Earth's Greatest Extinction Event May Have Been Caused by Thinning Ozone

It resulted in a world where plants could not reproduce.

The end-Permian extinction is one of the greatest mysteries in Earth’s history. Sure, the Cretaceous-Tertiary extinction event — the one that (almost) wiped out all the dinosaurs — was bad, but even it pales in comparison. The end-Permian extinction, which began about 251.9 million years ago, wiped out over 90 percent of marine species and more than two-thirds of terrestrial species in about 500 thousand years.

However, scientists don’t really know what caused it, having nothing but a theory that massive volcanic eruption kicked the whole event in motion.

But in a paper published Wednesday in the journal Science Advances, researchers from the Department of Integrative Biology and Museum of Paleontology at the University of California, Berkeley, provide experimental evidence that the end-Permian extinction, also known as the Permian-Triassic extinction, could have been caused, in large part, by something we’re all too familiar with: a depleted ozone layer.

Scientists say that a volcanic eruption could have thinned the ozone layer, which in turn let UV-B radiation reach the surface of the Earth, causing pine trees to become sterile.

Jeff Benca

They propose that increased UV-B radiation, which was let in by an ozone layer that had been thinned by the massive volcanic eruption, made it difficult or impossible for trees to reproduce. So, rather than directly killing off the animals, the volcanic activity kicked off a cascade that triggered deforestation, leading to food web collapses, and, eventually to animal extinction.

The evidence comes in the form of mutated pollen grains, which the researchers argue were the result of increased UV-B radiation — the kind that causes sunburns. The fossil record has turned up many specimens of mutated pollen from gymnosperms, the plants that dominated before the [rise of flowering plants), all of which date to the time of the end-Permian extinction. While scientists have hypothesized that these mutant pine, palm, and gingko pollen grains were the result of UV-B radiation, before now researchers hadn’t turned up any strong evidence.

To test their hypothesis, the researchers tried to mutate pollen grains themselves, attempting to recreate the effects of low-ozone conditions. They exposed 30 reproductively mature dwarf pines (Pinus mugo Columnaris), whose pollen is similar to that of end-Permian pines, to a range of light conditions: Six were left outside to serve as a control group, while the other 24 were placed indoors in growth chambers with higher levels of UV-B radiation.

All the plants survived, but the trees exposed to elevated levels of UV-B radiation developed mutated pollen grains and had cones that stopped growing before they were fertile. In other words, the plants were alive but could not reproduce.

Scientists found that pine trees exposed to elevated UV-B radiation like that which may have occurred during the end-Permian extinction event developed mutated pollen grains very similar to fossilized pollen from that time period.

Benca et al

The mutated pollen grains from the trees grown under UV-B conditions (designed to simulate those from the end-Permian extinction event) bear a striking resemblance to fossilized pollen grains from that time period.

This supports the hypothesis that volcanic activity during the end-Permian extinction event didn’t directly kill off animals on Earth but instead created conditions that were really bad for the plants and animals living here. These conditions led to a slow but certain decline over hundreds of thousands of years, as plants failed to reproduce, causing food crisis after food crisis for animals and eventual mass deaths.

The researchers warn that this could also serve as a cautionary tale for our current era. In a moment when ocean temperatures are rising and glaciers are melting, it’s possible that something like the cascades of ecological forces that happened millions of years ago could happen again today. In fact, some scientists say it’s a near certainty that we’ll see a mass extinction event in the next century. But hey, at least maybe someone will learn from our mistakes in a couple hundred million years.

Abstract: Although Siberian Trap volcanism is considered a primary driver of the largest extinction in Earth history, the end-Permian crisis, the relationship between these events remains unclear. However, malformations in fossilized gymnosperm pollen from the extinction interval suggest biological stress coinciding with pulsed forest decline. These grains are hypothesized to have been caused by enhanced ultraviolet-B (UV-B) irradiation from volcanism-induced ozone shield deterioration. We tested this proposed mechanism by observing the effects of inferred end-Permian UV-B regimes on pollen development and reproductive success in living conifers. We find that pollen malformation frequencies increase fivefold under high UV-B intensities. Surprisingly, all trees survived but were sterilized under enhanced UV-B. These results support the hypothesis that heightened UV-B stress could have contributed not only to pollen malformation production but also to deforestation during Permian-Triassic crisis intervals. By reducing the fertility of several widespread gymnosperm lineages, pulsed ozone shield weakening could have induced repeated terrestrial biosphere destabilization and food web collapse without exerting a direct “kill” mechanism on land plants or animals. These findings challenge the paradigm that mass extinctions require kill mechanisms and suggest that modern conifer forests may be considerably more vulnerable to anthropogenic ozone layer depletion than expected.
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