At the end of the Permian, around 252 million years ago, volcanoes in the Siberian Traps spewed lava over an area almost the size of Australia, igniting massive wildfires, burning vast oil and coal deposits, and pouring greenhouse gases into the atmosphere.
Global temperatures spiked by around 10°C, with the sea surface in the tropics approaching 40°C. The oceans acidified and lost oxygen, and 80%–95% of marine species were extinguished in the Great Dying—the worst mass extinction but not the only one our planet has endured.
After such apocalypses, Earth’s climate typically reverts more or less to normal within 80,000–200,000 years, largely thanks to the interaction of the global carbon and silica cycles via a process called silicate weathering.
But as the Late Permian gave way to the Triassic, Earth stayed warm for an unusually long time—around 5 million years—according to oxygen isotopes preserved in prehistoric fish teeth and other fossils. A new study published in the Proceedings of the National Academy of Sciences of the United States of America suggests that the reason lies in the Early Triassic’s near-lifeless oceans and a process called reverse weathering, or the formation of marine clay.
Not Business As Usual
Atmospheric carbon dioxide dissolves in rainwater and forms a weak acid that reacts with, or weathers, rocks on land, dissolving silica and producing ions of magnesium, calcium, and bicarbonate that then wash into the sea. There, marine creatures use the silica and bicarbonate to make their shells and exoskeletons.
The warmer atmospheres that accompanied many mass extinctions resulted in increased rainfall, leading to more weathering and thus converting atmospheric carbon dioxide (CO2) into dissolved carbon at higher rates. This process usually stabilizes the climate within several hundred thousand years.
Why the Early Triassic was an outlier—remaining so warm for so long—has been a mystery until now.
In search of an answer, doctoral student Sofia Rauzi and her supervisor, biogeochemist Terry Isson, both from the University of Waikato, chipped colorful chert samples from rocks on New Zealand’s Waiheke Island. They then traveled to Norway’s icebound Svalbard archipelago to collect similar-aged cherts and marine shales. A colleague sent some more from Japan.
“You can’t just find 250-million-year-old rocks anywhere, so you have to go to the places where they are.”
“You can’t just find 250-million-year-old rocks anywhere, so you have to go to the places where they are,” Rauzi said. The collection represented a range of ages before, during, and after the Early Triassic.
Back at the lab, the scientists analyzed the relative proportions of lithium isotopes trapped inside clay minerals within the samples. “When clay forms, it takes up the lighter lithium isotope and enriches the surrounding water with the heavier lithium isotope,” Rauzi said. “So you can track clay-forming processes by measuring the lithium isotopic composition of these rocks.”
The signatures from the 250-million-year-old rocks were “dramatic,” Isson said, showing limited clay formation before the mass extinction, a significant increase in clay formation afterward, and then a return to baseline after about 5 million years.
These findings lend support to a theory Isson and a colleague first proposed in 2018 and elaborated on in 2022 and again earlier this year. Along with alkaline metals, dissolved silica is an essential ingredient for marine clay formation, he explained. As it does today, the ocean during the Permian teemed with tiny organisms such as diatoms and radiolarians, which used silica to build their exoskeletons—so there was little left to precipitate into clay.
This precipitation is called reverse weathering because the chemical reaction that occurs emits the CO2 captured by weathering, Isson said. “It’s undoing what the weathering has done.”
So when the vast majority of those animals were wiped out in the Great Dying, the ocean’s dissolved silica became available for clay formation. The lithium ratios the group measured suggest that reverse weathering did, in fact, accelerate immediately following the extinction—releasing extra CO2 into the atmosphere, countering the carbon-trapping effect of weathering, and keeping Earth warmer for longer.
Tuning Earth’s Thermostat
“This study provides some of the first geochemical evidence that demonstrates that how much and where clays are forming is intimately connected with…the ecological success of marine ecosystems and the Earth’s climate system,” Isson said. “It gives us an insight into how the Earth’s thermostat really operates.”
In the past, other researchers have used carbonates to infer the lithium isotope ratios of seawater and thus how widely clay formation varied over time, Rauzi said—a less direct method. The lithium signatures she and her coauthors measured, however, contradict those derived from carbonates in previous studies, with implications for our understanding of the way the ocean’s lithium isotope composition has changed over time.
“This is really the first time where anyone’s pointed out that the siliceous archives tell you a drastically different story from the carbonates.”
“It’s provocative,” Isson said. “This is really the first time where anyone’s pointed out that the siliceous archives tell you a drastically different story from the carbonates.” This suggests that researchers should use carbonate archives with caution, Rauzi added. “Maybe they were not such a good recorder of seawater during this time.”
Using clay was an innovative approach, said Hana Jurikova, a biogeochemist at the University of St Andrews in the United Kingdom who was not involved in the study.
The study highlights the importance of accounting for reverse weathering in climate models, she said. “It’s a process that we don’t know enough about.” More research is needed to firmly establish whether the Early Triassic was the only time in the past 500 million years when the process played a significant role in Earth’s climate system and to fine-tune our understanding of how rocks get their lithium signatures, she said.
The work also is a reminder of the complex interconnections between life on this planet and the grand global systems that sustain it. “Sometimes we are surprised that we are able to affect the climate, but actually, many organisms have figured that out,” Jurikova said. “Everything is intertwined.”
—Kate Evans, Science Writer
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