A controversial new idea suggests that a large space rock
exploded over North America 13,000 years ago.
The blast may have wiped out one of America's first Stone Age
cultures as well as the continent's big mammals such as the mammoth
and the mastodon.
The blast, from a comet or asteroid, caused a major bout of
climatic cooling which may also have affected human cultures
emerging in Europe and Asia.
Scientists will outline their evidence this week at a meeting in
The evidence comes from layers of sediment at more than 20 sites
across North America.
These sediments contain exotic materials: tiny spheres of glass
and carbon, ultra-small specks of diamond - called nanodiamond - and
amounts of the rare element iridium that are too high to have come
All, they argue, point to the explosion 12,900 years ago of an
extraterrestrial object up to 5km across.
No crater remains, possibly because the Laurentide Ice Sheet,
which blanketed thousands of sq km of North America during the last
Ice Age, was thick enough to mask the impact.
Another possibility is that the comet or asteroid exploded in the air.
The rocks studied by the researchers have a black layer which,
they argue, is the charcoal deposited by wildfires which swept the
continent after the explosion.