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, published in Nature Geoscience on Aug. 17, 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 had found that the Labrador Sea has little impact on the strength of AMOC, but the study finds it plays a critical role in oxygen transport.

Credit: Laila Milevski/Cornell University

The research also sheds light on processes that may be helping the North Atlantic maintain its oxygen levels, as oxygen declines in oceans globally due to warming temperatures.

“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,” said first author Una Miller, assistant professor of earth and atmospheric sciences in the College of Agriculture and Life Sciences. “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.

Miller, working with a large team of researchers including senior author Jaime Palter at the University of Rhode Island, used data from 60 oxygen sensors attached for the first time to moorings that run along the bottom of the Labrador and western Irminger seas.

“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,” Palter said. “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.”

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.

“Then there was a multiyear process to just figure out how to use the data, because it was messy,” Palter said. “Una did so much of that work to develop methods to make this possible.”

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. Miller is continuing to study oxygenation in the Southern Ocean, around Antarctica, another critical region where the surface ocean connects to the deep ocean.

“It’s been really fun to think on such a big scale and to work with such a large system with such obvious impacts and importance,” she said.

Additional co-authors include Ellen Park, Isabela Le Bras, Hiroki Nagao and David Nicholson from the Woods Hole Oceanographic Institution; Dariia Atmanchuk from Dalhousie University; Kristen Fogaren, Hilary Palevsky and Meg Yoder from Boston College; Yao Fu from the University of South Florida; Johannes Karstensen from the GEOMAR Helmholtz Centre for Ocean Research; and Jannes Koelling from the University of Washington.

Funding for the study came from the National Science Foundation, the National Oceanic and Atmospheric Administration, the Canada Excellence Chair in Ocean Science and Technology and the Canada First Research Excellence Fund.