The oceans play a crucial role in slowing global warming. So far, they have diverted to their abyss a third of CO emissions2 of humanity. However, the mechanisms that regulate this immense carbon vacuum still hold many secrets for scientists.
On a Cape Cod beach, not far from the Woods Hole Institute of Oceanography (WHOI), a flock of cormorants and seagulls skilfully dive into the water and emerge with fish in their beaks. The tides deposit horseshoe crabs, brown arthropods straight out of prehistory, on the shore.
The upper layers of the ocean are full of life. The sun allows the growth of microscopic algae that absorb CO2 dissolved in water. These algae are at the base of the food chain. When the animals that eat them die or defecate, the residues – very rich in carbon – sink to the bottom of the seas.
These particles are nicknamed “marine snow”, underlines Ken Buesseler, a chemical researcher who welcomes The duty in his WHOI lab. While delivering his explanations, Mr. Buesseler wiggles his fingers to imitate soft falling snowflakes twirling. Marine snowfall is the most important vector of carbon from the surface to the depths of the ocean.
Note, however, that not all marine snow reaches the bottom of the sea. The majority of the flakes are the delight of the inhabitants of the “twilight layer” (” twilight zone in English), which extends from about 100 to 1000 meters deep. Fish, zooplankton, crustaceans and squid intercept it and feed on it.
The greatest migration on the planet
The animals of the twilight layer influence carbon fluxes in a second way: every night, they visit the surface of the ocean to eat algae there, sheltered from predators thanks to the darkness. This is, according to biologists, the greatest migration on the planet in terms of biomass. And this rapid ascent, on the back of a shrimp, constitutes a second very important vector of carbon towards the abyss.
Through these two mechanisms, billions of tons of carbon are transported each year to the bottom of the seas, where they remain isolated from the atmosphere for centuries. These phenomena were already occurring naturally before humans emitted greenhouse gases, but they have now shifted into high gear.
However, it is difficult to know whether, with climate change or overfishing, the carbon cycle in the seas will change. “I’ve been working on these questions for 35 years, but it’s very hard to tell because of the many pathways that carbon can take in the ocean,” says Buesseler.
Carbon fluxes vary greatly depending on geography, temperature, season, etc. In order to explain these variations, Mr. Buesseler’s team wants to multiply the measurements of marine snow flow in different environments. To achieve this, it has been relying on underwater cameras for several years.
“With a camera, we have information about what is happening inside the ocean 24 hours a day, seven days a week,” enthuses Elena Ceballos Romero, a WHOI postdoctoral researcher, interviewed by video since Europe, where she took part in scientific conferences.
Last August, Mr.me Ceballos Romero boarded the RV Endeavor, a research vessel, which sailed in the Northwest Atlantic to carry out several scientific missions on the twilight layer. She was in charge of the Twilight Zone Explorer, an autonomous underwater vehicle that can go down to 6,000 meters deep, equipped with a high-definition camera.
The principle is simple: the camera is oriented upwards. When the particles fall, they land on his lens. Thanks to the image collected over hours, days or even months, experts can count marine snowflakes, measure their size and guess their nature. The carbon content can then be estimated.
“Sediment traps”
“Our team went to the ship last year [en juillet 2021], but we had only seen very weak carbon movements. This year, on the other hand, we have observed very strong flows, but we do not yet know why,” says the young researcher. The timing of the cruise, in August rather than July, could be in question, but that’s just a guess.
More traditional methods also make it possible to measure the flow of marine snow. “Sediment traps”, for example, consist of cups placed at certain depths, which collect everything that falls there. However, these approaches require many manipulations from researchers. With the cameras, the ultimate goal is to deploy dozens of autonomous robots that could record data for months.
Knowledge of carbon fluxes in the ocean is not only used to advance basic science. They are also part of a body of applied science aimed at determining whether it is possible to artificially stimulate carbon sequestration by the oceans. The idea is to “fertilize” the seas with iron, an essential nutrient whose scarcity often limits the growth of microscopic algae near the surface.
Geoengineering is obviously controversial: will humanity hurt the planet more by trying to heal it? Buesseler, who has been interested in iron fertilization for years, believes the impact of this approach on marine ecosystems needs to be assessed, but believes the climate crisis is too serious to sit idly by.
He therefore created a group, Exploring Ocean Iron Solutions, which brings together dozens of scientists and whose objective is to evaluate, with complete objectivity and within an established ethical framework, whether iron fertilization is worth the effort. “The idea, he explains, is to do research before someone markets a stupid project for the wrong reasons. »
In the past, initiatives have evaluated the extent to which the addition of iron boosts plant growth (and therefore the uptake of CO2) on the surface. The next step is to understand, in different marine environments, what proportion of this carbon crosses the twilight layer towards the bottom of the seas. “We absolutely have to make sure that we don’t overestimate the benefits of this approach,” concludes Buesseler.