Tulane study finds Southern Ocean plays key role in global ocean oxygen levels

Oxygen and ocean water have a long-established relationship. The warmer oceans become, the less oxygen they are able to hold. When oxygen levels in the ocean, known as dissolved oxygen, decline, marine life can be threatened, and the cycling of carbon and nutrients in the ocean can be disrupted.  

But new research from Tulane University scientists found that, when it comes to declining dissolved oxygen levels, temperature may not tell the whole story.  

In a new study published in Nature Geoscience, Tulane researchers reconstructed global ocean oxygen levels during the Last Interglacial, a warm period about 130,000 to 115,000 years ago that scientists often study for clues about how Earth’s oceans respond to warmer conditions.

They found that ocean oxygen initially increased, reaching levels close to those of today, before declining later in the period despite the climate remaining warm. The study suggests that ocean oxygen can also depend heavily on how efficiently the Southern Ocean, which surrounds Antarctica, carries oxygen from the surface into the deep sea.  

“The main takeaway is that during the Last Interglacial, global ocean oxygen levels did not remain stable,” said lead author Fang Qian, a postdoctoral researcher in Tulane School of Science and Engineering’s Department of Earth and Environmental Sciences. “They initially rose but later declined, even though the climate remained warm.”

The study found that global oxygen changes closely tracked Antarctic temperatures and sea ice. When sea ice retreated, Southern Ocean ventilation strengthened, meaning more oxygen-rich surface water reached the deep ocean. Later, expanding sea ice and weaker ventilation likely reduced that oxygen supply.  

“What we found is that ocean deoxygenation in a warmer period is not only controlled by oxygen solubility in the water,” said co-author Yi Wang, assistant professor in Tulane’s Department of Earth and Environmental Sciences. “What happens far away in the Southern Ocean — including the amount of sea ice around Antarctica — is crucial for deep-water formation, which adds oxygen to the deep ocean as water sinks from the surface.”

The Last Interglacial period was one of the warmest intervals of the past 800,000 years. Mean ocean temperatures were about 1 degree Celsius warmer than today, while global sea level was 6 to 9 meters higher. That makes the period a useful natural laboratory for understanding how ocean oxygen responds during prolonged warm conditions.

To reconstruct those changes, researchers analyzed thallium isotopes preserved in seafloor sediments dating to the Last Interglacial from the Arabian Sea and off Southern California. The chemistry of thallium in seawater is linked to oxygen-sensitive manganese minerals, allowing scientists to use the isotopes as a record of broad changes in global ocean oxygen. Similar patterns at the two distant sites provided evidence that the researchers were detecting a global signal rather than local changes.

The findings add a new piece to scientists’ understanding of what controls oxygen in the global ocean over thousands of years.