Ancient deposits on the ocean floor could offer a clue as to how the seas will respond to global warming, scientists believe.
The deposits could also show whether oceans will release greenhouse gases from carbon stored within deep waters.
Researchers in the US analysed ocean oxygen level and its connections with carbon dioxide (CO2) in the Earth's atmosphere during the last ice age, which ended more than 11,000 years ago. They said their findings could offer an insight into how the oceans will respond as the world warms.
Oceans adjust atmospheric CO2 as ice ages transition to warmer climates by releasing carbon – and the researchers warned that carbon release from the deep sea may rise as the climate warms.
"The research reveals the important role of the Southern Ocean in controlling the global ocean oxygen reservoir and carbon storage," said Yi Wang, lead researcher and an assistant professor of earth and environmental sciences at Tulane University School of Science and Engineering in New Orleans. "This will have implications for understanding how the ocean, especially the Southern Ocean, will dynamically affect the atmospheric CO2 in the future."
How did they do it?
The team analysed sea floor sediments collected from the Arabian Sea to reconstruct average global ocean oxygen levels thousands of years ago. They precisely measured isotopes of the metal thallium trapped in the sediments, which indicate how much oxygen was dissolved in the global ocean at the time the sediments formed.
"Study of these metal isotopes on glacial-interglacial transitions has never been looked at before, and these measurements allowed us to essentially recreate the past," said Wang, who specialises in marine biogeochemistry and paleoceanography.
The thallium isotope ratios showed the global ocean lost oxygen overall during the last ice age compared to the current warmer interglacial period.
The study revealed a thousand-year global ocean deoxygenation during abrupt warming in the Northern Hemisphere. The ocean gained more oxygen when abrupt cooling occurred during the transition from the last ice age to today.
What could this mean for climate change?
The oceans release carbon during warmer periods, the researchers said, and it could mean that the Southern Ocean releases more carbon as the world warms.
The researchers attributed the observed ocean oxygen changes to Southern Ocean processes.
Sune Nielsen, associate scientist at Woods Hole Oceanographic Institution (WHOI) in Massachusetts and co-author of the research, said: "This study is the first to present an average picture of how the oxygen content of the global oceans evolved as Earth transitioned from the last glacial period into the warmer climate of the last 10,000 years.
"These new data are a really big deal because they show that the Southern Ocean plays a critical role in modulating atmospheric CO2. Given that high latitude regions are those most affected by anthropogenic climate change, it is troubling that these also have an outsize impact on atmospheric CO2 in the first place."