Palaeozoic World

A huge cosmic explosion could have caused a mass extinction on Earth 450 million years ago, according to an analysis by scientists in the US.

A gamma ray burst could have caused the Ordovician extinction (which occurred between 444-447 million years ago at the boundary between the Ordovician Period and the subsequent Silurian Period), killing 60% of marine invertebrates at a time when life was largely confined to the sea.

This was the second largest of Earth's major extinction events in terms of the percentage of life forms that became extinct.

Such cosmic blasts are the most powerful explosions in the Universe, and the scientists think a 10-second burst near Earth could deplete up to half of the planet's ozone layer.

With the ozone layer devastated, the Sun's ultraviolet radiation could have killed off much of the life on land and near the surface of oceans and lakes.

Gamma ray bursts are rare occurrences, but scientists estimate that at least one must have occurred near the Earth in the past one billion years.

Scientists think that gamma-ray bursts are generated in two principal scenarios. In one scenario, a star collapses in on itself, giving birth to a black hole and releasing a high-energy jet of material travelling at close to the speed of light.

The bursts could also be generated when two neutron stars collide. "A gamma ray burst originating within 6,000 light-years from Earth would have a devastating effect on life," said co-author Dr Adrian Melott, an astronomer at the University of Kansas, US.

"We don't know exactly when one came, but we're rather sure it did come - and left its mark. What's most surprising is that just a 10-second burst can cause years of devastating ozone damage."

Surface-dwellers

Dr Melott and his colleagues used computer models to calculate the effects of a nearby gamma ray burst on the Earth's atmosphere and its life forms.

They showed that up to half the ozone layer would be destroyed within weeks. Five years on, at least 10% would still be missing.

Although deep sea creatures would be protected from the effects of the burst, surface-dwelling plankton and other life near the top of the ocean would not survive.

This would have had huge implications for other life forms, because plankton form the foundation of the marine food chain; they provide for animals which are then eaten by larger species.

Bruce Lieberman, a palaeontologist at the University of Kansas, originated the idea that a gamma ray burst could have caused the Ordovician extinction. An ice age has been implicated by other scientists in the extinction.

The latest research shows that a gamma ray burst could have caused a fast die out early on and could also have triggered a drop in temperature similar to the effect of an ice age.

Swift, a Nasa space mission launched in November 2004, is currently investigating the phenomenon of gamma ray bursts from Earth orbit.

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