No asteroids needed: ancient mass extinction tied to ozone loss, warming climate - Science Magazine

The end of the Devonian period, 359 million years ago, was an eventful time: Fish were inching out of the ocean, and fernlike forests were advancing on land.

The world was recovering from a mass extinction 12 million years earlier, but the climate was still chaotic, swinging between hothouse conditions and freezes so deep that glaciers formed in the tropics.

And then, just as the planet was warming from one of these ice ages, another extinction struck, seemingly without reason.

Now, spores from fernlike plants, preserved in ancient lake sediments from eastern Greenland, suggest a culprit: The planet’s protective ozone layer was suddenly stripped away, exposing surface life to a blast of mutation-causing ultraviolet (UV) radiation.

Just as the extinction set in, the spores became misshapen and dark, indicating DNA damage, John Marshall, a palynologist at the University of Southampton, and his co-authors say in a paper published today in Science Advances.

It’s evidence, he says, that “all of the ozone protection is gone.”.

But 2 years ago, researchers found evidence that in Earth’s worst extinction—the end-Permian, 252 million years ago—volcanoes lofted Siberian salt deposits into the stratosphere, where they might have fed chemical reactions that obliterated the ozone layer and sterilized whole forests.

Now, spores from the end-Devonian make a compelling case that, even without eruptions, a warming climate can deplete the ozone layer, says Lauren Sallan, a paleobiologist at the University of Pennsylvania.

“Because the evidence is so strong, it will make people rethink other mass extinction events.”.

The end-Devonian die-off has long sat in the shadow of the Late Devonian extinction 12 million years earlier, one of the planet’s largest.

There was no evidence for volcanism or a giant impact, but one alluring clue was seen in the rapid formation and disappearance of rock deposits associated with glaciers, Sallan says.

As the climate warmed after the Devonian’s last ice age, lakes formed and filled with sediment that slowly turned to mudstone, recording conditions before and during the extinction.

The damage Marshall saw is consistent with such exposure, says Jeffrey Benca, an experimental paleobotanist who has linked such damage to the end-Permian extinction.

Marshall argues that the warming climate drove more powerful summer thunderstorms, which could have injected an ozone-depleting mix of water and salts into the stratosphere.

Marshall’s scenario could explain not just the extinction, but also the many natural gas deposits dating from the period, says Sarah Carmichael, a geochemist at Appalachian State University.

But atmospheric scientists can barely agree on whether these ozone depletions are happening now, let alone hundreds of millions of years ago.

Carmichael, for her part, would like to see evidence beyond the pollen grains that UV drove the extinction.