Volcanoes Blamed for Mass Extinctions

July 16, 2008 -- Ninety-three million years ago, Earth was a reshuffled jigsaw of continents, a hothouse where the average temperature was nearly twice that of today.

Palm trees grew in what would be Alaska, large reptiles roamed in northern Canada and the ice-free Arctic Ocean warmed to the equivalent of a tepid swimming pool.

So our planet was balmy -- but hardly a biological paradise, for it was whacked by a mass die-out. The depths of the ocean suddenly became starved of oxygen, wiping out swathes of marine life.

The extinction was so spectacular that, helped by a suddenly sluggish shift in ocean circulation, the remains of the tiny victims littered the sea bed in thick layers, and over geological time became transformed into oil.

After the extinction, levels of greenhouse gases in the atmosphere dropped and Earth lurched into a sudden, but short-lived, period of cooling.

Earth scientists have pondered for years as to how this extraordinary "anoxic event" of the late Cretaceous took place.

The answer to the catastrophe, contend scientists from the University of Alberta, Canada, lies in fire fountains that erupted on the ocean floor, altering the chemistry of the sea and possibly of the atmosphere too.

Steven Turgeon and Robert Creaser, of the university's Department of Earth and Atmospheric Sciences, say the clue can be found in isotope levels of the element osmium, a telltale of volcanism in seawater, that were analysed in black shale rocks, drilled off the coast of South America and mountains in Italy.