A study literally sheds light on the question of which factors led to the catastrophic mass extinction at the end of the Permian period: the detection of sunscreen substances in fossil plant pollen shows that increased UV radiation probably also played a role. The pollution was apparently due to depletion of the ozone layer by gases from increased volcanism 250 million years ago. In addition to the strong global warming, the increased UV radiation could also have contributed to the extinction of many terrestrial species, the scientists say.
The mass extinction that wiped the dinosaurs off the face of Earth's history at the end of the Cretaceous is arguably the most famous. But even before that there had been catastrophic losses in biodiversity. The most drastic case is the mass extinction at the end of the Permian period around 250 million years ago. At that time, about 80 percent of marine and terrestrial species suddenly disappeared. Various indications indicate that this slump was due to the consequences of gigantic volcanic eruptions in present-day Siberia. They released large amounts of carbon dioxide from the earth's interior into the atmosphere, which in a short time led to strong global warming.
In addition, acidification caused by the input of volcanic substances was clearly causing problems for marine life. As far as animals and plants on land are concerned, there were already indications that they were also exposed to increased UV radiation in addition to climate change. Because certain substances that were released during the course of the volcanic activity could have destroyed the ozone layer at the time. Findings of fossil pollen grains from the era that exhibited deformities such as those caused by strong exposure to UV-B radiation were interpreted as an indication. However, certain volcanic pollutants could also have caused this, which led to a lack of clarity.
On the trail of increased UV exposure
An international team of researchers has now provided new indications of increased UV-B exposure. They have been able to detect traces of certain phenolic substances (UACs) in fossil pollen grains, which are known to help plants protect their tissues from ultraviolet radiation. “Plants need sunlight for photosynthesis, but they need to protect themselves, and especially their pollen grains, from the harmful effects of UV-B radiation. To do this, the plants load the outer walls of the pollen grains with compounds that act like sunscreens to protect the delicate cells and ensure successful reproduction,” explains co-author Barry Lomax from the University of Nottingham.
The researchers have now applied their infrared microspectroscopy detection method to pollen grains from a site in Tibet. They can be assigned to plants that grew around the time of the mass extinction around 250 million years ago. The scientists explain that certain substances in the layers clearly show the traces of the then increased volcanism. This enabled them to compare the pollen characteristics before and after the changes. "We found significantly elevated concentrations of the phenolic compounds in fossil pollen grains from Tibet, formed during the mass extinction event and peak volcanic activity," said lead author Feng Liu of the Chinese Academy of Sciences in Nanjing. This means that the plants at that time had to protect these structures, which are important for reproduction, particularly intensively against UV-B radiation. This is therefore a clear indication that the ozone layer was so depleted that it was no longer able to shield the earth's surface from UV-B radiation as before.
Catastrophic cascading effects
According to the researchers, this could have had far-reaching consequences for the entire Earth system. Model simulations show that increased UV-B stress can lead to a significant decrease in plant biomass production. This may have reduced terrestrial carbon storage at the time, further amplifying global warming from volcanic activity. In addition, there may have been a direct effect on late Permian wildlife: elevated levels of phenolic compounds are known to make plant tissues less digestible. In addition to the strong climate changes, this could have put additional strain on herbivores, the researchers say.
As they sum up, it is now becoming apparent in what complex way volcanism plunged the living environment of the earth into a catastrophe 250 million years ago. "Volcanism of this magnitude affects all aspects of the Earth system, from direct chemical changes in the atmosphere, to changes in carbon sequestration, to disrupting the nutritious food sources available to animals," says co-author Wes Fraser of Oxford Brookes University.
Source: University of Nottingham, Chinese Academy of Sciences, Article: Science Advances, doi: 10.1126/sciadv.abo6102