Scientists Think They Know What Caused the Deadliest Mass Extinction in the History of the Earth

Humanity can have a love/hate relationship with itself, but there’s no denying that we’re the pinnacle of evolution on Earth as things stand now. But it took an awfully long time for evolution to produce beings such as we. Several times, life had to drag itself back from near annihilation.

The largest extinction setback was the Permian-Triassic extinction, also called the “Great Dying,” some 252 million years ago. Up to 96% of all marine species and 70% of terrestrial vertebrate species went extinct.

What happened?

There’s some uncertainty surrounding the cause or causes of the extinction. Many of the usual suspects have been accused: impact events, climate change brought on by methane-producing bacteria, massive volcanic eruptions, as well as some lesser-known potential causes like a fungal spike. There’s some evidence for each hypothesis, but it’s still controversial. The uncertainty around the Great Dying also extends to the timeline and sequence of events, including how long Earth took to recover.

Now, the authors of a new study say they have finally, conclusively figured it out. The paper is titled “Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations.” The lead author is Dr. Hana Jurikova from the School of Earth and Environmental Sciences at St. Andrews University. The study is published in the journal Nature Geoscience.

“Our research provides the first precise reconstruction of the carbon source and with it the trigger of the crisis, as well as uncovers the subsequent chain of processes that resulted in Earth’s largest mass extinction.”

Dr. Hana Jurikova, Lead Author, School of Earth and Environmental Sciences at St. Andrews University.

The team of scientists behind this research includes researchers from Germany, Italy, and Canada. They’re part of an EU-funded project called BASE-LiNE Earth. BASE-LiNE Earth is led by Prof. Dr. Anton Eisenhauer from GEOMAR Helmholtz Centre for Ocean Research Kiel in cooperation with the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences.

BASE-LiNE stands for Brachiopods As SEnsitive tracers of gLobal mariNe Environment. The effort focuses on brachiopod fossils, which, according to a press release, haven’t received much attention and are sometimes overlooked. Brachiopods are a group of creatures that have hard shells on their upper and lower surfaces. They’re different from bivalve molluscs like clams, which have shells on the sides. Brachiopods are still around today, but are far less numerous than they were in the Paleozoic.

Many brachiopod species went extinct during the Great Dying. But some survived, and the team of researchers found brachiopod shells that span the critical time period during the Permian-Triassic extinction event.

The shells are a record over time of the ocean’s pH level. The pH level reflects the amount of absorbed carbon dioxide (CO2) in the ocean when the animal made the shell. When combined with carbon isotope constraints, the team was able to construct a timeline of not only the amount of atmospheric CO2 but also their sources at the time of the extinction.

The researchers determined that a large pulse of CO2 triggered the Permian-Triassic extinction. The pulse originated in Siberia, where a huge volcanic eruption created a massive flood basalt province. All of that activity released an enormous amount of CO2 into the atmosphere; 100,000 billion tonnes (= 1 × 10¹⁴ tonnes).

That is an almost incomprehensible amount of carbon injected into the atmosphere in a short (geologically speaking) period of time. A press release announcing the paper says that’s “more than 40 times the amount of all carbon available in modern fossil fuel reserves including carbon already burned since the industrial revolution.”

All of that CO2 threw Earth’s biogeochemical makeup out of balance and spelled doom for most of the species on Earth. The team used innovative modeling to determine exactly what that sudden injection of CO2 meant for our planet.

There’s already a lot of evidence showing that Siberian volcanic activity led to the extinction. But the evidence has never been this unified. “The exact causes and consequences, however, remain controversial, and a coherent unifying scenario for the environmental evolution over this important interval in Earth’s history is still lacking,” the researchers write.

“Our findings enable us to assemble a consistent biogeochemical reconstruction of the mechanisms that resulted in the largest Phanerozoic mass extinction,” the authors write in their paper.

The CO2 led to extreme warming of the Earth’s atmosphere and lethal acidification of the oceans. For marine animals that build their own shells, like the brachiopods at the heart of this research, it was devastating. As the ocean became more acidic, the carbonate that they need to build shells becomes unavailable, locked into the ocean’s new chemistry. Some brachiopods survived, creating the evidential record that enabled this study.

The extreme climate warming was the second severe blow for life on Earth. The greenhouse effect created pronounced changes in weathering on the land, and on nutrient input and cycling in the ocean. The result was vast deoxygenation of the Earth’s oceans. It likely also poisoned the oceans with sulfides, killing other groups of organisms.

The oceans, birthplace of life on Earth, became a place of death.

While volcanic activity has been proposed as a cause of the Great Dying before, this study is a more complete timeline of events. Then, as now, life on Earth relies on global cycles of nutrients, carbon, nitrogen, and other things. In only a few thousand years, those cycles were upended by the CO2 from the Siberian eruptions. Climate warming, ocean acidification, ocean deoxygenation, and sulfide poisoning came one after another, creating the most severe extinction in the Earth’s history.

“These findings lead us to view the PTB <Permian-Triassic Boundary> mass extinction as a cascading marine collapse, triggered by a multimillennial-scale voluminous injection of carbon to the atmosphere by the emplacement of Siberian Traps sill intrusions,” the authors write. “Its magnitude profoundly altered the biogeochemical processes and set off a chain of events that selectively eliminated different groups of marine organisms.”

In a press release, lead researcher Dr. Jurikova said: “Our research provides the first precise reconstruction of the carbon source and with it the trigger of the crisis, as well as uncovers the subsequent chain of processes that resulted in Earth’s largest mass extinction.”

“It took several hundreds of thousands to millions of years for the ecosystem to recover from the catastrophe, which profoundly altered the course of evolution of life on Earth.”

It’s natural to wonder if these findings can tell us anything about our current predicament, where our own carbon emissions are warming the climate and acidifying the oceans.

In their paper’s conclusion the authors write: “Given the vastly differing timescales and carbon budgets involved, LIP <Large Igneous Province> carbon cycle dynamics is a poor analogy for present-day fossil fuel emissions; notwithstanding that, the modern geological carbon reservoirs are insufficient for anthropogenic release beyond centennial time scales.”

Alarmingly, we may still be emitting more carbon than during the Siberian volcanic activity that triggered the Permian-Triassic extinction. “It is, however, noteworthy that even the peak emissions rate during the largest known mass extinction is still more than 14 times less than the current anthropogenic rate.” 

During the Great Dying, it took thousands of years for the effects of all of that carbon to unfold. But we’re already seeing the effects of our carbon emissions, and we’re barely out of the Industrial Revolution. “The environmental deterioration during the PTB took several thousands of years to unfold, whereas the current, unprecedented emissions rate has already started to take a toll on the marine ecosystems,” the authors point out. 

The paper ends with what could be an ominous understatement: “A coupled increase in atmospheric CO2 and decrease in surface ocean pH, global warming, changes in productivity and oxygen depletion have been reported worldwide, which suggests that the scenario outlined here for the PTB may also be relevant to understanding future environmental and climatic trends.”

More:

• Press Release: Great Dying – what caused Earth’s biggest mass extinction?
• New Research: Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations
• Universe Today: During Mass Extinction Events, Volcanoes Were Releasing About the Same Amount of CO2 as We Are Today