A deadly pulse of ultraviolet (UV) radiation may have played a role in Earth’s largest mass extinction, fossilized pollen grains reveal.
Pollen dating back to the time of the Permian-Triassic mass extinction, around 250 million years ago, produced “sunscreen” compounds that protected against harmful UV-B rays, the analysis found. At that time, about 80% of all marine and terrestrial species died.
For the study, which was published Jan. 6 in the journal Science Advances (opens in a new tab)a team of international scientists has developed a new method of using a laser beam to examine the tiny grains, which measure about half the width of a human hair and have been found embedded in rocks discovered in southern Tibet, according to a press release (opens in a new tab).
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Plants depend on photosynthesis to convert sunlight into energy, but they also need a mechanism to block harmful UV-B rays.
“As UV-B is bad for us, it is just as bad for plants,” Barry Lomax (opens in a new tab), study co-author and professor of plant paleobiology at the University of Nottingham in the UK, told Live Science. “Instead of going [the pharmacy], plants can alter their chemistry and create their own equivalent version of sunscreen compounds. Their chemical structure acts to dissipate high-energy wavelengths of UV-B light and prevent it from penetrating the preserved tissues of the pollen grains.”
In this case, the radiation spike didn’t “kill the plants instantly, but rather slowed them down by decreasing their ability to photosynthesize, making them sterile over time,” Lomax said. “You then end up with an extinction driven by a lack of sexual reproduction rather than UV-B which fry the plants instantly.”
Experts have long speculated that the Permian-Triassic extinction, listed as one of the top five extinction events on Earth, was in response to a “paleoclimate emergency” caused by the eruption of the Siberian Traps, a large volcanic event in what is now modern Siberia. The catastrophic incident forced plumes of carbon deep inside the Earth into the stratosphere, resulting in a global warming event that “led to a collapse of the Earth’s ozone layer”, according to the researchers.
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“And when you thin the ozone layer, you end up with more UV-B,” Lomax said.
In their research, the scientists also found a link between the burst of UV-B radiation and how it altered the chemistry of plant tissues, which led to “a loss of insect diversity”, said Lomax.
“In this case, the plant tissues became less palatable to herbivores and less digestible,” Lomax said.
Because plant leaves had less nitrogen, they were not nutritious enough for the insects that ate them. This may explain why insect populations plummeted during this extinction event.
“Often insects emerge unscathed during mass extinction events, but that was not the case here,” Lomax said.