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Volcanoes fingered in oceanic mass extinction

Dramatic late Cretaceous anoxic event

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Scientists reckon they've confirmed what caused an oceanic mass extinction during the late Cretaceous period, 93 million years ago, provoked by a sudden, and previously unexplained "anoxic event" which deprived bottom-dwelling species of oxygen.

According to a report in Nature, the cataclysm was provoked by massive undersea volcanic eruptions in a plateau under the present-day Caribbean which "devastated animals living on the sea-floor such as foraminifera, millimetre-sized creatures that lie at the bottom of the oceanic food-chain".

While scientists had previously suspected a volcanic link to the extinction, Steven Turgeon, a geologist at Canada's University of Alberta, said: "What was missing was a clear link in the sedimentary record."

To make the case, Turgeon and colleague Robert Creaser looked at levels of two isotopes of osmium in sedimetary shale samples from the sea bed off South America and from mountains in Italy. They found that in both cases "the ratio of osmium-187 to osmium-188 dropped dramatically just before the extinction event", indicating "a huge amount of molten magma, carrying a higher proportion of the heavier isotope, was discharged into the oceans".

The figures represent, Turgeon said, a sudden and "astounding" 3,000–5,000 per cent increase in global volcanic activity. The Caribbean plateau was "the only volcanic site large enough to spew forth that much magma in the geologically-fleeting space of a few hundred of thousand years", the researchers concluded.

Tim Bralower, a marine geologist at Pennsylvania State University, agreed the study has "nailed the coffin shut" on the matter of whether the plateau was the source of the event, but noted that it remained to be proved quite what drove the mechanism of extinction.

Nature notes: "One theory is that minerals in the magma fertilized the ocean and led to a huge bloom of creatures that then died and decayed, sucking up oxygen in the process."

Of the eruption itself, Bralower added: "We really don't know why then or why there." He suggested the cause of the big bang "may lie in the mantle, or perhaps even at the outer edge of earth's core", and concluded: "That is now the $64 million dollar question." ®

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