Main findings

• The Arabian Sea was richer in oxygen 16 million years ago than it is today, despite warmer climate conditions.
• Monsoons, ocean circulation, and ocean gateways play important roles and add complexity when trying to predict future ocean oxygenation.
• In the very long term, future ocean oxygenation may improve, but the impact on marine biology is unknown.

A new study suggests that the world’s oxygen-depleted oceans could return to high oxygen concentrations in the coming centuries, even as the climate continues to warm.

Researchers from the University of Southampton (UK) and Rutgers University (USA) examined plankton fossils in the Arabian Sea and found that despite dramatic global warming 16 million years ago, oxygen levels were higher than they are today. It wasn’t until four million years later, when the climate cooled, that the oceans became truly oxygen-starved.

The researchers also found that this region off the west coast of India behaved differently from similar low-oxygen regions in the Pacific Ocean, suggesting that other regional systems, such as strong winds, ocean currents, and outflow from marginal oceans, may have slowed the process.

Scientists’ discoveries are published in Nature magazine Communication Earth and Environment.

“Dissolved oxygen in our oceans is essential for sustaining marine life and promoting greater biodiversity and stronger ecosystems. But over the past 50 years, as global temperatures have risen, 2 per cent of the oxygen in the world’s oceans has been lost every 10 years,” explains co-author Dr Alexandra Orderset from the University of Southampton.

She added: “The Miocene climate optimum (MCO), about 17 to 14 million years ago, had temperatures and atmospheric conditions similar to those we predict will occur after 2100. We took a snapshot of ocean oxygenation during the MCO to understand how things will develop more than 100 years from now.”

Scientists examined tiny fossil plankton called foraminifera extracted from core samples provided by the Ocean Drilling Program (ODP). The remains of these organisms contain important chemical information that can indicate the concentration of oxygen in seawater over millions of years.

Researchers found that from the early Miocene (19 million years ago) until about 12 million years ago, an oxygen minimum zone (OMZ) existed in the Arabian Sea, with oxygen concentrations below about 100 micromoles per kilogram of water.

However, oxygen levels at this time were not low enough to cause the process of nitrogen being released from the water into the atmosphere (a condition observed in the Arabian Sea today). Rather, this process was delayed and did not occur until after 12 million years.

“Currently, parts of the Arabian Sea are ‘suboxic’, with minimal oxygenation and a limited number of marine species. The same area during the MCO was hypoxic under similar climate conditions, so the oxygen content is relatively moderate and a wider range of species is present,” Dr. Orderset said.

Co-author Dr Anya Hess of George Mason University (formerly of Rutgers University and Woods Hole Oceanographic Institution) added: “The MCO is the closest comparison to climate warming since 2100 under high emissions scenarios. One of our previous studies shows that the eastern tropical Pacific was actually well oxygenated during this period, in contrast to the deoxygenation trends seen today.”

“During the MCO period, the Arabian Sea was also more oxygen-rich, but not as much as the Pacific Ocean, with moderate oxygen and eventual decline about 2 million years later than in the Pacific Ocean.”

Dr. Orderset concluded: “Our results suggest that the already ongoing ocean oxygen loss is strongly shaped by regional oceanography. Global models that focus solely on climate warming run the risk of failing to capture regional factors that may amplify or offset more general trends.”

“Our study shows that the ocean’s response to a warming climate is complex. This means we need to be ready to adapt to changing ocean conditions.”

end

Note to editor

1. The paper “Contrasting evolution of oxygen minimum zones in the Arabian Sea and Pacific Ocean during the Miocene” was published in the journal Communication Earth and Environment: https://www.nature.com/articles/s43247-025-03112-4
2. For interviews or further information, please contact Peter Franklin, University of Southampton Media Manager. press@soton.ac.uk +44 23 8059 3212
3. Download image here: https://safesend.soton.ac.uk/pickup?claimID=MPwt2BeKMrpdqqe4&claimPasscode=ptdNzvynfPSYQvHj&emailAddr=259933
4. Find out more about Marine and Earth Sciences at the University of Southampton. https://www.southampton.ac.uk/about/faculties-schools-Departments/school-of-ocean-and-earth-science
5. The University of Southampton promotes original thinking, turning knowledge into action and impact, and creating solutions to the world’s challenges. We are included in the top 100 institutions in the world (QS World University Rankings 2026). Our academics are leaders in their fields, forge connections with high-profile international companies and organizations, and inspire a 25,000-strong community of high-achieving students from more than 135 countries around the world. Through quality education, the University supports students on a journey of discovery to realize their potential and join a global network of more than 300,000 alumni. www.southampton.ac.uk
6. For more information about Rutgers University, please see below. https://www.rutgers.edu/

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