A team of researchers from the University of Southampton (England) has shown that an extinction event 360 million years ago, which wiped out much of Earth’s freshwater plants and aquatic life , was caused by a brief but severe breakdown of the ozone layer that protects the Earth from harmful ultraviolet (UV) radiation. This recently discovered extinction mechanism has profound implications for our current warming world.
Remember that our planet has gone through a series of mass extinctions in the geological past. The best known was caused by an asteroid that hit Earth 66 million years ago and precipitated the dinosaurs extinction (it wiped out 80% of all living animal species at that time and with a devastating effect on the plant world as well. ). Three other extinctions, including the end of the Great Permian Death, 252 million years ago, were caused by massive volcanic eruptions on a continental scale that destabilized Earth’s atmospheres and oceans.
Now, scientists have found evidence showing that high levels of UV radiation collapsed forest ecosystems and killed many species of fish and tetrapods at the end of the Devonian geological period, 359 million years ago. This damaging burst of UV radiation occurred as part of one of Earth’s climate cycles, rather than being caused by a large volcanic eruption.
The experts collected rock samples from various locations in Greenland, which were closer to the equator during the late Devonian, and studied them for fossilized plant spores – which have been preserved in rocks for hundreds of millions of years. They dissolved the rocks in hydrofluoric acid, releasing spores from microscopic plants, like pollen, but from fern-shaped plants that had no seeds or flowers. And they found that many of the spores exhibited signs of damage from ultraviolet radiation. Many of the spores had strange spines on their surface, a response to UV radiation that damages their DNA. And or after they were darker in color, probably a protective pigmentation developed to defend against such strong ultraviolet radiation.
The scientists concluded that the ozone layer had shrunk, increasing the amount of ultraviolet radiation that bathed the surface, destroying a large number of plant species. And since plants make up the foundation of the food web, this has a cascading effect that kills herbivores first and then carnivores that eat them.
This newly discovered extinction mechanism was therefore caused by changes in Earth’s temperatures and the climate cycle, leading to the deadly breakdown of ozone. The authors of the study in the journal Science Advances warn that we could face a similar scenario as we move towards global temperatures similar to those that existed 359 million years ago due to climate change.
“Our ozone layer is naturally in a state of flux, constantly being created and lost, and we have shown that this happened in the past as well, without a catalyst such as a continental-scale volcanic eruption,” the authors write.
As it is, the fossil record suggests a dramatic increase in ultraviolet radiation, caused by a temporary depletion of the ozone layer as the world warmed; a conclusion, clearly worrying, as it would show that ozone depletion could be a natural response to global warming. And we all know that the world is heating up at a devastating rate right now …
Referencia: John E. A. Marshall, Jon Lakin, Ian Troth, Sarah M. Wallace-Johnson. UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism. Science Advances, 2020; 6 (22): eaba0768 DOI: 10.1126/sciadv.aba0768
