Mass Extinction By Erosion

End-Devonian Mass Extinction Caused by Erosion of Ozone Layer, New Study Finds

High levels of damaging ultraviolet-B (UV-B) radiation collapsed forest ecosystems and killed off many species of fish and tetrapods at the end of the Devonian period, 359 million years ago, according to new research from the University of Southampton and the Sedgwick Museum of Earth Sciences.

Devonian-period seafloor. Image credit: University of Michigan Museum of Natural History.

Devonian-period seafloor. Image credit: University of Michigan Museum of Natural History.

“Our ozone shield vanished for a short time in this ancient period, coinciding with a brief and quick warming of the Earth,” said University of Southampton’s Professor John Marshall, lead author of the study.

“Ozone layer is naturally in a state of flux — constantly being created and lost — and we have shown this happened in the past too, without a catalyst such as a continental scale volcanic eruption.”

Professor Marshall and colleagues collected rock samples during expeditions to mountainous polar-regions in East Greenland, which once formed a huge ancient lake bed in the arid interior of the Old Red Sandstone Continent, made up of Europe and North America.

The researchers also collected rock samples from the Andean Mountains above Lake Titicaca in Bolivia. These South American samples were from the southern continent of Gondwana, which was closer to the Devonian South Pole.

They held clues as to what was happening at the edge of the melting Devonian ice sheet, allowing a comparison between the extinction event close to the pole and close to the equator.

On microscopic examination, the researchers found many of the spores had bizarrely formed spines on their surface — a response to UV-B radiation damaging their DNA.

Also, many spores had dark pigmented walls, thought to be a kind of protective tan, due to increased and damaging UV-B levels.

Normal (left) and malformed (right) spores from East Greenland. Image credit: John Marshall.

Normal (left) and malformed (right) spores from East Greenland. Image credit: John Marshall.

“During a time of rapid global warming, the ozone layer collapsed for a short period, exposing life on Earth to harmful levels of UV-B radiation and triggering a mass extinction event on land and in shallow water at the Devonian-Carboniferous boundary,” the scientists said.

“Following melting of the ice sheets, the climate was very warm, with the increased heat above continents pushing more naturally generated ozone destroying chemicals into the upper atmosphere. This let in high levels of UV-B radiation for several thousand years.”

During the extinction, plants selectively survived, but were enormously disrupted as the forest ecosystem collapsed. The dominant group of armored fish became extinct. Those that survived — sharks and bony fish — remain to this day the dominant fish in our ecosystems.

These extinctions came at a key time for the evolution of our own ancestors, the tetrapods.

“Current estimates suggest we will reach similar global temperatures to those of 360 million years ago, with the possibility that a similar collapse of the ozone layer could occur again, exposing surface and shallow sea life to deadly radiation,” Professor Marshall said.

“Our findings have startling implications for life on Earth today. This would move us from the current state of climate change, to a climate emergency.”

The study was published in the journal Science Advances.

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John E.A. Marshall et al. 2020. UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism. Science Advances 6 (22): eaba0768; doi: 10.1126/sciadv.aba0768

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