Add a new suspect to the list of what might have killed the ice-age megafauna of North America – a barrage of debris from a disintegrating comet. Instead of a four-kilometre comet blowing a single crater in the North American ice sheet 13,000 years ago, the infalling debris would have filled the sky with a series of megatonne-scale explosions like the Tunguska event of 1908.
A series of fiery blasts could be every bit as deadly as one big impact. That's why planetary scientists have warned that blowing up an asteroid about to hit the Earth could still cause major damage. But others hold that no cosmic catastrophe is needed to explain the demise of mammoths and mastodons, and the end of Clovis culture.
The new proposal comes from Bill Napier, an astronomer at Cardiff University, who in 1982 co-authored a book titled The Cosmic Serpent. In it, he and Victor Clube suggest that the outer planets sometimes divert giant comets into the inner solar system, where they fragment over thousands of years. The theory of coherent catastrophism claims that we live at a time of increased impacts, because one of those giant comets entered the inner solar system 20,000 to 30,000 years ago, leaving behind the Taurid meteor shower, Comet Encke and other debris.
In a paper now at Arxiv.org that is to appear in the Monthly Notices of the Royal Astronomical Society, Napier argues the real catastrophe was the Earth hitting a clump of debris from a 50- to 100-kilometre comet. "Running into thousands of fragments from this comet is a much more likely event than a single large collision," he says. "It gives a convincing match to the major geophysical features at this boundary," including the deposition of nanodiamonds contained in the fragments.
But recent studies continue to raise doubts about the impact theory. In December, researchers reported that they had found no geochemical signatures of an impact in the layer of sediment formed at the time.
Meanwhile, a different kind of catastrophe has been found 13,000 years ago - the sudden drainage of a huge Canadian lake. Meltwater at the base of the North American ice sheet had accumulated in the lake because it had nowhere else to go. Eventually whatever held the water in place along the edge of the glacier broke or melted, causing the water to flow away.
Geologists had looked in vain for evidence of drainage through the Saint Lawrence River into the Atlantic Ocean. Wallace Broeckner of the Lamont-Doherty Earth Observatory had suggested this would have caused a sudden dramatic cooling by shutting down the global conveyor belt of ocean circulation.
But a paper just out in Nature shows that instead the water drained north through the Mackenzie River, dumping some 9500 cubic kilometres of water into the Arctic Ocean. Broeckner says such an Arctic release might have been an even better climate trigger, obviating the need for an impact to cause the cold spell that occurred about 13,000 years ago.
Source: New Scientist