Double asteroid-hit may have caused mass extinction

EVOLUTION

Two huge asteroid impacts on the Earth may have led to mass extinction of species 35 million years ago, according to new research. The collisions occurred within a few hundred thousand years of each other - a brief pause, in geological terms.

The "dinosaur-killer" asteroid, which hit the Gulf of Mexico, had already occurred: that happened about 65 million years ago. But the "double whammy" of asteroid impacts would have led to widespread species extinction by throwing up huge quantities of dust into the atmosphere, changing the global climate abruptly.

Scientists had known for some time of the impacts, the first of which was in Siberia, visible now as the 100-kilometre wide Popigai crater. But they disagreed about the date, putting it at anything between five million and 65 million years. But now a new study of rocks melted by the impact has narrowed the date down to 35.7 million years ago. A few hundred thousand years later, another huge object from space smashed into Chesapeake Bay in the US. The collisions were just before the so-called "Eocene cataclysm" - marked by an abrupt reduction in biodiversity. The findings, by a team led by Richard Grieve of the Geological Survey of Canada in Ottawa, appear today in the science journal Nature.