[TheDude]

Story in the Economist.

$this->bbcode_second_pass_quote('', 'A')RCHAEOLOGISTS know little of what life was like for the first people in America. One thing they thought they knew, however, was that these people hunted mammoths and other large mammals until none was left. Once that had happened, they disappeared themselves.

But the Clovis people of 13,000 years ago, named after the place their characteristic arrowheads and spear-points were first found, may not be a bell-tolling example of the danger of technological progress after all. They may have gone, along with mammoths, short-headed bears, ground sloths and camels, because a comet exploded over their heads.

If that were true, North America would have suffered the first world-changing impact of an extraterrestrial object in the era of modern man. And because it happened so recently—“like yesterday” in the words of James Kennett of the University of California, Santa Barbara—any amateur-archaeologist could inspect the evidence for himself by digging deep enough. In many places in the United States and Canada, at a depth corresponding to 12,900 years ago, a few centimetres of charcoal will appear. This, according to Dr Kennett, is the product of wildfires that spanned the continent after an object roughly a kilometre across grazed the Earth, broke up into ever smaller pieces and deposited all of its oomph as heat into the atmosphere.

Mixed in with the charcoal, as Dr Kennett and a number of his colleagues reported to a meeting of the American Geophysical Union in Acapulco this week, are other bits that point to such an impact. There are glass-like beads made of carbon. To melt carbon requires a temperature of around 4,000°C. Such heat is hard to come by on Earth. And only this month it was discovered, by looking at the beads in an electron microscope, that they are chock full of diamonds a micron or less across. Finding the temperature and pressure to create such diamonds on the surface of the Earth would be impossible, unless a huge hammer-blow from space is meted out.

Something the researchers have not found in this layer is iridium, or at least not a lot of it. This metal is the fingerprint, the world over, of the impact some 65m years ago of an asteroid, in what is now Mexico. That collision caused the demise of the dinosaurs. A lack of iridium is the reason Dr Kennett and his colleagues think the collision they believe they have discovered was with a comet rather than an asteroid. Comets consist of “dirty ice”, rather than solid rock, and so contain little iridium.

That something big happened around 13,000 years ago was already known. Palaeontologists have a name for the sudden cooling of the climate of the northern hemisphere that started right then and lasted for over a thousand years. They call it the Younger Dryas.

Before the Younger Dryas, the world had been warming up as the last ice age came to an end. It was this warming that gave the Clovis people and their prey a chance to thrive. But as for the cause of the Younger Dryas, the eyes of science were, until now, directed not up at the sky, but down into the ocean.

Dr Kennett himself contributed to the body of work that suggests changing ocean currents caused the cooling. There is evidence that currents which bring warm water from the tropics to the Arctic, as the Gulf Stream does now, can be pushed around. Dr Kennett worked on the idea that at the start of the Younger Dryas a huge lake of fresh water suddenly breached a wall of ice that had confined it, and flowed into the northern Atlantic Ocean. That would have altered the heat- and salinity-driven dynamics of the area rapidly, stopping the warm current from travelling so far north.

[Tanada]

Oh I dunno, there is considerable evidence mounting that a lot people lived here before the Clovis groups took over. If this Dryas comet event took place as described it would explain how the Clovis people were able to supplant the prior peoples and leave almost nothing but archeological digs to show they had been here before.