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Palaeozoic World

Gamma Rays and the Ordovician-Silurian Extinction

Edited from BBC News, 11 April 2005. Updated 17 December 2017

An analysis by scientists in the USA in 2005 discovered that a huge cosmic explosion could have caused a mass extinction on Earth 450 million years ago.

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 sixty percent of marine invertebrates at a time at which 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 which became extinct. Such cosmic blasts are the most powerful explosions in the universe, and the US scientists judged that a ten-second burst near Earth could deplete up to half the planet's ozone layer.

With the ozone layer devastated by the cosmic blast, 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 Earth in the past one billion years. They also 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 of Earth would have a devastating effect on life, according to co-author of the report, Dr Adrian Melott, an astronomer at the University of Kansas, US.

It isn't known when such a burst occurred, but the scientists knew that one did occur - and left its mark. What's most surprising is that just a ten 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 Earth's atmosphere and its life forms. They showed that up to half the ozone layer would have been destroyed within weeks. Five years on, at least ten percent would still be missing.

Although deep sea creatures would have been protected from the effects of the burst, surface-dwelling plankton and other life near the top of the ocean would not have survived. 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 in the extinction by other scientists. The latest research showed 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.

The answer could be that both were responsible, gamma rays and then ice age.



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