This article was adapted from .
Researchers have pinpointed the source of oxygen that sustains deep sea life in the North Atlantic Ocean: the churning waters in the Labrador Sea.
The study, , gives heightened significance to the Labrador Sea, sandwiched between Greenland and Newfoundland. There, waters from the Atlantic Meridional Overturning Circulation (AMOC), the ocean’s major current system, turn in a gyre, and oxygen-rich surface waters mix with deeper waters. Previous research found that the Labrador Sea has little impact on the strength of AMOC, but the new study shows it plays a critical role in oxygen transport.
The research also sheds light on processes that help the North Atlantic maintain its oxygen levels, while oxygen declines in oceans globally due to warming temperatures.
“This is the first work that directly observes dissolved oxygen transport in the Labrador Sea, a critical region with active deep convection to ventilate the deep North Atlantic Ocean,” said Yao Fu, assistant professor at the USF College of Marine Science and co-author of the paper. “This is a critical step toward a more comprehensive understanding of the role of ocean circulation in driving deep ocean oxygen ventilation.”
First author , assistant professor of earth and atmospheric sciences in Cornell’s College of Agriculture and Life Sciences, explained the results: “We found that the Labrador Sea exports enough oxygen to meet the biological need across a vast part of the deep North Atlantic Ocean, so it’s very likely crucial to sustain these deep sea ecosystems. Our finding shows that if we’re going to understand the future, especially in the face of these deoxygenation trends, you can’t just look at the strength of AMOC, you also have to understand processes in the Labrador Sea.”
Investigating AMOC at a critical time
The study comes amid debate about the vulnerability of AMOC, as the current has weakened over the last 75 years. AMOC carries warm water from the tropics to the North Atlantic and carbon dioxide and oxygen throughout the deep sea; the movement of warmer waters results in a more temperate Europe, and the gases sustain life and store carbon. Scientists have warned that a collapse of the system could cause major disruption in weather and devastate ecosystems.
A large team of researchers contributed to the study, using data from 60 oxygen sensors attached for the first time to moorings that run along the bottom of the Labrador and western Irminger seas.
“This work relies on the accurate measurement and production of the velocity, temperature, and salinity fields to calculate the dissolved oxygen transport in the Labrador Sea,” Fu said. “My involvement was primarily producing the velocity, temperature, and salinity fields along the Overturning in the Subpolar North Atlantic Program (OSNAP) West section and ensure the application of these data in the oxygen transport calculation was appropriate.”
Senior author from the University of Rhode Island described the challenges researchers overcame: “No one’s successfully sustained multiple years of oxygen measurements on moorings like these before, so that was one breakthrough, along with a machine learning method to fill in gaps so we could map these oxygen values. Now we know the rate of oxygenation, we know the processes, and we can link it with other work to show that the current needs to take this last step in the Labrador Sea in order for ecosystems to function.”
Oxygen is hard to come by in the deep ocean, Palter said. Layers of ocean water, at different temperatures and densities, largely don’t mix. She described the Atlantic as having a lid on it, which means oxygen entering from the air largely stays in the surface layer. But when currents circulate into the subpolar North Atlantic and the Labrador Sea, they become colder and denser — and they sink, carrying oxygen and carbon.
“That becomes the lower limb of AMOC, which spreads through the deep interior of the Atlantic Ocean,” Miller said. “In terms of gases, that’s really important, because there’s no photosynthesis below a certain depth — the only atmospheric oxygen in the deep ocean is really from this overturning circulation, this injection of waters that were at the surface and flowed through the Labrador Sea.”
The researchers were able to quantify the amount of oxygen the Labrador Sea waters carry: more than 27 teramoles per year, enough oxygen to sustain breathing for every person on earth for at least two months. The team found that the amount of oxygen matches estimates of the respiration rates of microbes and animals across the North Atlantic deep sea. The correlation strongly suggests deep sea life relies on the Labrador Sea, which is one of very few regions where this mixing of waters occurs.
“Animals really suffer when oxygen dips below a certain threshold,” Palter said. “The supply of oxygen from these processes balances the oxygen consumption over pretty much the whole deep North Atlantic.”
This map shows the path of oxygen transport from the Labrador Sea. Credit: Laila Milevski/Cornell University
The researchers said the study, like much of oceanography, was a game of patience — after installing the sensors in 2020, the team left them on the moorings for two years, not knowing whether they would even survive the deep sea environment.
The researchers said many questions remain about the relationship between the strength of AMOC and oxygenation processes, and what would happen if one or both were to weaken.
Additional co-authors include , , and from the Woods Hole Oceanographic Institution; from Dalhousie University; , and from Boston College; from the GEOMAR Helmholtz Centre for Ocean Research; and .
Funding for the study came from the , the , the Canada Excellence Chair in Ocean Science and Technology and the .