Layers of clues: The Contessa Quarry in central Italy where rock for part of the study was drilled from. The different coloured sediment layers represent deposition of organic carbonate under oxygenated versus anoxic ocean conditions.

Credit: S.Turgeon

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PARIS: 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 mysterious mass extinction. As the fossil record attests, the depths of the ocean suddenly became starved of oxygen, wiping out swathes of marine life.

Spectacular extinction

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 researchers from the University of Alberta, Canada, lies in fiery fountains that erupted on the ocean floor, altering the chemistry of the sea and possibly of the atmosphere too.

Earth scientists Steven Turgeon and Robert Creaser argue that the answer can be found in isotope levels of the element osmium, a telltale clue of volcanism in seawater. These levels were analysed in black shale rocks, drilled off the coast of South America and Italian mountains.

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, said the researchers. They detailed their findings last week in the British journal Nature.

Two competing theories

Two theories, which are not mutually exclusive, emerge to explain the chemistry of what happened next, commented Tim Bralower, a geologist at Pennsylvania State University, in the U.S., 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 said. 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, said Bralower, 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 said 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.