Supernova could explain extinctions at the end of the Devonian period

Researchers suggest radiation from a supernova located 65 light-years away could have caused a series of extinction events some 360 million years ago. Photo by Jesse Miller

Aug. 18 (UPI) -- New research suggests harmful cosmic rays from a nearby supernova might have caused the extinction events that form the boundary between the Devonian-Carboniferous periods.

Around 360 million years ago, a lengthy period of biodiversity declines culminated in a series of extinction events that saw 19 percent of all families and 50 percent of all genera disappear.
Scientists have previously unearthed a diversity of Late Devonian plant spores that show evidence of being burnt by ultraviolet light, signs of a prolonged ozone-depletion event.

"Earth-based catastrophes such as large-scale volcanism and global warming can destroy the ozone layer, too, but evidence for those is inconclusive for the time interval in question," lead researcher Brian Fields said in a news release.

"Instead, we propose that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone," said Fields, professor of astronomy and physics at the University of Illinois, Urbana-Champaign.

Today, the closest supernova threat is the star Betelgeuse, located 600 light-years away. Scientists estimate a supernova would have to occur within 25 light-years to present a significant threat to life on Earth.

Millions of years ago, however, a variety of closer stars may have presented a graver threat, researchers contend.

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Scientists determined that other cosmic threats, like a gamma-ray burst, solar eruption or meteorite explosion, are too short-lived to account for Devonian-Carboniferous extinctions. Some researchers estimate consisted of a half-dozen different events spread out over thousands, even millions, of years.

According to the new study, published Tuesday in the journal PNAS, a supernova could have delivered a one-two punch of electromagnetic energy.

After an initial blast of UV, X-rays and gamma rays, a barrage of supernova debris can sustain a constant supply of irradiation. The effects of a single supernova, scientists estimated, could affect Earth for up 100,000 years.

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The researchers suggest a series of supernovas might have poisoned Earth with ultraviolet rays for a few hundred thousands years.

"This is entirely possible," said grad student Jesse Miller. "Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion."

Currently, the possibility that supernova radiation triggered the Devonian-Carboniferous extinctions is only a theory. But researchers claim it's a theory that could be confirmed by the discovery of radioactive isotopes plutonium-244 and samarium-146 in rocks from the period.

"When you see green bananas in Illinois, you know they are fresh, and you know they did not grow here. Like bananas, Pu-244 and Sm-146 decay over time," Fields said. "So if we find these radioisotopes on Earth today, we know they are fresh and not from here -- the green bananas of the isotope world -- and thus the smoking guns of a nearby supernova."

Fields and his colleagues are currently working out what Pu-244 or Sm-146 isotope concentrations might look like in Devonian-Carboniferous era rocks, so that scientists will know what to look for if and when they go prospecting.

"The overarching message of our study is that life on Earth does not exist in isolation," Fields said. "We are citizens of a larger cosmos, and the cosmos intervenes in our lives -- often imperceptibly, but sometimes ferociously."