What caused the ‘Great Dying’ mass extinction? We may finally know

The LPME, also known as the “Great Dying,” was the largest mass extinction event in Earth’s history, occurring around 252 million years ago. It killed off about 96 percent of all marine species and 70 percent of land vertebrates, ending the Permian and marking the start of the Triassic period (252-201 million years ago).

There is still a lot of disagreement about what exactly caused the extinction. Still, some theories include a meteor strike, a lot of volcanic activity, a sudden change in climate, and the lack of oxygen in the oceans. But, even though the LPME occurred more than 250 million years ago, Frank shows parallels between the current big climate changes.

“It’s relevant to understanding what might happen in the future. The main cause of climate change is related to a massive injection of carbon dioxide into the atmosphere around the time of the extinction, which led to rapid warming,” he said in the UConn press release.

Also, according to Frank, it is widely believed that the major volcanism in the Siberian Traps Large Igneous Province (STLIP), a gigantic lava deposit, caused the rapid warming associated with the LPME. However, there still needs to be factual data to support this claim.

A tragedy of tragedies: How does mercury fit into all of this?

In the geological record, volcanoes give helpful hints. A significant amount of gases, such as CO₂ and methane, as well as particles and heavy metals, were also released and deposited globally with the lava eruption.

“However, it’s hard to link something like that to the extinction event directly,” says Frank. “As geologists, we’re looking for a signature of some kind—a smoking gun—so we can point to the cause,” he added.

According to the researchers, mercury, one of the heavy metals connected to volcanic eruptions, was the “smoking gun” they needed to focus on. Finding places where that record is still present is the tricky part.

Frank says that the sediments in marine environments keep a record of Earth’s history in a way that is almost like a tape recorder. This is because deposits are buried and protected right away. These sediments tell us a lot about how the extinction happened in the oceans and what species went extinct.

Finding such well-preserved records from this era on land is more challenging.

Frank uses Connecticut as an example. The state is rich in metamorphic rocks that are 400–500 million years old and found at or near the surface, covered by glacial deposits that date to around 23,000 years ago.

“There’s a big gap in the record here. You have to be lucky to preserve terrestrial records, and that’s why they aren’t as well studied, because there are fewer out there,” says Frank.

A world-first: Focusing on LPME evidence in the southern hemisphere

Previous research on the LPME has primarily focused on places in the northern hemisphere, and not all parts of the world have geological records with such significant gaps. However, two southern hemisphere regions with excellent occurrence records are the Sydney Basin in eastern Australia and the Karoo Basin in South Africa. Frank and Fielding have previously investigated both of these regions.

To obtain samples for analysis of mercury isotopes, a colleague and co-author named Jun Shen from the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences contacted Frank, Fielding, and other co-authors.

Frank said that Shen could look at the mercury isotopes in the samples and put all the pieces together.

“It turns out that volcanic [mercury emissions] have a very specific isotopic composition of the mercury that accumulated at the extinction horizon. Knowing the age of these deposits, we can more definitively tie the timing of the extinction to this massive eruption in Siberia,” he said.

“What is different about this paper is we looked not only at mercury, but the isotopic composition of the mercury from samples in the high southern latitudes, both for the first time,” he said.

Scientists have tried to get a better handle on this, but Fielding says that the more we learn, the more complicated it gets.

“As a starting point, geologists have pinpointed the timing of the major extinction event at 251.9 million years with a high degree of precision from radiogenic isotope dating methods. Researchers know that is when the major extinction event happened in the marine environment, and it was assumed that the terrestrial extinction event happened [simultaneously],” Frank explained.

Land animals may have died out first

But Frank and Fielding found in an earlier study that the land-based extinction happened 200–600,000 years earlier.

“That suggests that the event itself wasn’t just one big whammy that happened instantaneously. It wasn’t just one very bad day on Earth, so to speak; it took some time to build, and this feeds well into the new results because it suggests the volcanism was the root cause,” says Fielding.

“That’s just the first impact of the biotic crisis that happened on land, and it happened early. It took [time for it] to be transmitted into the oceans. The event 251.9 million years ago was the major tipping point in environmental conditions in the ocean that had deteriorated over some time,” he added.

According to Frank, reconstructing the events requires knowledge from numerous geologists who are experts in separate fields, including sedimentology, geochemistry, paleontology, and geochronology.

“This type of work requires a lot of collaboration. It all started with fieldwork when a group of us went down to Australia, where we studied the stratigraphic sections that preserved the time interval. The main point is that we now have a chemical signature in the form of mercury isotope signatures that definitively ties the extinction horizon in these terrestrial sections that provide a record of what was happening on land due to Siberian Traps volcanism,” Frank said.

You can read the study for yourself in the journal Nature Communications.

Study abstract:

“The latest Permian mass extinction (LPME) was triggered by magmatism of the Siberian Traps Large Igneous Province (STLIP), which left an extensive record of sedimentary Hg anomalies at Northern Hemisphere and tropical sites. Here, we present Hg records from terrestrial sites in southern Pangea, nearly antipodal to contemporaneous STLIP activity, providing insights into the global distribution of volcanogenic Hg during this event and its environmental processing. These profiles (two from Karoo Basin, South Africa; two from Sydney Basin, Australia) exhibit significant Hg enrichments within the uppermost Permian extinction interval as well as positive Δ199Hg excursions (to ~0.3‰), providing evidence of long-distance atmospheric transfer of volcanogenic Hg. These results demonstrate the far-reaching effects of the Siberian Traps as well as refine stratigraphic placement of the LPME interval in the Karoo Basin at a temporal resolution of ~105 years based on global isochronism of volcanogenic Hg anomalies.”