Rain as acidic as lemon juice may have contributed to ancient mass extinction

Rain as acidic as undiluted lemon juice may have played a part in killing off plants and organisms around the world during the most severe mass extinction in Earth’s history.

About 252 million years ago, the end of the Permian period brought about a worldwide collapse known as the Great Dying, during which a vast majority of species went extinct.

The cause of such a massive extinction is a matter of scientific debate, centering on several potential causes, including an asteroid collision, similar to what likely killed off the dinosaurs 186 million years later; a gradual, global loss of oxygen in the oceans; and a cascade of environmental events triggered by massive volcanic eruptions in a region known today as the Siberian Traps.

Now scientists at MIT and elsewhere have simulated this last possibility, creating global climate models of scenarios in which repeated bursts of volcanism spew gases, including sulfur, into the atmosphere. From their simulations, they found that sulfur emissions were significant enough to create widespread acid rain throughout the Northern Hemisphere, with pH levels reaching 2 – as acidic as undiluted lemon juice. They say such acidity may have been sufficient to disfigure plants and stunt their growth, contributing to their ultimate extinction.

“Imagine you’re a plant that’s growing happily in the latest Permian,” says Benjamin Black, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It’s been getting hotter and hotter, but perhaps your species has had time to adjust to that. But then quite suddenly, over the course of a few months, the rain begins to sizzle with sulfuric acid. It would be quite a shock if you were that plant.”

Black is lead author of a paper reporting the group’s results, which appears in the journal Geology. Co-authors include Jean-Fran├žois Lamarque, Christine Shields, and Jeffrey Kiehl from the National Center for Atmospheric Research and Linda Elkins-Tanton of the Carnegie Institution for Science.

Lemon juice spike

Geologists who have examined the rock record in Siberia have observed evidence of immense volcanism that came in short bursts beginning near the end of the Permian period and continuing for another million years. The volume of magma totaled several million cubic kilometers – enough to completely blanket the continental United States. This boiling stew of magma likely released carbon dioxide and other gases into the atmosphere, leading to gradual but powerful global warming.

The eruptions may also have released large clouds of sulfur, which ultimately returned to Earth’s surface as acid rain. Black, who has spent several summers in Siberia collecting samples to measure sulfur and other chemicals preserved in igneous rocks, used these measurements, along with other evidence, to develop simulations of magmatic activity in the end-Permian world.

The group simulated 27 scenarios, each approximating the release of gases from a plausible volcanic episode, including medium eruptions, large eruptions, and magma erupted through explosive pipes in the Earth’s crust. The researchers included a wide range of gases in their simulations, based on estimates from chemical analyses and thermal modeling. They then tracked water in the atmosphere, and the interactions among various gases and aerosols, to calculate the pH of rain.

The results showed that both carbon dioxide and volcanic sulfur could have significantly affected the acidity of rain at the end of the Permian. Levels of carbon dioxide and other greenhouse gases may have risen rapidly at the time, in part because of Siberian volcanism. According to their simulations, the researchers found that this elevated carbon dioxide could have increased rain’s acidity by an order of magnitude.

Adding sulfur emissions to the mix, they found that acidity further spiked to a pH of 2 – as acidic as undiluted lemon juice – and that such acidic rain may have fallen over most of the Northern Hemisphere. After an eruption ended, the researchers found that pH levels in rain bounced back, becoming less acidic within one year. However, with repeated bursts of volcanic activity, Black says the resulting swings in acid rain could have greatly stressed terrestrial species.

“Plants and animals wouldn’t have much time to adapt to these changes in the pH of rain,” Black says. “I think it certainly contributed to the environmental stress which was making it difficult for plants and animals to survive. At a certain point you have to ask, ‘How much can a plant take?'”

Life as an end-Permian organism

In addition to acid rain, the researchers modeled ozone depletion resulting from volcanic activity. While ozone depletion is more difficult to model than acid rain, their results suggest that a mix of gases released into the atmosphere may have destroyed 5 to 65 percent of the ozone layer, substantially increasing species’ exposure to ultraviolet radiation. The greatest ozone depletion occurred near the poles.

Going forward, Black hopes paleontologists and geochemists will consider the results as a point of comparison for their own observations of the end-Permian mass extinction. In the meantime, he says he now has a much more vivid picture of that catastrophic time.

“It’s not just one thing that was unpleasant,” Black says. “It’s this whole host of really nasty atmospheric and environmental effects. These results really made me feel sorry for end-Permian organisms.”

Land animals, ecosystems walloped after Permian dieoff

<IMG SRC="/Images/182956516.jpg" WIDTH="350" HEIGHT="220" BORDER="0" ALT="Lystrosaurus, a relative to mammals, was one of a handful of ‘disaster taxa’ to escape from the rubble of the Permian Period, along with the meter-high spore-tree Pleuromeia. Low diversity of animals delayed the full recovery of land ecosystems by millions of years. – Victor Leshyk”>
Lystrosaurus, a relative to mammals, was one of a handful of ‘disaster taxa’ to escape from the rubble of the Permian Period, along with the meter-high spore-tree Pleuromeia. Low diversity of animals delayed the full recovery of land ecosystems by millions of years. – Victor Leshyk

The cataclysmic events that marked the end of the Permian Period some 252 million years ago were a watershed moment in the history of life on Earth. As much as 90 percent of ocean organisms were extinguished, ushering in a new order of marine species, some of which we still see today. But while land dwellers certainly sustained major losses, the extent of extinction and the reshuffling afterward were less clear.

In a paper published in the journal Proceedings of the Royal Society B, researchers at Brown University and the University of Utah undertook an exhaustive specimen-by-specimen analysis to confirm that land-based vertebrates suffered catastrophic losses as the Permian drew to a close. From the ashes, the survivors, a handful of genera labeled “disaster taxa,” were free to roam more or less unimpeded, with few competitors in their respective ecological niches. This lack of competition, the researchers write, caused vicious boom-and-bust cycles in the ecosystems, as external forces wreaked magnified havoc on the tenuous links in the food web. As a result, the scientists conclude from the fossil record that terrestrial ecosystems took up to 8 million years to rebound fully from the mass extinction through incremental evolution and speciation.

“It means the (terrestrial ecosystems) were more subject to greater risk of collapse because there were fewer links” in the food web, said Jessica Whiteside, assistant professor of geological sciences at Brown and co-author on the paper.

The boom-and-bust cycles that marked land-based ecosystems’ erratic rebound were like “mini-extinction events and recoveries,” said Randall Irmis, a co-author on the paper, who is a curator of paleontology at the Natural History Museum of Utah and an assistant professor of geology and geophysics at Utah.

The hypothesis, in essence, places ecosystems’ recovery post-Permian squarely on the repopulation and diversification of species, rather than on an outside event, such as a smoothing out of climate. The analysis mirrors the conclusions reached by Whiteside in a paper published last year in Geology, in which she and a colleague argued that it took up to 10 million years after the end-Permian mass extinction for enough species to repopulate the ocean – restoring the food web – for the marine ecosystem to stabilize.

“It really is the same pattern” with land-based ecosystems as marine environments, Whiteside said. The same seems to hold true for plants, she added.

Some studies have argued that continued volcanism following the end-Permian extinction kept ecosystems’ recovery at bay, but Whiteside and Irmis say there’s no physical evidence of such activity.

The researchers examined nearly 8,600 specimens, from near the end of the Permian to the middle Triassic, roughly 260 million to 242 million years ago. The fossils came from sites in the southern Ural Mountains of Russia and from the Karoo Basin in South Africa. The specimen count and analysis indicated that approximately 78 percent of land-based vertebrate genera perished in the end-Permian mass extinction. Out of the rubble emerged just a few species, the disaster taxa. One of these was Lystrosaurus, a dicynodont synapsid (related to mammals) about the size of a German shepherd. This creature barely registered during the Permian but dominated the ecosystem following the end-Permian extinction, the fossil record showed. Why Lystrosaurus survived the cataclysm when most others did not is a mystery, perhaps a combination of luck and not being picky about what it ate or where it lived. Similarly, a reptilian taxon, procolophonids, were mostly absent leading to the end-Permian extinction, yet exploded onto the scene afterward.

“Comparison with previous food-web modeling studies suggests this low diversity and prevalence of just a few taxa meant that links in the food web were few, causing instability in the ecosystem and making it susceptible to boom-bust cycles and further extinction,” Whiteside said.

The ecosystems that emerged from the extinction had such low animal diversity that it was especially vulnerable to crashes spawned by environmental and other changes, the authors write. Only after species richness and evenness had been re-established, restoring enough population numbers and redundancy to the food web, did the terrestrial ecosystem fully recover. At that point, the carbon cycle, a broad indicator of life and death as well as the effect of outside influences, stabilized, the researchers note, using data from previous studies of carbon isotopes spanning the Permian and Triassic periods.

“These results are consistent with the idea that the fluctuating carbon cycle reflects the unstable ecosystems in the aftermath of the extinction event,” Whiteside said.