Fossilized plankton study gives long-term hope for oxygen depleted oceans

University of Southampton

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Scanning electron image of fossilised planktonic foraminifera.

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Credit: Anya Hess

Key findings

• Arabian Sea was better oxygenated 16 million years ago than it is today, despite warmer climate conditions.

• Monsoons, ocean circulation, and ocean gateways play an important role, adding complexity as we try to predict future ocean oxygenation.

• In the very long-term, future sea oxygenation may improve, with unknown consequences for marine biology.

A new study suggests the world’s oxygen depleted seas may have a chance of returning to higher oxygen concentrations in the centuries to come, despite our increasingly warming climate.

Researchers at the University of Southampton (UK) and Rutgers University (USA) examined fossilised plankton from the Arabian Sea and found that despite dramatic global warming 16 million years ago, oxygen levels were higher than today. The sea only became truly oxygen deficient four million years later, as the climate cooled.

In addition, the team revealed the region, off the west coast of India, behaved differently than a similar low-oxygen area in the Pacific, suggesting other local systems, such as strong winds, ocean currents, and outflow from marginal seas may have delayed the process.

The scientists’ findings are published in the Nature journal Communications Earth & Environment.

“Oxygen dissolved in our oceans is essential for sustaining marine life, promoting greater biodiversity and stronger ecosystems. However, over the past 50 years, two percent of oxygen in the seas worldwide has been lost each decade as global temperatures rise,” explains co-lead author, Dr Alexandra Auderset of the University of Southampton and formerly of Max Planck Institute of Chemistry, Mainz.

She adds: “The Miocene Climatic Optimum (MCO), a period approximately 17 to 14 million years ago, had similar temperatures and atmospheric conditions to those we predict will occur after 2100. We have taken a snapshot of sea oxygenation during the MCO to help understand how things might develop a-hundred years or more from now.”

The scientists examined tiny fossilised plankton called foraminifera (forams) extracted from core samples provided by the Ocean Drilling Program (ODP). The remains of these creatures hold important chemical information that can indicate oxygen concentrations in sea water over millions of years.

The researchers found that an Oxygen Minimum Zone (OMZ) existed in the Arabian Sea, with oxygen levels below about 100 micromol per kilogram of water, from the early Miocene (19 million years ago) to around 12 million years ago.

However, the oxygen levels at this time were not so low as to trigger a process where nitrogen is expelled from the water and into the atmosphere – a state that is observed nowadays in the Arabian Sea. Rather, this process was delayed and did not  occur until later, after the 12 million year mark and beyond.

“Today parts of the Arabian sea are ‘suboxic’, supporting only limited marine life due to minimal oxygenation. This same region during the MCO, under similar climatic conditions, was hypoxic – so comparatively moderate oxygen content, supporting a wider range of organisms,” says Dr. Auderset.

Co-lead-author, Dr Anya Hess of George Mason University, and formerly of Rutgers University and Woods Hole Oceanographic Institution, adds: “The MCO is the closest comparison we have to climate warming beyond 2100 under a high-emissions scenario. One of our previous studies shows the eastern tropical Pacific was actually well oxygenated during this period, in contrast to the deoxygenation trend we see today.

“The Arabian Sea was also better oxygenated during the MCO, but not as much as the Pacific, with moderate oxygenation and an eventual decline that lagged behind the Pacific by about 2 million years.”

Dr Auderset concludes: “Our results suggest that ocean oxygen loss, already underway today, is strongly shaped by local oceanography. Global models that focus solely on climate warming, risk not capturing the regional factors that may either amplify or counteract those more general trends.

“Our research shows ocean response to climate warming is complex, and this means that we will need to be ready to adapt to changing ocean conditions.”

Ends

Notes to editors

1. The paper ‘Contrasting evolution of the Arabian Sea and Pacific Ocean oxygen minimum zones during the Miocene’ is published in the journal Communications Earth & Environment: https://www.nature.com/articles/s43247-025-03112-4
    
2. For interview or further info contact Peter Franklin, Media Manager, University of Southampton. press@soton.ac.uk +44 23 8059 3212
    
3. Download images here: https://safesend.soton.ac.uk/pickup?claimID=MPwt2BeKMrpdqqe4&claimPasscode=ptdNzvynfPSYQvHj&emailAddr=259933
    
4. More about Ocean and Earth Science at the University of Southampton can be found here: https://www.southampton.ac.uk/about/faculties-schools-departments/school-of-ocean-and-earth-science
    
5. The University of Southampton drives original thinking, turns knowledge into action and impact, and creates solutions to the world’s challenges. We are among the top 100 institutions globally (QS World University Rankings 2026). Our academics are leaders in their fields, forging links with high-profile international businesses and organisations, and inspiring a 25,000-strong community of exceptional students, from over 135 countries worldwide. Through our high-quality education, the University helps students on a journey of discovery to realise their potential and join our global network of over 300,000 alumni. www.southampton.ac.uk
    
6. For more about Rutgers University visit: https://www.rutgers.edu/

Alexandra Auderset and Alfredo Martinez-Garcia in laboratory at Max Planck Institute for Chemistry.

Credit

Simone Moretti

Graphic showing modern (top) and MCO (bottom) oxygenation.

Credit

University of Southampton

Journal

Communications Earth & Environment

DOI

10.1038/s43247-025-03112-4 

Method of Research

Computational simulation/modeling

Subject of Research

Not applicable

Article Title

Contrasting evolution of the Arabian Sea and Pacific Ocean oxygen minimum zones during the Miocene

Article Publication Date

26-Jan-2026