Loss of oxygen in ocean caused mass extinction of marine life

US and Italian researchers have discovered a clue in limestone that helps explain a mass extinction of marine life around 183 million years ago and may provide warnings about how oxygen depletion and climate change could impact today’s oceans. Their work has been published in the journal PNAS.

During the Jurassic Period, when marine reptiles like ichthyosaurs and plesiosaurs thrived, volcanic activity in modern South Africa released an estimated 20,500 gigatons of carbon dioxide (CO₂) over 500,000 years. This heated the oceans, causing them to lose oxygen. The result was the suffocation and mass extinction of marine species.

“It’s an analogue, but not a perfect one, to predict what will happen to future oxygen loss in oceans from human-made carbon emissions and the impact that loss will have on marine ecosystems and biodiversity,” said study co-author Mariano Remírez, an assistant research professor at George Mason University.

Studying limestone sediment that carries chemicals dating back to the time of the volcanic outburst, the researchers were able to estimate the change in oxygen levels in ancient oceans. At one point, oxygen was completely depleted in up to 8% of the ancient global seafloor — an area roughly three times the size of the United States.

“This event, and events like it, are the best analogues we have in Earth’s past for what is to come in the next decades and centuries,” said study co-author Michael A Kipp, an assistant professor at Duke University.

Since the Industrial Revolution began in the 18th and 19th centuries, human activity has released CO₂ emissions equivalent to 12% of what was released during the Jurassic volcanism. But Kipp said that today’s rapid rate of atmospheric CO₂ release is unprecedented in history, making it hard to predict when another mass extinction might occur or how severe it might be.

“We just don’t have anything this severe,” Kipp said. “We go to the most rapid CO₂-emitting events we can in history, and they’re still not rapid enough to be a perfect comparison to what we’re going through today. We’re perturbing the system faster than ever before.

“We have at least quantified the marine oxygen loss during this event, which will help constrain our predictions of what will happen in the future.”

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