Earth's last ocean crisis wiped out coral reefs - will it happen again?

Around 201 million years ago, acidification turned Earth’s oceans from cradles of life into harbingers of extinction.

New research from the University of St Andrews and the University of Birmingham has confirmed for the first time that a sharp and rapid drop in ocean pH directly contributed to a major extinction event. The cause? A massive surge in atmospheric carbon dioxide.

The study represents the first full reconstruction of ancient ocean pH using boron isotopes in fossil oysters. It shows that the acidification during the Triassic-Jurassic boundary was not only severe but prolonged.

Earth’s oceans thrived before the fall

In the seas, early modern corals, ichthyosaurs, ammonites, and bivalves thrived. Coral reefs flourished until the acidification triggered a “reef gap” – a period of reef absence that lasted hundreds of thousands of years. This collapse wasn’t subtle.

It reshaped marine ecosystems, reducing carbonate sedimentation and leaving many marine organisms unable to form shells or skeletons.

Using oysters from Lavernock Point, Wales, researchers detected a 0.29 unit or greater drop in ocean pH. It may have exceeded 0.41 units. This translates to more than a doubling of atmospheric CO₂, with levels climbing over 1300 ppm. For context, today’s CO₂ levels hover near 420 ppm.

Drastic changes, fast consequences

The pH drop happened in tandem with a sharp decline in the carbon isotope ratio (δ¹³C), known as the “main carbon isotope excursion (CIE).”

This points to an enormous carbon release event. The study’s authors estimate that over 10,000 gigatons of carbon may have entered the system, mainly from Earth’s mantle. These emissions likely came from volcanic activity as the supercontinent Pangaea began breaking apart.

“The geological record tells us that major CO₂ release transforms the face of our planet, acidifying the ocean, and causing mass extinction. We have to act fast to avoid these outcomes in our future,” said study co-author Dr. James Rae.

Volcanoes linked to ocean acidification

Using the cGENIE Earth system model, the researchers tested scenarios with different carbon sources. The model confirmed that the likely carbon input was mantle-derived.

The size and speed of the event eliminated methane clathrates as the dominant cause, although they may have contributed. The data also ruled out biomass burning and indicated that volcanic CO₂ and contact metamorphism played the lead roles.

Carbonate saturation dropped drastically. Marine organisms used to high-saturation seas struggled as saturation levels fell below safe thresholds.

Initial drop hit harder

Interestingly, the more catastrophic acidification may have happened earlier, during the “initial CIE”. Though no oyster samples span that exact interval, the data suggest that the initial event was faster and possibly two to three times more intense in terms of pH drop.

This would align with the wider collapse of carbonate sedimentation and widespread death of shell-building marine life.

Earth’s oceans slowly bounce back

Despite the devastation, ocean pH eventually recovered. Around 201.28 million years ago, pH levels began to rise again.

This rebound may have resulted from the emergence of silica-producing organisms. These organisms disrupted a process called reverse weathering.

This process had previously prolonged the high-CO₂, low-pH conditions by locking alkalinity away in clays. Its sudden stop helped restore ocean chemistry.

History warns of a repeat

“The mass extinction event during the Triassic-Jurassic period was over a much longer timeframe, whereas modern ocean acidification is happening at a much quicker rate,” said study co-author Dr. Sarah Greene.

“This warning from the past should give us fresh cause to step up efforts to reduce human greenhouse gas emissions that could otherwise see acidification reach or exceed levels seen during these mass extinction events.”

The data show this is not an isolated case. Ocean acidification has now been confirmed in at least three of Earth’s five mass extinctions. These include the Permian–Triassic, Toarcian, and now the Triassic–Jurassic.

According to the study, the observed pH drops in these events are strikingly similar in size to worst-case predictions for 2100 under IPCC’s RCP8.5 scenario.

Acid oceans could return to Earth

Mass extinctions don’t just come from space rocks or ice ages. Earth can end its own chapters when the carbon cycle breaks down.

The Triassic–Jurassic event tells us that prolonged CO₂ release, even from natural sources, can tip the balance. It can turn oceans hostile and trigger a global loss of life.

Today, the same process is unfolding, only much faster. The past offers no comfort. It offers a clear call to action.

The study is published in the journal Nature Communications.

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