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Did Gamma Rays Cause Ordovician Mass Extinction?

By Ernie Tretkoff

A gamma ray burst may have caused the Ordovician extinction, suggests Brian Thomas and colleagues at the University of Kansas. This mass extinction, the second largest ever, took place about 440 million years ago and wiped out about two-thirds of all species.

Scientists have blamed the extinction on a sudden ice age that occurred at the end of the Ordovician period. Thomas agrees that the ice age clearly contributed to the extinction, but suggests that a gamma ray burst could account for both the onset of the ice age and other effects such as ozone depletion that may have also been a factor in the mass die-off. Thomas and colleagues first reported their hypothesis in a paper posted on-line last September (astro-ph/0309415). They presented further details at the April Meeting.

Gamma ray bursts, the most powerful explosions known, are believed to come from super- novae, and are observed about once a day. A gamma ray burst within about ten thousand light years of Earth would pose a threat to life, Thomas and colleagues estimate. He believes such an event occurs about once in a billion years.

The intense radiation of a gamma ray burst could have depleted about forty percent of the ozone layer, according to Thomas's recent calculations, presented at the April Meeting. The ozone layer would take about ten years to recover from such a blast, said Thomas.

The loss of such a large fraction of the protective ozone layer would have allowed harmful ultraviolet radiation to reach Earth. Because ultraviolet flux is attenuated through water, marine organisms that dwelt closest to the surface would have received the most UV radiation, and thus would have been killed at higher rates than those that lived deeper, said Thomas. Indeed, geological evidence confirms that species living near the top of the water column were hit hardest in the Ordovician extinction.

In addition to depleting the ozone layer, a gamma ray burst may have initiated the sudden episode of global cooling that began at about the time of the Ordovician extinction. Gamma rays break up nitrogen and oxygen molecules in the atmosphere and convert them to nitrogen dioxide. Nitrogen dioxide, the brown gas that makes up smog, blocks out sunlight, thereby darkening and cooling Earth, and possibly setting off an ice age.

Some fossil evidence suggests that some species, including surface-dwelling plankton, began to die off before the ice age began, lending support to the idea that something other than cooling contributed to the mass extinction, said Thomas.

Thomas and colleagues considered other possible causes for the Ordovician mass extinction, such as an asteroid impact like the one believed to have killed the dinosaurs. But no evidence has been found for such an event at the end of the Ordovician period.

Thomas admits that there is also no "smoking gun evidence" for a gamma ray burst at that time, but he believes a gamma ray burst is a good explanation for the pattern of extinction and the cause of the sudden ice age.

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