UV radiation might be behind the planet’s biggest mass extinction

Above 10 miles in the sky lies a layer of ozone—a form of orange gas with molecules of three atoms, rather than two. This ozone layer is a crucial shield that protects all life from the sun’s barrage of ultraviolet radiation. So what happens if something in the ozone layer goes horribly wrong? 

The results can be catastrophic. And we have prehistoric proof that might support that.

It comes from the time of the worst mass extinction in Earth’s history—252 million years ago, at the end of the Permian period when an apocalyptic cascade of volcanic eruptions may have turned the world toxic. And it comes in the form of fossilized pollen grains with signs of exposure to a high-energy type of ultraviolet known as ultraviolet B (UV-B) radiation. In a paper published today in the journal Science Advances, an international group of geologists and botanists used the deformed specimens to piece together a possible course of deadly events.

“I would say the elevated UV-B radiation probably played a part in the extinction of some terrestrial life,” says Feng Liu, a geologist at the Nanjing Institute of Geology and Palaeontology in China and one of the paper’s authors. Scientists have long suspected that a drop in ozone levels and spike in ultraviolet rays might have played a role in this catastrophe, and now they have data to show for it.

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One prime suspect for the end-of-the-Permian devastation is the Siberian Traps. These igneous rocks coat central Siberia (which, at the time, was one of the northernmost chunks of the supercontinent Pangaea) and were spewed from a truly colossal complex of volcanoes. Experts think that for more than a million years, the Siberian Traps belched greenhouse gases like carbon dioxide into Earth’s atmosphere. 

In the wake of constant volcanic activity, teeming ancient oceans would have acidified and deoxygenated, turning toxic and sentencing more than 80 percent of their resident marine species to extinction. Life would of course recover, but it needed millions of years more to return to its pre-extinction abundance.

That explains much of the prehistoric carnage in the water, but what about on land? What types of terrestrial organisms died, and why? The fossil record there is much less clear.

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Researchers had previously dug up clues of some immense destruction. For instance, several parts of that ancient world were once covered with forests of great ferns. Both of these biomes vanish from the fossil record around the end of the Permian, suggesting that ground dwellers suffered worldwide. 

Still, other experts contend that the fossil record could be misleading, and the extinctions were more regional. “It’s a case of compiling lots of pieces of information from different places, and trying to build it together into a coherent—albeit incomplete—picture,” says Phillip Jardine, a paleobotanist at the University of Münsterin in Germany and author on the new paper. So far, that picture doesn’t tell us what, exactly, caused the deaths on land.

But these scientists may have found a missing piece. In 2014, Liu collected samples from rocks under what is now southern Tibet. When he and his colleagues studied the rock closely, they found ancient grains of conjoined and misshapen pollen.

To understand what caused the damage, the team analyzed the pollen and sought out particular compounds containing carbon, oxygen, and nitrogen. Plants would have created these chemicals to protect themselves from UV-B radiation, which consists of shorter wavelengths than visible light and therefore, higher energies. As a result, UV-B rays can cause more damage to living cells than UV-A.

Scientists like Jardine had used the same technique to study UV-B levels that reached Earth’s surface a few hundred thousand years ago. But this was the first time anybody had tried to look for these compounds from 252 million years ago. And Jardine and Liu’s group did find them.

“I think the key thing is that we have definite evidence that plants were affected by this,” says Jardine. “The increase in UV-B-absorbing compounds that we have observed shows that plants were biochemically responding to this situation.”

The hunch is that at the Permian period’s end, volcanic activity unleashed gases known as halocarbons, which contain atoms of halogens like chlorine and bromine. The chemicals might have eaten away at the ozone layer, allowing more UV-B travel to the ground. That, in turn, would have stunted plant growth and reproduction, possibly leading to fewer flora pulling toxic carbon dioxide out of the air.

“Whilst it would be pre-emptive of me to suggest ozone depletion or elevated UV radiation were the only cause of these mass extinctions, it certainly seems plausible that increasing UV radiation at a time when the global ecosystem is already under considerable stress is likely to exacerbate negative impacts on life on Earth,” says Wesley Fraser, a geologist at Oxford Brookes University in the UK and another one of the study authors.

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If UV-B really did make the planet more unlivable in that period, the devastation may have happened globally. Of course, scientists will need to find hard evidence of that. “These data only came from one locality, so we need to find more from the same time interval to validate these findings,” says Jardine.

Though the mass extinction at the end of the Permian is considered the deadliest, there were more. Scientists have identified similar mortality events at the ends of the Devonian (around 360 million years ago) and the Triassic (around 201 million years ago) periods. And according to Fraser, scientists have found traces of ultraviolet poisoning in those extinctions, too.

“There may be a common thread involving UV radiation spanning different mass extinction events,” says Fraser. Even if ultraviolet radiation wasn’t the primary killer, it might have been the accomplice that helped do in much of the world’s terrestrial life.

And while the Permian is ancient history, we’re still wrestling with the problem of UV-B radiation today. It was not too long ago that the world was in alarm over an ozone hole over Antarctica, caused by compounds known as chlorofluorocarbons (CFCs) leaching into the atmosphere from the refrigerators and air conditioners that once used them. Many were concerned that the ozone hole would expand and leave large parts of the globe exposed to burning UV radiation.

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After governments came together in 1987 to craft the Montreal Protocol and ban CFCs, the ozone hole began to heal. But the damage was done, and it continues to affect plants today.

With that in mind, learning about how UV-B exposure affected plants in the past could inform scientists about what may happen in the near future. And vice versa, Fraser explains. “I think deep-time and modern-day research on UV-B radiation go hand-in-glove.”