Fossilized pollen provides a glimpse of Earth’s broiling past

Chemical clues in fossilized pollen grains suggest the world’s greatest mass extinction was a sunny affair, according to a new analysis (Sci. Adv. 2023, DOI: 10.1126/sciadv.abo6102). As the Permian period gave way to the Triassic period about 250 million years ago, massive volcanic eruptions in present-day Siberia triggered a cataclysmic combination of greenhouse gas emissions, widespread mercury pollution, and a host of other atmospheric disturbances. Yet the full picture remains murky, including the possibility that damage to the ozone layer may have cooked many of the species already struggling to survive concurrent calamities, says Wesley T. Fraser, an organic geochemist at Oxford Brookes University.

To investigate, Fraser and his colleagues turned their attention to tiny grains of fossilized pollen from southern Tibet. This collection contains around 800 pollen grains that come from the ancient relatives of ferns and conifers—plant types which survived the extinction event—and date back to time points spanning the Permian-Triassic boundary period. The tough outer coating of the pollen grains not only helped them stand the test of time in the fossil record, but also preserved traces of para-coumaric acid and ferulic acid. These phenolic compounds absorb ultraviolet B light, which helped shield the pollen from harmful solar radiation. Using Fourier transform infrared microspectroscopy, the researchers found that the abundance of UVB-absorbing compounds in their pollen-based chronology spiked around the same time as mercury and other signs of violent volcanic activity did.

Cindy Looy, a paleobotanist at the University of California, Berkeley, who was not involved in the study, says it’s rare to look at the UV-absorbing compounds in pollen grains from that far in the past, much less across such a broad timeframe during an extinction event. By weaving together multiple lines of evidence “to squeeze the most data out of these samples”, the researchers have brought science one step closer to solving the biggest murder mystery of all time, Looy says.

It’s becoming clearer that this massive mystery was set in a complicated scene in which scorching solar radiation driven by ozone depletion was yet another death knell for much of life on Earth, Fraser says. Conditions would have been brutal, prompting this advice from Fraser: “If you’re going to build a time machine, don’t go to the end-Permian,” he says.