Researchers at the University of Southampton have shown that an extinction event 360 million years ago, which killed much of the earth’s aquatic and freshwater life, was caused by a brief degradation of the ozone layer that protects the earth from damaging ultraviolet (UV) radiation. This is a newly discovered extinction mechanism that has profound consequences for our warming world today.
There have been a number of mass extinctions in the geological past. Only one was caused by an asteroid hitting the earth, which was 66 million years ago when the dinosaurs became extinct. Three of the others, including the late Permian Great Dying, 252 million years ago, were caused by huge continental-scale volcanic eruptions that destabilized the Earth’s atmosphere and ocean.
Researchers have now found evidence that high levels of UV radiation collapsed forest ecosystems and killed many fish species and tetrapods (our four limb ancestors) at the end of Devon’s geological period, 359 million years ago. This harmful eruption of UV radiation occurred as part of one of the Earth’s climate cycles, rather than being caused by a huge volcanic eruption.
Ozone collapse occurred when the climate quickly warmed up after an intense ice age and the researchers suggested that the earth could reach comparable temperatures today, possibly triggering a similar event. Their results are published in the journal Scientific progress.
The team collected rock samples during expeditions to mountainous polar regions of East Greenland, which once formed a huge old lake bed in the dry interior of the old red sandstone continent, consisting of Europe and North America. This lake was on the southern hemisphere of the earth and would have resembled its nature as the modern lake Chad on the edge of the Sahara desert.
Other stones were collected from the Andes Mountains above Lake Titicaca in Bolivia. These South American specimens came from the southern continent of Gondwana, which was closer to Devon’s southern pole. They kept clues as to what was happening at the edge of the melted Devonian ice sheet, enabling a comparison between the extinction event near the pole and near the equator.
Back in the lab, the rocks were dissolved in flow acid, releases microscopic plant spores (like pollen, but from fern-like plants that did not have seeds or flowers) that had been laying down for hundreds of millions of years. By microscopic examination, the researchers found that many of the spores had formed strange spines on their surface – a response to UV radiation damaging their DNA. Many spores also had dark pigmented walls, believed to be a kind of protective “sunburn”, due to increased and harmful UV levels.
The researchers concluded that during a period of rapid global warming, the ozone layer collapsed for a short period, exposing life on Earth to harmful levels of UV radiation, triggering a mass extinction event on land and in shallow water at the Devonian-Carboniferous border.
After the melting of the ice sheet, the climate was very warm, with the increased heat over continents that squeezed more naturally generated ozone that destroys chemicals in the upper atmosphere. This allows high levels of UV-B radiation for thousands of years.
Leading researcher Professor John Marshall, University of Southampton’s School of Ocean and Earth Science, who is a National Geographic Explorer, comments: “Our ozone shield disappeared for a short time during this ancient period, coinciding with a brief and rapid global warming. Our ozone layer is naturally in a flow state – constantly created and lost – and we have shown that this has happened before too, without a catalyst such as volcanic eruption on a continental scale. “
During the extinction, plants survived selectively but were greatly disturbed when the forest ecosystem collapsed. The dominant group of armored fish was extinct. Those who survived – sharks and bone fish – today remain the dominant fish in our ecosystems.
These extinctions came at an important time for the development of our own ancestors, tetrapods. These early tetrapods are fish that were developed to have limbs rather than fins, but still lived mostly in water. Their limbs had many fingers and toes. The eradication restores the direction of their development with survivors being grounded and with the number of fingers and toes reduced to five.
Professor Marshall says his team’s results have startling consequences for life on Earth today: “Current estimates suggest that we will reach similar global temperatures as 360 million years ago, with the possibility that a similar collapse of the ozone layer could occur again and expose the surface and shallow marine life to deadly radiation. This would move us from the current climate change situation, to a climate emergency situation. “
The distant places visited in eastern Greenland are very difficult to access, with trips by light aircraft that can land directly on the tundra. Transport in the large field area took place with inflatable boats equipped with outboard engines, all of which must fit into the small aircraft.
All field logistics were organized by CASP, an independent charity based in Cambridge specialized in remote geological field work. Mike Curtis, CEO of CASP says: “We have a history of helping research geologists like John Marshall and colleagues gain access to remote field areas and we are particularly pleased that their research has proven to have such potential profound consequences.”
Reference: “UV-B Radiation was the Devonian-Carboniferous Boundary Method for Earth Extinguishing Murders” by John E. A. Marshall, Jon Lakin, Ian Troth and Sarah M. Wallace-Johnson, May 27, 2020, Scientific progress.
DOI: 10.1126 / sciadv.aba0768
The research was partly funded by a grant from the National Geographic Society.
The research was conducted in collaboration with The Sedgwick Museum of Earth Sciences at the University of Cambridge.