Around 252 million years ago, the Earth experienced its largest mass extinction. Known as the “Great Dying,” this cataclysmic event wiped out more than 81% of marine species and 70% of life on land. 

Currently, the prevailing theory is that the extinction both in the oceans and on land was triggered simultaneously by massive volcanic eruptions. As greenhouse gases and toxic substances were pumped into the atmosphere, they kickstarted a global catastrophe that included devastating acid rain, runaway global warming, ozone layer destruction and oxygen-deficient waters. 

“Essentially, the planet’s life-support systems were pushed past a breaking point,” said Jianbo Chen, a former UC Davis postdoctoral scholar who worked with Isabel P. Montañez, a Distinguished Professor in the Department of Earth and Planetary Sciences.     

In a new study appearing Oct. 27 in the Proceedings of the National Academy of Sciences, Chen, Montañez and a team of researchers challenge the idea that these volcanic eruptions triggered a single global, environmental collapse all at once. Rather, the devastation and environmental collapse on land happened regionally and in stages.   

“Our study in paleo-tropical Southwest China shows the local terrestrial ecosystems collapsed several hundred thousand years after a similar collapse happened in southern high-latitude areas like the Sydney Basin in Australia,” said Chen, who is now an associate professor at Yunnan University’s Institute of Paleontology located in Kunming, China.

“This time lag is a crucial clue,” Chen added. “The die-offs weren’t caused by a single, global ‘hammer blow,’ but by a series of regional collapses that happened at different times.” 

A new look at an ancient catastrophe

In the paper, the team analyzed a well-studied drill core from an ancient tropical peatland in Southwest China called HK-1. Measuring roughly 671 meters long and nine centimeters wide, the HK-1 drill core is a continuous rock record that captures a landmass transitioning from a purely terrestrial forest to a swampy wetland inundated by a rising sea and then to a full-blown marine environment. 

“What it captures is the ‘crime scene’ of the extinction on land,” Chen said. 'We get to see the last stand of a great, ancient tropical rainforest apparently thriving up to the Permian-Triassic boundary and then pinpoint the exact moment soon after the boundary when it collapsed.”

In essence, it tells an extinction story in a geochemical script that’s written in its sediments.     

To read this story, the team conducted geochemical analyses of the core’s carbon and multiple sulfur isotopes, its total organic carbon contents and mercury in bulk sediments.

The team searched for dramatic changes in a sulfur isotope, specifically a geochemical fingerprint called a mass-independent isotope anomaly. This signature is associated with major volcanic eruptions. 

“Think of it as a ‘smoking gun’ for a specific kind of catastrophe,” Chen said. “When a massive volcanic eruption blasts sulfur high into the stratosphere, above the ozone layer, it is affected by intense UV radiation.”

“This process leaves a very distinct chemical fingerprint in the sulfur isotopes,” Chen added. “This specific fingerprint has been found in the rock record for other major mass extinctions, like the one that killed the dinosaurs.” 

While this signature was near absent from the team’s analyses, two other sulfur signatures were not. This led the team to conclude that the sulfur remnants found in HK-1 came from a non-volcanic source.

“The discovery pointed us directly to the land itself,” Chen said. “The sulfur was being washed in from the catastrophic erosion of deforested soils.” 

A progressive unraveling, not a single collapse

Further research revealed that the signatures of this ecosystem’s collapse and its disruptive effects on the carbon-sulfur cycle coincided with similar records from high-latitude areas like Australia’s Sydney Basin. But the latter event occurred at least 600,000 years prior to the event recorded in HK-1. 

“If a single, giant volcanic event was the direct trigger as has long been proposed, then you would expect the collapse to be much more simultaneous,” Chen said.   

Chen described the research as investigating the “main event” of the end-Permian crisis. What’s still a mystery is the event’s prologue. Were there warning signs to these catastrophes? 

“We now want to look deeper into the 671-meter drill core at the rock layers from before the extinction,” he said. “We will analyze the carbon, sulfur and mercury to see if the ecosystem was already sick or becoming unstable long before it finally tipped over.” 

In follow-up research, the team will use their analyses techniques to investigate other famous extinction sites associated with the end-Permian crisis, such as the Karoo Basin in South Africa and other locations in North China. 

“The ultimate goal is to build a truly global, high-resolution timeline,” Chen said. “By comparing these different continents, we can finally move from seeing the extinction as a single snapshot to understanding it as a complex, rolling cascade or regional collapses.”

This study was supported jointly by the National Natural Science Foundation of China, the Yunnan Province Science and Technology Department and the Yunnan Science and Technology Champion Project.