hunters would drive a fairly large variety of animals into extinction fairly suddenly at 13,000 years ago," Douglas Kennett, an archaeologist at the University of Oregon in Eugene, said in a telephone interview.
Douglas is James Kennett's son and a project team member.
"We think a better explanation is this dramatic [impact] event, which would be coupled with the fallout and major changes in climate and environments that would have associated with it," he continued.
Douglas added that the impact may be linked to the disappearance of the Clovis culture. Clovis people are considered to be among the earliest Americans and at one time were thought to be the first.
According to Douglas, archaeological evidence suggests Clovis populations, which were spread out across the Americas prior to 12,900 years ago, became fragmented.
The archaeological record shows a time gap between the presence of Clovis people and later cultures, particularly in the Great Lakes region where the comet is thought to have struck, he added.
"The basic story seems to be, there's more of a gap in the East and less of a gap in the West," he said.
David Meltzer is an archaeologist at Southern Methodist University in Dallas, Texas, who is not part of the research team. He said the theory is far from proven.
First, he said, the team must prove a comet did in fact hit Earth 12,900 years ago, an issue that geologists will eventually resolve.
Then, if an impact is demonstrated, the team has to show what the effects were.
"At the moment, the issues are far more complicated than all animals died at once and people suffered tremendously," he said.
For example, some of the big animals went extinct well before the proposed impact, and others disappear later.
Nor does Meltzer see evidence for the disappearance of Clovis populations.
"At least out on the [Great] Plains, populations are booming [at the time of impact], they're not declining at all despite this horrific global conflagration," he said.
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