- The Arabian Sea contained more oxygen about 16 million years ago than it does today, even though Earth’s climate was warmer at the time.
- Powerful monsoons, shifting ocean currents, and connections between seas strongly influence oxygen levels, showing that ocean health depends on more than temperature alone.
- Over very long timescales, oxygen levels in the oceans could increase again, although what that would mean for marine life remains uncertain.
Ancient Oceans May Hold Clues to Future Oxygen Recovery
A new study suggests that parts of the world’s oxygen depleted oceans could regain higher oxygen levels in the centuries ahead, even as global temperatures continue to rise.
Scientists from the University of Southampton (UK) and Rutgers University (USA) analyzed fossilized plankton preserved in sediments from the Arabian Sea. Their results show that during a period of intense global warming about 16 million years ago, ocean oxygen levels in this region were actually higher than they are today. Severe oxygen depletion did not emerge until roughly four million years later, after the climate began to cool.
Why the Arabian Sea Behaved Differently
The researchers also found that the Arabian Sea, located off India’s west coast, followed a different path than a comparable low oxygen region in the Pacific Ocean. This contrast points to the importance of regional influences, including strong monsoon winds, ocean circulation patterns, and water exchange from nearby seas. These local factors appear to have slowed the loss of oxygen in the Arabian Sea.
The findings were published in the Nature journal Communications Earth & Environment.
Oxygen Loss Is Already Underway Today
“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.”
Fossil Plankton Reveal Long Term Oxygen History
To reconstruct ancient ocean conditions, the team studied microscopic fossilized plankton known as foraminifera (forams). These fossils were collected from sediment cores provided by the Ocean Drilling Program (ODP). Chemical signals preserved in the shells of these organisms allow scientists to estimate oxygen levels in seawater across millions of years.
The analysis showed that an Oxygen Minimum Zone (OMZ) was present in the Arabian Sea from the early Miocene, about 19 million years ago, until roughly 12 million years ago. During this time, oxygen concentrations stayed below around 100 micromol per kilogram of water.
Delayed Onset of Severe Oxygen Depletion
Despite these low oxygen levels, conditions were not extreme enough to trigger the release of nitrogen from seawater into the atmosphere, a process that occurs in the Arabian Sea today. That shift did not happen until after 12 million years ago, indicating that the most severe oxygen loss was delayed.
“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.
Regional Ocean Forces Shape Oxygen Outcomes
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.”
Why Future Ocean Predictions Are So Complex
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.”
