Researchers have discovered a key clue in Italian limestone that sheds light on the mass extinction of marine life millions of years ago. The discovery could also shed light on how current ocean conditions may be affected by oxygen depletion and climate change.

“This event and events like it are the best analogs in Earth’s past for things to come in the coming decades and centuries,” said Michael A. Kipp, an assistant professor of earth and climate sciences at Duke University. Kipp is a co-author of a study published June 24 in the journal Nature. Proceedings of the National Academy of SciencesA measurement of the loss of oxygen in the oceans that caused the extinction of marine species 183 million years ago.

During the Jurassic period, when marine reptiles such as ichthyosaurs and plesiosaurs evolved, volcanic activity in what is now South Africa released an estimated 20,500 gigatons of carbon dioxide (CO2) over 500,000 years. This warmed the oceans, causing them to lose oxygen.

The result was the drowning and mass extinction of marine species.

“This is an analog for predicting what will happen to future ocean oxygen losses due to anthropogenic carbon emissions and the impact of those losses on marine ecosystems and biodiversity, but it is not ideal,” said co-author Mariano Ramírez, an Assistant Research Professor at George Mason University.

Estimating Ancient Oxygen Levels

By studying limestone sediments containing chemicals from the time of the volcanic eruption, the researchers were able to estimate changes in oxygen levels in ancient oceans. At one point, 8% of the ancient global seafloor, an area about three times the size of the United States, was completely depleted of oxygen.

Since the beginning of the Industrial Revolution in the 18th and 19th centuries, human activities have resulted in CO2 emissions equivalent to 12% of that emitted during the Jurassic volcanism. But Kipp said the rate of CO2 released into the atmosphere today is unprecedented, making it difficult to predict when another mass extinction might occur or how severe it might be.

“We don’t have anything that serious,” Kipp said. “We are moving toward the fastest CO2 emissions events in history, but they are still not fast enough to compare perfectly with what we are experiencing today. We are breaking down the system faster than ever.”

“At least we measured the loss of oxygen in the sea during this event, which will help constrain our predictions of what will happen in the future,” Kipp said.