Image Credit: Laila Milevski/Cornell University
Scientific Frontline: Extended "At a Glance" Summary: Deep-Sea Oxygenation in the Labrador Sea
The Core Concept: The Labrador Sea acts as a crucial "lung" for the deep North Atlantic, mixing oxygen-rich surface waters with deeper currents to sustain deep-sea ecosystems.
Key Distinction/Mechanism: Unlike most of the ocean, where layers of water at different temperatures and densities remain separate (keeping oxygen trapped near the surface), the subpolar North Atlantic and Labrador Sea cool and densify the currents. This cooling causes the oxygen-rich surface waters to sink, injecting essential oxygen into the deep-sea environment.
Major Frameworks/Components:
- Atlantic Meridional Overturning Circulation (AMOC): The major ocean current system that carries warm water from the tropics to the North Atlantic and distributes oxygen and carbon dioxide throughout the deep sea. The sinking water in the Labrador Sea forms the lower limb of this circulation.
- Gyre Mixing: The churning motion of the AMOC in the Labrador Sea facilitates the crucial mixing of oxygenated surface water with the oxygen-depleted deep water.
- Respiration Correlation: The estimated 27 teramoles of oxygen exported annually by the Labrador Sea closely aligns with the estimated respiration rates of microbes and animals in the deep North Atlantic.
Branch of Science: Oceanography, Marine Biology, Climatology, Earth Science, and Atmospheric Sciences.
Future Application: Understanding the specific mechanics of the Labrador Sea's oxygen export provides a critical metric for predicting the health of deep-sea ecosystems and the broader impacts of global ocean deoxygenation linked to climate change.
Why It Matters: While previous research indicated the Labrador Sea had little influence on the overall strength of the AMOC, this study proves it is the primary engine for oxygenating the deep North Atlantic. As global ocean temperatures rise and oxygen levels decline, identifying and monitoring this specific oxygenation pathway is vital for anticipating how deep-sea life will fare if the AMOC continues to weaken.
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 August 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 found that the Labrador Sea has little impact on the strength of the AMOC, but the study finds it plays a critical role in oxygen transport.
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 rising 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 the AMOC, you also have to understand processes in the Labrador Sea.”
Investigating the AMOC at a Critical Time
The study comes amid debate about the vulnerability of the AMOC, as the current has weakened over the last 75 years. The 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 disruptions 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 the 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 the 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.
Funding: 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.
Published in journal: Nature Geoscience
Title: Central role of Labrador Sea convection for transport of oxygen into the deep North Atlantic Ocean
Authors: Una Kim Miller, Jaime Palter, Ellen Park, Dariia Atamanchuk, Kristen Fogaren, Yao Fu, Johannes Karstensen, Jannes Koelling, Isabela Le Bras, Hiroki Nagao, David Nicholson, Hilary Palevsky, and Meg Yoder
Source/Credit: Cornell University | Caitlin Hayes
Edited by: Scientific Frontline
Reference Number: es081726_01