Mass extinction event 260 million years ago resulted from climate change, studies say

The Capitanian mass extinction was once lumped in with the “Great Dying” of the end-Permian mass extinction, but the lesser-known extinction occurred 8–10 million years earlier. It may not have been great, but it was quite lethal, seeing as many as 62 percent of species go extinct, according to one estimate. Two new papers by different teams shed new light on the event, revealing a pattern of cause and effect that’s seen in other mass extinctions: huge volcanic eruptions, global warming, the collapse of the terrestrial ecosystem, and the spread of oxygen-starved ocean dead zones.

Ocean dead zones

Huyue Song of China University of Geosciences and colleagues from China, the US, and the UK studied mid-Permian-age rocks at a site called Penglaitan, about 300 miles west of Hong Kong. They found that there were two distinct pulses of Capitanian extinction, one about 262 million years ago and another around 260 million years ago. Those are both well before the more famous “Great Dying” end-Permian extinction, which occurred 252 million years ago, and Song’s team set out to uncover what happened.

“In a way, the extinction losses have been hiding in the shadow of the end-Permian extinction,” said Paul Wignall, a professor at the University of Leeds and a co-author on Song’s paper. “It wiped out a lot of genera of all the usual things in the sea,” adding, “a bunch of animals died on land,” as well.

Previous studies have found evidence of Capitanian extinctions in places as far afield as Ellesmere Island and Spitsbergen in the Arctic, China, Iran, Texas, New Mexico, North Dakota, South Africa, and Antarctica. The extinctions hit corals, mollusks, forams, and calcareous algae in the seas, as well as land plants and animals such as the dinocephalians (meaning “terrible heads”), a group of large reptiles related to the ancestors of mammals.

In the time leading up to the extinctions, the Penglaitan area was like the Bahamas, Wignall told Ars, with a warm shallow sea and reefs. But then the environment soured.

To find out why, they analyzed uranium isotopes, along with carbon and oxygen isotopes, in rocks from Penglaitan. When seawater has limited oxygen, microbes in the seabed obtain electrons for their metabolism from other elements, including uranium. Since the microbes prefer uranium-238 over uranium-235, they alter the balance of uranium isotopes between the seabed and seawater. When this imbalance is found preserved in rocks, it tells us that there were oxygen-starved dead zones in the global oceans at that time.

Song and colleagues used this approach to discover that each Capitanian marine extinction pulse coincided with widespread oxygen starvation in the ocean, called anoxia. “Oxygen levels were getting weaker, which is to the detriment of animals,” said Wignall.

Professor Bas van de Schootbrugge of Utrecht University, who was not involved in the study, agrees with the anoxia explanation but questions the extinctions: “This data seems robust. As for the presumed mass extinction, especially the global nature of it, I am less convinced,” he told Ars.

Song and colleagues accept that judgment, writing: “The timing and number of episodes of the Capitanian biocrisis remain controversial.”

Carbon isotopes in the same rocks showed that the anoxia was coincident with large shifts in the carbon cycle, and oxygen isotopes revealed there was global warming at each of the two extinction pulses.

The question is, what caused this environmental upheaval?

Deforestation

The second paper, by Kunio Kaiho of Tohoku University in Japan, with colleagues from China and Canada, offers an answer.

Kaiho’s paper concentrates on the second pulse of extinction and provides hints about what led to the anoxia in the oceans. Kaiho and colleagues analyzed chemicals called “polycyclic aromatic hydrocarbons” (PAHs) extracted from rocks at Penglaitan. PAH molecules are produced by burning, with specific and distinct molecular structures formed depending on the temperature of the fire that made them. One of these—coronene—is only made in exceptionally hot fires with temperatures over 1,200° C. Coronene was found by Kaiho and colleagues, providing a sign of “high-temperature combustion of organic matter,” as they say in their paper.

Crucially, Kaiho also found a chemical tracer for soil erosion, called “dibenzofuran,” in the same rocks. That data shows that there was a lot of soil erosion, which indicates a terrestrial vegetation collapse at the time of the mass extinction. This was likely the cause of the anoxia. “Soil erosion events cause eutrophication [i.e., oxygen starvation] of seawater and mass mortality of near-shore animals,” explained Kaiho.

Interestingly, coronene is also found in rocks formed in the end-Permian mass extinction, linked to wildfires and ecosystem collapse at that time, too.

Destabilized climate

“The Kaiho paper, the increased burning… indicates a climate that's becoming more prone to drought,” said Wignall. “You're losing your forests on land, basically.”

Both papers point to the Emeishan large igneous province as the initial trigger of the environmental destruction. Also located in China, the Emeishan LIP was the product of repeated magma floods and was in the process of erupting at the time. It was of the same type of igneous paroxysm that has been linked to most other mass extinctions in the geological record, such as the Siberian Traps that triggered the end-Permian extinction and the Central Atlantic Magmatic Province that initiated the end-Triassic mass extinction. All these eruptions emitted prodigious quantities of CO₂ and sulfur dioxide and may have damaged the ozone layer and stressed vegetation with acid rain and mercury.

“High-temperature wildfires, such as the recent ones witnessed in Australia, can contribute to the formation of coronene,” Kaiho told Ars, but “most... coronene comes from volcanic eruptions.” This ties volcanic eruptions to the die-off when those rocks were forming.

Kaiho thinks the volcanic eruptions disrupted the climate, bringing rainfall deluges: “Global warming causes an increase in precipitation, which induces [a] large amount of soil erosion,” he said.

A recurring pattern echoed today

Wignall thinks the Capitanian fits a recurring pattern seen in most mass extinction events:

“They all link to giant volcanism, the large igneous provinces. They all link with rapid global warming events as well, and there's the anoxia story as well, the ocean stagnation that you see happening at the same time,” said Wignall. “So yeah, that's a recurrent pattern!”

But Van de Schootbrugge is less confident the Capitanian extinctions fit that pattern: “There are doubts about the timing of the biotic crisis, and the timing of the presumed LIP volcanism. All in all, a lot of uncertainties that clearly need more attention.”

Mass extinctions in the geological record happened on a larger scale and over far longer timeframes than human-caused climate change, but in many respects, they are remarkably similar. Instead of volcanic CO₂, we have CO₂ from fossil fuels, and we have methane leaks adding to global warming. Climate change is causing mass tree die-offs from droughts; anoxic ocean dead zones, while smaller than in the Capitanian, are spreading. Kaiho’s diagnosis of deforestation, deluges, and soil destruction are echoed today by the muddy floods this summer in the US, Japan, India, China, Turkey, Cuba, Italy, Spain, Chile, Brazil, Ivory Coast, and more.

“It just becomes more unstable on land, I think, which may be all part of the destabilization of the climate associated with these warming events,” said Wignall.

Earth and Planetary Science Letters, 2023. DOI: doi.org/10.1016/j.epsl.2023.118128 (About DOIs).

Palaeogeography, Palaeoclimatology, Palaeoecology, 2023. DOI: doi.org/10.1016/j.palaeo.2023.111518