Ozone erosion 360 million years ago responsible for a mass extinction

Erosion of the ozone layer 360 million years ago was responsible for a mass extinction event that wiped out much of the Earth’s plants and freshwater animals

  • The team found evidence of the impact of ultraviolet radiation in rock samples 
  • They say that the Earth rapidly warmed up at the time of the ozone breakdown 
  • Authors say this could happen again as temperatures rise due to climate change 

A mass extinction 360 million years ago that killed off many of the Earth’s plants and freshwater animals was caused by a breakdown of the ozone layer.

Scientists from the University of Southampton found evidence that it was high levels of ultraviolet radiation that destroyed the ancient forest ecosystem.  

This newly discovered extinction mechanism was caused by changes in the Earth’s temperatures and climate cycle – this led to the deadly ozone breakdown. 

Study authors warn that we could face a similar scenario as we head towards similar global temperatures that existed 359 million years ago due to climate change.

They dissolved the rocks in hydrofluoric acid, releasing microscopic plant spores preserved for hundreds of millions of years. Many had bizarrely formed spines in response to UV radiation (right) when compared to normal spores (left)

Lead researcher Professor John Marshall said the ozone shield – that protects the Earth from harmful UV radiation – vanished for a short period of time.

He said when it vanished it coincided with a brief and quick warming of the Earth.

‘Our 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.’

The scientists found evidence that high levels of UV radiation destroyed forest ecosystems and killed many species of fish and tetrapods.

Prof John Marshall (left), taking samples in Spitsbergen. Those samples helped them discover pores that were damaged by UV radiation

This was at the end of the Devonian geological period, 359 million years ago.

Other types of mass extinctions include an asteroid hitting the Earth 66 million years ago causing dinosaur extinction and a huge scale continental volcanic eruption that destabilised the Earth’s atmosphere and ocean 252 million years ago.

The team discovered that this damaging burst of UV radiation occurred as part of one of the Earth’s climate cycles, rather than by a huge volcanic eruption.

They found that the ozone collapse occurred as the climate rapidly warmed following an intense ice age.

Study authors warned that the Earth could reach similar temperatures, triggering a possible extinction event on the same scale – if something isn’t done to slow the pace of climate change.

The team collected rock samples from mountainous polar-regions in East Greenland, the lake was situated in the Earth’s southern hemisphere 350 million years ago.

At the time the lake would have been similar in nature to modern day Lake Chad on the edge of the Sahara Desert.

THE DEVONIAN PERIOD: A TIME OF CONTINENTAL FORESTS AND FISH 

The Devonian period – from 419 million to 359 million years ago – ended with a mass extinction. 

Researchers now believe that extinction was caused by UV radiation.

This increase in solar radiation came due to the erosion of the ozone layer caused by a spike in temperatures. 

The period is named after the county of Devon, England where rocks from the period were first studied. 

This is the period where life on dry land started to expand and form extensive forests covering whole continents. 

Other rocks were collected 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 dissolved the rocks in hydrofluoric acid, releasing microscopic plant spores, like pollen, but from fern-like plants that didn’t have seeds or flowers. 

These spores have lain preserved in rocks for hundreds of millions of years.

The scientists found many of the spores had bizarrely formed spines on their surface – a response to UV radiation damaging their DNA.

Many spores had dark pigmented walls believed to be a ‘protective tan’ due to increased and damaging UV levels.

The scientists concluded that, during a time of rapid global warming, the ozone layer collapsed for a short period, exposing life on Earth to harmful levels of UV radiation.

This then trigged a mass extinction event on land and in shallow water at the Devonian-Carboniferous boundary.

The ice sheets melted and the climate became very warm – resulting in naturally generated ozone destroying chemicals in the upper atmosphere.

This let in high levels of UV-B radiation for several thousand years – killing some plants and fish, the team wrote.

Researchers visited Greenland to get samples from ancient lakes in order to identify the cause of the mass extinction 359 million years ago

During the extinction, plants selectively survived, but were enormously disrupted as the forest ecosystem collapsed. 

The dominant group of armoured 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 – four legged vertebrates. These early tetrapods are fish that evolved to have limbs rather than fins, but still mostly lived in water. 

Their limbs possessed many fingers and toes at the time of the extinction.

The extinction reset the direction of their evolution with the post-extinction survivors being terrestrial and with the number of fingers and toes reduced to five. 

Professor Marshall said: ‘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.’

He said if this happens it would result in ‘exposing surface and shallow sea life to deadly radiation.’

‘This would move us from the current state of climate change, to a climate emergency.’

The findings are published in Science Advances.

WHEN WERE EARTH’S FIVE GREAT EXTINCTION EVENTS?

Five times, a vast majority of the world’s life has been snuffed out in what have been called mass extinctions.

End-Ordovician mass extinction
The first of the traditional big five extinction events, around 540 million years ago, was probably the second most severe. Virtually all life was in the sea at the time and around 85% of these species vanished.

Late Devonian mass extinction

About 375-359 million years ago, major environmental changes caused a drawn-out extinction event that wiped out major fish groups and stopped new coral reefs forming for 100 million years.

Five times, a vast majority of the world’s life has been snuffed out in what have been called mass extinctions. The most famous may be the End-Cretaceous, which wiped out the dinosaurs. Artist’s impression

End-Permian mass extinction (the Great Dying)
The largest extinction event and the one that affected the Earth’s ecology most profoundly took place 252 million years ago. As much as 97% of species that leave a fossil record disappeared forever.

End-Triassic mass extinction
Dinosaurs first appeared in the Early Triassic, but large amphibians and mammal-like reptiles were the dominant land animals. The rapid mass extinction that occurred 201 million years ago changed that.

End-Cretaceous mass extinction

An asteroid slammed down on Earth 66 million years ago, and is often blamed for ending the reign of the dinosaurs.

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