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The end of the Devonian period, 359 million years ago, was adventurous: fish emerged from the ocean, and spore forests spread across the land. The world has tried to recover from a mass extinction 12 million years ago, but the climate remained chaotic, oscillating between the heat of the greenhouse and the cold that glaciers form in the tropics. And then, as the planet warmed up after another ice age, another mass extinction occurred for no apparent reason. Now, surviving spores in the sediments of ancient lakes in eastern Greenland point to the culprit: the planet’s protective ozone layer was ripped off abruptly, and life on earth was affected by mutational ultraviolet (UV) radiation.
As soon as the extinction began, the spores deformed and darkened, indicating DNA damage, says John Marshall, a palynologist * at the University of Southampton, co-author of an article in Science Advances. It’s proof that “all the ozone protection is gone,” he says.
Scientists have long believed, at least until humanity became an extinct force, that life on Earth could disappear in only two ways: an asteroid strike or global volcanic eruptions. But 2 years ago, researchers found signs that the volcanoes that caused the worst extinction on Earth, the Permian period, 252 million years ago, ejected Siberian salt deposits into the stratosphere, where they could cause chemical reactions that destroyed the layer. ozone and sterilized forests. Spores from extreme Devon convincingly show that even without eruptions, warmer weather can deplete the ozone layer, says Lauren Sallan, a paleobiologist at the University of Pennsylvania. “Because the evidence is so strong, people will have to rethink other mass extinctions.”
Devon’s end extinction has long been in the shadow of the late Devonian extinction of one of the largest in the history of the planet, 12 million years earlier. Probably caused by volcanic eruptions, whose gases dramatically cooled and heated the planet, they killed most of the corals and the many shell creatures in the sea. But 10 years ago, the work of Sallan and others showed that the end of the Devonian extinction was also strong, destroying many plants and vertebrates, including most tetrapods, four-legged fish, for which the fingers began to develop. . Only the five-fingered tetrapods survived. “Our evolution began,” says Marshall. “All archaic lines have been removed.”
However, it was not known why the end of the Devonian extinction occurred. There were no signs of volcanic activity or strong cosmic body shocks, just an indication of the rapid formation and disappearance of glacial rock deposits, Sallan says. “The weather was really confused at the time.”
For the past 3 decades, Marshall has been exploring surviving rocks from that era in eastern Greenland. At the time, the region was far from the Arctic, at lower latitudes, trapped in a deserted area of land called the Old Continent of Red Sandstone. After the last Devonian Ice Age, as the weather warmed, the lakes formed and filled with sediment, gradually becoming a cobblestone, where conditions were recorded before and during the extinction. In 2017, Marshall excavated an ideal 6-meter-long alumina core.
Capture surprising transformations: Healthy needle fossils, covered with characteristic symmetrical needles, are suddenly replaced by warped and uneven needles. Spore fossils are common because they are protected by a shell, but they are damaged by UV rays, just like humans; When exposed to UV rays, spores can even “tan”. The damage Marshall saw corresponds to that experienced by such exposure, says Jeffrey Benca, an experimental paleobotanist who linked such damage to the extinction of the Permian extreme. “His proposal seems very likely,” he says.
According to Marshall, hot climates have intensified summer storms, which may have dumped a mixture of water and ozone-depleting salts into the stratosphere. After deforestation by UV rays, its unrestricted nutrients leaked into the sea, where they could cause intense plankton and algae blooms, creating even more ozone-depleting salts through positive feedback. “It is like an ideal storm,” he says.
The Marshall scenario could explain not only the extinction, but also many of the natural gas deposits that formed during that period, says Sarah Carmichael, geochemist at Appalachian State University. They were formed from the decomposition of organic matter, but no one explained what led to the acceleration of plankton growth necessary for them. Nutrients filtered from dead forests could fertilize marine life.
It is also a sign of what can happen in today’s warming world, where powerful storms sometimes “jump” into the troposphere and bring moisture to the cold, dry stratosphere. Along with aerosol particles and chlorine molecules, moisture can destroy ozone.
But atmospheric scientists disagree on whether that ozone depletion is happening now, let alone a hundred million years ago. There are more jumps now than expected, but it’s still unclear whether they’re triggering ozone-depleting reactions. Elliot Atlas, an atmospheric chemist at the University of Miami who studies this dynamics, is skeptical of Marshall’s theory. Much more in-depth modeling is needed, he says. “Isn’t that possible? I couldn’t say that. “
Carmichael, meanwhile, would like more signs than just pollen that the extinction was caused by UV rays. “I would refrain from saying that UV radiation is the only reason,” she says. “But I think that is one of the reasons.”
Prepared by sciencemag.org
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