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Biogeochemistry

Deep Breathing Underwater

Atamanchuk, D., Koelling, J., Send, U., Wallace, D. Rapid transfer of oxygen to the deep ocean mediated by bubbles. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0532-2

 

The Labrador Sea is one of the lungs of the ocean, breathing in oxygen that supports underwater life across the planet.  A new study finds that climate models drastically underestimate how much oxygen it absorbs—and reveals that the deep ocean is even more vulnerable to climate change than thought.

Birds-eye view of the Labrador Sea. Source: Lauren Dauphin, NASA Earth Observatory

 

Ocean Lungs and Arteries

With every breath you take, you can thank the ocean.  Marine plants such as phytoplankton, algal plankton, and kelp produce up to 70% of the oxygen in the atmosphere.  They also absorb vast quantities of carbon dioxide, slowing the buildup of this greenhouse gas in the atmosphere and—at least temporarily—reducing the impacts of climate change.

One of the ocean’s lungs is the Labrador Sea.  This sea, which comprises the left arm of the North Atlantic Ocean, is one of the few places where the deep ocean exchanges gases directly with the atmosphere.  Here, oxygen-rich water at the surface becomes so dense that it can sink up to two kilometers, where powerful currents transport it around the globe.  This system of arteries allows oxygen breathed in by the Labrador Sea to support sea life up to thousands of kilometers away.

 

The Case of the Missing Oxygen

The ocean exchanges oxygen and carbon dioxide with the atmosphere. Source: VITALS

Dissolved oxygen concentrations in the ocean are controlled by biological processes and the exchange of gases between the sea’s surface and the air above it.  Over the past fifty years, the ocean’s inventory of oxygen has decreased by 2%.  Only about 15% of this deoxygenation can be attributed to ocean warming, which decreases the amount of gas it can hold—the same reason why your warm soda tastes flat.  This begs the question of where on earth (or rather, in the ocean) the remaining oxygen is disappearing to.

To tackle such a tricky question, scientists turn to climate models.  Such models allow us to pare down the complexity of nature and simulate important processes.  Scientists construct models based on the laws and equations of physics, chemistry, and biology (i.e., lots and lots of math).  For processes we don’t yet understand well, scientists rely on “parameterizations”, which are approximations of each process.  Parameterizations are one of the main sources of uncertainty in climate models.

However, our climate models are unable to account for the missing oxygen.  In some regions—particularly high-latitude seas such as the Labrador Sea—model predictions of how much oxygen the ocean absorbs are much lower than observed.  Theories abounded for why this might be the case.  In this study, the scientists suspected that the parameterization of gas exchange was missing a key component.

Deployment of the SeaCycler in May 2016 in the central Labrador Sea. Source: Kat Fupsova

 

Enter the SeaCycler

To test their hunch, researchers from Dalhousie University (Nova Scotia) and the Scripps Institution of Oceanography (California) teamed up to develop a novel sensor system called the SeaCycler.  Costing $1 million, it is the only platform of its kind in the world.  In May 2016, the scientists deployed the SeaCycler at the bottom of the Labrador Sea, 150 meters underwater.  The platform contained an instrument float, which measured key parameters such as oxygen and carbon dioxide as it ascended to the surface every twenty hours.  Meanwhile, a separate communication float transmitted data to a satellite.  Over the course of its yearlong deployment, the SeaCycler made millions of measurements as it quietly listened to the sea breathe.

The SeaCycler’s results were astonishing.  Its data revealed that the Labrador Sea is absorbing 10 times more oxygen than estimated by previous climate models.  Furthermore, two-thirds of this oxygen is absorbed during just 40 days in winter.  The scientists explained this phenomenon as a seasonal “trap door”, which opens during winter when intense storms inject air bubbles into the sea—sort of like a large-scale SodaStream.  Previous gas exchange parameterizations hadn’t included this bubble-injection mechanism, explaining their large underestimations.

 

Asthma Attack

These findings highlight the vulnerability of ocean ecosystems to climate change.  As human-induced ocean warming causes our ice sheets to melt, more and more fresh water is being added to the Labrador Sea.  Because this water is less dense than seawater, the surface water won’t sink as much as it normally does—and as lead author Dariia Atamanchuk states, the ocean’s “breathing becomes shallower”.  This spells trouble for fish and other creatures that require oxygen to survive in the deep sea, and could also impact ocean nutrient cycles and marine habitats.  In turn, we would feel the impacts on our fisheries and coastal economies.

While alarming, we don’t have to worry that the ocean’s lungs will fail tomorrow—it takes hundreds of years for water formed in the Labrador Sea meander throughout the ocean basins.  Still, as Atamanchuck states, “It is important that we get all these processes right and to predict what will happen in the future with the deep-sea ecosystems.”

Climate change threatens ocean ecosystems. Source: Carbon Brief

 

Breathe In, Breathe Out

There are two morals to the SeaCycler story.  First, obtaining long-term measurements is imperative to understanding the complex ocean—scientists were only able to capture the trap-door effect through vigilant monitoring of the Labrador Sea over a year.  Second, we need innovative ocean sensors to act as our eyes and ears in the ocean, particularly in harsh regions like the northern seas.  No ship-based mission could have come close to the amount of data collected by the SeaCycler.

Based on the success of the first mission, the team is already hard at work refurbishing the SeaCycler and building a second platform.  Armed with even more sensors, these platforms will be redeployed in the Labrador Sea this September.  The scientists’ ultimate goal is to establish a permanent underwater observatory there so we can monitor the ocean’s health, one breath at a time.  ■

 

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