The health of the oceans in a future world with increased global warming may be revealed by an analysis of the oxygen concentrations in Earth’s oceans.
Ocean oxygen levels in a crucial region were greater during the Miocene warm period, some 16 million years ago when Earth’s temperature was higher than it is today, according to a Rutgers-led study that examined ocean sediment and was published in Nature.
Ocean oxygen levels have been dropping over the past few decades, which has led to worries that oxygen-deficient zones in some of the world’s oceans would widen and worsen conditions for marine life.
Scientists have linked the pattern to rising temperatures brought on by climate change, which have an impact on how much oxygen can be taken in from the atmosphere.
“Our study shows that the eastern equatorial Pacific, which today is home to the largest oxygen-deficient zone in the oceans, was well oxygenated during the Miocene warm period, despite the fact that global temperatures at that time were higher than at present,” said Anya Hess, the lead author of the study and a Rutgers doctoral student working with Yair Rosenthal, a Distinguished Professor focused on marine and Earth sciences with the Rutgers School of Art and Sciences and the School of Environmental and Biological Sciences.
Hess added: “This suggests that current oxygen loss may ultimately reverse.”
Oxygen-deficient zones have had the fastest rates of oxygen loss recently, and it is anticipated that they will continue to grow and get shallower, endangering fisheries by reducing fish habitat. The researchers decided to conduct further research since climate models forecast different outcomes for these zones after 2100.
Our study shows that the eastern equatorial Pacific, which today is home to the largest oxygen-deficient zone in the oceans, was well oxygenated during the Miocene warm period, despite the fact that global temperatures at that time were higher than at present.
Anya Hess
The mid-Miocene, when climate conditions were comparable to those anticipated for the following few centuries in the current age of climate change, was chosen by researchers to test current climate models. Researchers looked at ocean sediments that were deposited in the eastern equatorial Pacific during the mid-Miocene.
Scientists aboard the National Science Foundation-funded research vessel JOIDES Resolution as part of what is now known as the International Ocean Discovery Program (IODP) recovered the sediments from the seafloor.
Foraminifera are microscopic organisms that dwell in the water column and have fossilized remnants that are the size of a single sand grain. The researchers examined the foraminifera’s chemical makeup, which corresponds to the chemical composition of the ancient ocean. They used isotopes of the element nitrogen that have a distinct relative atomic mass as detectors to determine the oxygen content of prehistoric waters.
The isotopes are sensitive to a process called denitrification that only occurs at very low oxygen levels. They also used an analytical technique that compares calcium and iodine levels and provides nuanced data that can distinguish between circumstances with high and low oxygen levels.
The techniques demonstrated that the region was well-oxygenated during the peak of the Miocene warmth, even approaching present levels seen in the South Pacific’s open ocean.
“These results were unexpected and suggest that the solubility-driven loss of oxygen that has occurred in recent decades is not the end of the story for oxygen’s response to climate change,” Rosenthal said.
Other authors on the study include Ken Miller, a Distinguished Professor in the Department of Earth and Planetary Sciences in the Rutgers School of Arts and Sciences, Alexandra Auderset and Alfredo Martinez-Garcia of the Max Planck Institute for Chemistry in Germany, Daniel Sigman of Princeton University and Xiaoli Zhou of Tongji University in China.
