A team of scientists led by a Tulane University oceanographer has discovered that sediments deep under the sea reveal a way to measure ocean oxygen content and its connection to carbon dioxide in Earth's atmosphere. The findings, published Jan. 19 in the journal Science Advances, help explain the ocean's role in past glacier melt cycles and could improve predictions of how the ocean's carbon cycle responds to global warming.
As ice ages transition into a warmer climate, the oceans regulate atmospheric carbon dioxide by releasing carbon stored in the deep sea. The study shows a surprising correlation between global ocean oxygen levels and atmospheric carbon dioxide from the last ice age to today - and as the climate warms, the release of carbon from the deep ocean is likely to increase.
"This study reveals the important role of the Southern Ocean in controlling global ocean oxygen pools and carbon storage," said lead researcher Yi Wang, assistant professor of earth and environmental sciences in Tulane's College of Science and Engineering. She specializes in marine biogeochemistry and paleoceanography. "This will have implications for understanding how the ocean, particularly the Southern Ocean, will dynamically affect atmospheric carbon dioxide in the future."
Wang conducted the research with colleagues at the Woods Hole Oceanographic Institution, the world's leading independent nonprofit organization dedicated to ocean research, exploration and education. She worked at the Institute before joining Tulane University in 2023.
The team analyzed seafloor sediments collected from the Arabian Sea to reconstruct the average oxygen content of the global ocean thousands of years ago. They precisely measured isotopes of the metal thallium in the sediments, which showed how much oxygen was dissolved in the global ocean when the sediments were formed.
"The study of these metal isotopes during glacial-interglacial transitions has never been done before, and these measurements allow us to essentially reconstruct the past," Wang said.
Thallium isotope ratios show that the global ocean lost oxygen overall during the last glacial period compared with the current, warmer interglacial period. Their research shows that the world's oceans experienced a millennium-long deoxygenation during a sudden warming in the northern hemisphere, and that the oceans gained more oxygen during a sudden cooling that transitioned from the last ice age to today. The researchers attributed the observed changes in ocean oxygen to the evolution of the Southern Ocean.
"This study is the first to show the average evolution of global ocean oxygen content from Earth's transition from the last ice age to the warming period of the past 10,000 years," said Sune Nielsen, associate scientist at the World Health Organization's Institute for Science and co-author of the study. "These new data are indeed significant because they show that the Southern Ocean plays a crucial role in regulating atmospheric carbon dioxide. Given that high latitudes are the areas most affected by anthropogenic climate change, it is troubling that these regions also have a large impact on atmospheric carbon dioxide in the first place."