Ancient pollen grains acting like ‘sunscreen’ may have contributed to mass extinction

NOTTINGHAM, United Kingdom — A meteor may have killed the dinosaurs, but it seems the Sun was the real terror back in the day. Geologists from the University of Nottingham have found that UV radiation contributed to the mass extinction of about 80 percent of marine and land species 250 million years ago.

At first glance, it seems impossible to know how intense the Sun was millions of years ago. However, when study authors examined the structure and composition of 250-million-year-old rocks from Tibet, they discovered pollen. Standing at half the width of a human hair, the fossilized pollen grains were wedged and preserved inside the rocks. Using a new method to study plants, the team could analyze the pollen’s composition, which had compounds that acted as a natural “sunscreen” against the Sun’s harsh ultraviolet rays.

“Plants require sunlight for photosynthesis but need to protect themselves and particularly their pollen against the harmful effects of UV-B radiation,” explains study co-author Barry Lomax in a university release. “To do so, plants load the outer walls of pollen grains with compounds that function like sunscreen to protect the vulnerable cells to ensure successful reproduction.”

Diagram showing how UV radiation in sunlight affects Earth
Schematic representation of how elevated UV-B radiation may have affected the broader terrestrial ecosystem. Image drawn by Conor Haynes-Mannering, University of Nottingham.

The pollen formed as an evolutionary adaptation against the Sun’s UV rays, which suggests they were so strong they threatened the ancient plants’ existence. While the pollen blocked the UV rays, not every species in that time period had a natural sunblock.

Did ancient pollen cut off vital food sources?

The rocks date back 250 million years ago, meaning the pollen appeared during the Permian period. This era in history occurred 299 to 251 million years ago and was the last period of the Paleozoic era. It is most known for having one of the five biggest mass extinction events. Researchers believe the end-Permian mass extinction event wiped out almost 80 percent of sea and land species and was the result of a volcanic eruption that covered what is now present-day Siberia. Volcanic eruptions release a high amount of carbon into the Earth’s atmosphere, contributing to greenhouse warming and weakening the ozone layer.

Microscopic image of an ancient pollen grain
Alisporites tenuicorpus, the pollen grain used in the study. The samples analyzed are about half the width of a human hair. Photograph is courtesy of Prof Liu Feng from Nanjing Institute of Geology and Paleontology

“We have developed a method to detect these phenolic compounds in fossil pollen grains recovered from Tibet, and detected much higher concentrations in those grains that were produced during the mass extinction and peak phase of volcanic activity,” adds Liu Feng, a professor from Nanjing Institute of Geology and Paleontology.

The pollen findings suggest the increased greenhouse gases may have strengthened the Sun’s UV rays on Earth, further contributing to the mass extinction crisis. Previous studies have found that massive amounts of UV radiation decrease plant biomass and terrestrial carbon storage, worsening global warming problems. Having more pollen and sunscreen-like chemicals covering plants make them more difficult to eat and digest, reducing a major food source for herbivores.

“Volcanism on such a cataclysmic scale impacts on all aspects of the Earth system, from direct chemical changes in the atmosphere, through changes in carbon sequestration rates, to reducing volume of nutritious food sources available for animals,” says study co-author Wes Fraser, a professor at Oxford Brookes University.

The study is published in the journal Science Advances.

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About the Author

Jocelyn Solis-Moreira

Jocelyn is a New York-based science journalist whose work has appeared in Discover Magazine, Health, and Live Science, among other publications. She holds a Master’s of Science in Psychology with a concentration in behavioral neuroscience and a Bachelor’s of Science in integrative neuroscience from Binghamton University. Jocelyn has reported on several medical and science topics ranging from coronavirus news to the latest findings in women’s health.

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