Scientists blame supernova for global death of marine life

Cosmic rays might have caused mass extinction 2 million years ago

January 09, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON - Two scientists from Baltimore say they've found evidence that a global extinction of some marine life about 2 million years ago might have been triggered by the explosion of a passing star.

The blast would have showered the planet with cosmic rays for decades, or perhaps as long as 1,000 years, the researchers said.

The radiation would have upset the atmospheric ozone chemistry that shields Earth from the sun's ultraviolet radiation, allowing enough to reach the surface to kill off many microscopic ocean plankton and some of the bigger creatures that eat it.

"If it is confirmed that this extinction was really provoked by a supernova, it is the best information we have about the destruction of the ozone layer," said Narciso Benitez, an associate research scientist in astronomy at the Johns Hopkins University.

He described his interdisciplinary scientific sleuthing yesterday at the annual meeting of the American Astronomical Society.

In the wake of his work, Benitez said, climatologists might use the fossil record to study the potential impact of ozone depletion far greater than the seasonal "ozone hole" now affecting Earth's circumpolar regions. And astronomers might search ocean sediments for traces of past supernova explosions.

Benitez began looking at the issue after a colleague - Jesus Maiz-Apellaniz, of the Space Telescope Science Institute in Baltimore - finished tracing 10 million years of movement by a nearby group of supernova-prone stars, only a few hundred light years from Earth. That work, too, was presented this week to the AAS meeting.

These hot, young stars, part of a larger group called the Scorpius-Centaurus OB Association (Sco-Cen), are now 300 to 500 light-years from Earth.

That's still nearby in a galaxy 100,000 light-years across but far enough away to cause no harm if any of them blew up.

But Maiz-Apellaniz discovered that during the past 10 million years some in the group had passed as close to Earth as 150 light-years, near enough for the effects of a supernova blast to be felt.

Were there any effects?

Benitez learned that in 1999 a team of German astronomers led by Klaus Knie found traces of a rare form of iron, called iron 60, in two separate layers of deep ocean sediments. One layer was less than 2.8 million years old, the other 3.7 million to 5.9 million years old.

Iron 60 is created in supernova explosions and blown like soot across interstellar space, at speeds of thousands of miles per second.

But where could the blasts have come from? Knie's team proposed a single blast, from an unknown nearby source. But "they couldn't find a good explanation for their result," Benitez said.

The Sco-Cen stars suddenly offered an explanation. A supernova blast or two among them while they were closest to Earth could have sent iron 60 debris raining down on the planet.

Both the amount of iron 60 laid down and the timing were a good match for an explosion or two among the Sco-Cen stars, Benitez said.

More important to Earth's living things, however, the supernova blast would also have sent a shower of cosmic rays zipping through space at nearly the speed of light.

A closer explosion - say, 10 light-years from Earth - and the cosmic radiation might have killed exposed living things directly, Maiz-Apellaniz said.

But at 150 light-years, Benitez said, the radiation would only have disrupted production of ozone in Earth's upper atmosphere, depleting it by as much as 20 percent in the tropics and 80 percent in polar regions.

"The impact of that is you have a huge increase in ultraviolet radi- ation" reaching Earth's surface, with deadly effects on some life, Benitez said.

Was there any evidence for such an effect?

Matilde Canelles, a microbiologist and immunologist at the National Institutes of Health who is Benitez' wife, said there was.

Canelles found previous research that had identified fossil evidence for a widespread extinction of plankton, and in particular the mollusks and other marine life dependent on plankton, about 2 million years ago.

Scientists continue to debate the cause of the extinction. But Benitez said it appears to have no ready explanation in the sorts of cataclysms thought to have caused other extinctions, such as climate change, volcanic activity or asteroid impacts.

On the other hand, he said, "it fits well with what we would predict with a supernova-provoked extinction."

Happily, he said, the next Sco-Cen star primed for a supernova explosion, sometime in the next 100,000 years, is Antares. And it is a safe 500 light-years away.

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