In the past 500 million years, there have been five mass extinction events on the earth. The most serious one occurred 250 million years ago, the end-Permian mass extinction event, which caused the disappearance of about 80% of marine and terrestrial species.

Scientists believe that the worst mass extinction event was caused by a paleoclimate emergency caused by a large-scale basic volcanic eruption in Siberia.

But how has the global increase in greenhouse gases caused by volcanic eruptions affected terrestrial ecosystems and led to extinctions? People don't know yet.

A research paper published online in the international academic journal "Science Advances" in the early morning of January 7, 2023 stated that "sunscreens" such as coumaric acid and ferulic acid in fossil pollen grains from southern Tibet The change in the material content is the direct evidence that the destruction of the atmospheric ozone layer led to the mass extinction of terrestrial organisms at the end of the Permian.

"Ozone depletion is the most plausible explanation after all possible causes have been ruled out"

Liu Feng, a researcher at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, and colleagues in the United Kingdom and Germany, found that during this period, the content of "sunscreen" substances such as coumaric acid and ferulic acid in plant pollen grains increased significantly, so it is speculated that During this period, the ultraviolet radiation of the earth's land increased significantly, and it was closely related to volcanic eruptions and mass extinctions.

"Fossil studies around the world have shown that many organisms disappeared at the end of the Permian, and now the debate is mainly focused on the extinction mechanism. We have seen through research that the coumaric acid content of pollen increased together with the mercury content in the formation. It will be extinct." On January 6, Liu Feng told The Paper that the research work provided an explanation for the extinction of the terrestrial ecosystem at the end of the Permian. Many previous mass extinction mechanisms could only explain ocean extinction events, but the increase in global ultraviolet radiation can not only explain land, but also explain the above phenomena in oceans, providing a new way of thinking.

The research team used Fourier transform infrared spectroscopy to quantitatively measure the coumaric acid and ferulic acid in 1011 Ali-type pollen found in the Permian-Triassic transition section in southern Tibet, my country. Through the big data analysis of the infrared spectra produced by these pollen, they found that the content of coumaric acid and ferulic acid in the outer wall of the pollen during the mass extinction at the end of the Permian was significantly higher than the content of these compounds in the fossil pollen before and after the extinction . This provides direct evidence of a global increase in UV radiation during the end-Permian mass extinction.

Late Permian black shale in the Qubu section of southern Tibet. Photo provided by the interviewee

Liu Feng told The Paper that the chemical properties of the outer wall of sporopollen are very "lazy". Some Japanese scholars believe that the outer wall of sporopollen is the most corrosion-resistant organic matter known in the world. "Our samples have been preserved in the formation for 250 million years." "There are many factors that affect the intensity of ultraviolet light, in addition to the ozone hole mentioned in our article, there are latitude, altitude, and the influence of the solar cycle. The ultraviolet rays near the equator on the earth are the most Strong, places with high altitudes such as Tibet have strong ultraviolet rays. The materials we studied were collected from southern Tibet. This place was a small island 250 million years ago. The latitude and altitude were very stable, and it 'symbiotic' with the pollen we studied There are also many snails and bivalves, which can also prove this. Therefore, the phenomenon of increased ultraviolet rays we found cannot be explained by changes in altitude and latitude. In addition, the solar cycle is also difficult to explain, because we found that The variation of the UV intensity does not show periodicity, and the correlation with the solar cycle can be ruled out. After ruling out all possible causes, the most reasonable explanation is the destruction of the ozone layer."

direct evidence

Liu Feng told The Paper reporter, "Mercury is very low in strata in places like Tibet where there is no industrial pollution, but we found mercury with a high content in the middle of the mass extinction at the end of the Permian in Tibet." Layers. Coincidentally, the content of coumaric acid in the outer wall of the pollen we obtained from these formations with high mercury content is also high. This kind comes from different research objects, one is organic, the other is inorganic, and chemical Compositional fluctuations are difficult to explain by chance or chance, and there must be a high correlation between the two, so we propose that volcanic ozone depletion and increased ultraviolet radiation were an important factor in the extinction."

Effects of increased ultraviolet radiation caused by ozone depletion on terrestrial food chains.

Previously, some palynologists found some deformed spores and pollen in the continental Permian-Triassic transition section, speculating that these deformed spores or pollen may be caused by the halide elements ejected from the Siberian large igneous rock province to cause global ozone layer destruction induced by increased ultraviolet radiation. However, the research on the deformed spores and pollen produced by modern plants shows that the environmental background of these deformed pollen and spores is very complicated. Drought, air pollution and plant trauma may induce plants to produce deformed spores or pollen. Therefore, the small amount of deformed spores and pollen found in the continental Permian-Triassic transition section cannot directly prove that there was an increase in ultraviolet radiation caused by the global ozone layer hole during the end-Permian mass extinction.

But scientists such as Liu Feng found more direct evidence: they detected an abnormal increase in "sunscreen" substances in pollen grains during this period.

Professor Barry Lomax from the University of Nottingham explained that plants need sunlight for photosynthesis, but also need to protect themselves from the harmful effects of UV radiation. To do this, plants load the outer walls of pollen grains with sunscreen-like compounds to protect the delicate pollen cells and ensure successful reproduction.

Compounds such as coumaric acid and ferulic acid function much like "sunscreen". These compounds form resonance-stabilized phenolic radicals that resist UV-induced oxidation, thereby protecting the delicate spores and pollen that support the reproduction of land plants.

Increased levels of UV radiation in the air have profound effects on entire terrestrial ecosystems. Ultraviolet rays not only have a strong killing effect on the reproductive cells of plants, but also damage the mesophyll cells of plants. In order to resist the damage of ultraviolet rays to mesophyll cells, plants synthesize a large amount of lutein, coumaric acid and ferulic acid in their leaves, correspondingly reducing the synthesis of chlorophyll, which leads to the weakening of plant photosynthesis, which in turn makes plants less sensitive to The weakening of greenhouse gas absorption capacity further exacerbates the global increase in greenhouse gases caused by volcanic eruptions at the end of the Permian. In addition, lutein, coumaric acid and ferulic acid in mesophyll are compounds that are difficult to digest and have low nutritional value for herbivores and insects. Therefore, the increase in ultraviolet radiation indirectly affected the terrestrial food chain, which may be the main reason for the mass extinction of terrestrial herbivores and insects at the end of the Permian.

Pollen fossils from the Permian-Triassic transitional strata in the Qubu section.

Dr Wes Fraser of Oxford Brookes University commented: "Volcanism on such a catastrophic scale has implications for every aspect of the Earth system. From immediate chemical changes in atmospheric composition, to changes in carbon sequestration rates, to changes in to a reduction in the amount of nutrients available to animals."

More possible "murderers"

Liu Feng said that global warming will not directly cause the increase of ultraviolet radiation. However, the halogen elements in the gas released by volcanic eruptions can destroy the atmospheric ozone layer. It and global warming may be simultaneous processes. The mass extinction of life on land at the end of the Permian was actually caused by the superimposition of the increase of greenhouse gases and the enhancement of ultraviolet radiation. Learning from the past, "Our current greenhouse gases are still increasing, but we still have to pay attention to the intensity of ultraviolet radiation (at the same time) to avoid the superposition of the two, otherwise it may bring about a very large ecological disaster."

Liu Feng said, "Governments of all countries are now emphasizing carbon neutrality and carbon peaking. But we cannot ignore that plant photosynthesis is the largest carbon reduction and carbon sequestration process in the world, and this process is easily inhibited by increased ultraviolet intensity. After the implementation of the Montreal Agreement, countries began to reduce the use of freon and other halogen refrigerants. At present, it has been observed that the hole in the ozone layer over Antarctica is being repaired. But we are not sure whether this hole is repaired due to our environmental protection activities or Other environmental factors are repaired. We are not sure whether this repair process is stable."

In addition, scientists have proposed a variety of explanations for the mass extinction event at the end of the Permian, such as the impact of asteroids on the earth and the rapid decomposition of combustible ice on the seabed.

According to the Science and Technology Daily report on April 13, 2018, the team of Professor Xiao Yilin and Professor Shen Yan'an from the University of Science and Technology of China concluded that the rapid increase in continental weathering led to changes in seawater composition, which was an important environment for the mass extinction event at the end of the Permian factor.

Schematic diagram of the formation process of copper-rich sediments during the Permian mass extinction in South China.

According to the official website of the Chinese Academy of Sciences, scientists have discovered a new "killer" of the mass extinction of life at the end of the Permian. On November 17, 2021, a paper published online in "Science Advances" showed that a research team composed of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing University and other domestic and foreign units discovered that large-scale acidic volcanoes around South China Eruption is one of the important triggers of the end-Permian mass extinction, and the large-scale basic volcanic eruption in Siberia that was once considered was not the only driver of the end-Permian mass extinction. According to the abundance and distribution range of copper elements in the strata, the research speculates that the volcanic activities around South China released at least more than 1.9 billion tons of copper and billions of tons of sulfur dioxide. This eruptive flux is the annual flux of sulfur dioxide in modern volcanic activities. 10-200 times. A large amount of sulfur-rich aerosol released by acidic volcanic eruptions penetrates the troposphere and stays in the stratosphere, leading to a global millennium-scale "volcanic winter". more deadly. Combined with the extensive Permian-Triassic acidic volcanic eruption records in eastern China, the Kunlun Mountains, the Jinsha River Basin, eastern Australia, and western South America, it proves that the special The environmental disasters caused by the acidic volcanic eruptions of continental magma arcs in the Tethys Ocean and around Pangaea are far beyond previous understanding, and may be one of the main "culprits" of the largest biological extinction event 252 million years ago.

According to previous reports by The Paper, on July 2, 2022, researchers from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences published a paper in the academic journal Science Advances, proposing for the first time that the mass extinction event of terrestrial life at the end of the Triassic Among them, the feathers with thermal insulation function and the long-awaited adaptation to the polar cold climate are the main reasons why dinosaurs escaped the "volcanic winter" at the end of the Triassic and quickly occupied the ecological dominance of the Jurassic.