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.

The eruptions -- so violent that stacks of lava flowed out to form the bed of the Caribbean -- preceded the extinction by up to 23,000 years, according to their research, which appears on Thursday in the London-based weekly science journal Nature.

Two theories, which are not mutually exclusive, emerge to explain the chemistry of what happened next, says Tim Bralower, a geologist at Pennsylvania State University, who reviewed the paper.

One possibility is that the volcanoes spewed out metal-rich fluids that seeded the upper level of the ocean with micronutrients, he says.

Tiny plantlife on the sea surface, called phytoplankton, gorged on the food, and storing up carbon as they grew. They then sank to the sea floor and decayed, stripping the ocean of oxygen.

The other is that the volcanoes disgorged clouds of carbon dioxide to the atmosphere, stoking global warming to the extent that Earth's ocean circulation system ground to a near-halt. Beyond the surface layers, water was no longer turned over and anoxia resulted.

Bralower says that figuring out the post-volcanism scenario could help scientists wrestling with unknowns about global warming today.

The knowledge gaps include the impact of higher temperatures on marine circulation and whether controversial schemes to sow the ocean with iron filings, to spur phytoplankton growth and thus soak up carbon dioxide from the atmosphere, would ease warming or cause oxygen starvation in the sea depths.

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