A study led by the ICM-CSIC and the BSC-CNS reveals that the mixing of surface and deep waters that occurs in winter acts as a shortcut to transport fresh organic matter to depths of more than 1,000 meters, injecting it directly into the ocean circulation and nourishing deep ecosystems.
Processes that capture carbon from the atmosphere and store it in the deep ocean are crucial for climate balance. Evaluating these mechanisms, however, is often complicated because measurements from the surface to great ocean depths are needed.
Now, a new study published in the journal Science Advances and led by the Institut de Ciències del Mar (ICM-CSIC) in collaboration with the Barcelona Supercomputing Center – Centro Nacional de Supercomputación (BSC-CNS) has allowed the observation and quantification of a key transport channel: the large downwellings of surface water that occur in winter inject nutrient-rich organic particles in an express way to depths of more than 1,000 meters.
In regions such as the subpolar North Atlantic, cold temperatures and strong winter winds cool the surface water, making it much denser and heavier, causing it to sink rapidly and mix with deeper waters. This process, known as deep convection, feeds the currents that connect all oceans over the centuries, and helps regulate the global climate and carbon storage in the ocean.
The work demonstrates that this "underwater cascade" is a mechanical shortcut that drags living microalgae and organic remains towards the seabed much faster and to a greater depth than previously thought. This transport is intermittent and regional in scope, but much more efficient than the gravitational settling of particles (the dominant transport mechanism).
Underwater robotics and supercomputing
To conduct the study, the scientific team combined real data and highly powerful computer simulations. On one hand, they analyzed information from a fleet of autonomous underwater robots (called Biogeochemical-Argo floats) that navigate and take measurements while drifting at a depth of about 1,000 meters. Thanks to these devices, surprising chlorophyll peaks have been detected in the depths of the Labrador and Irminger seas, coinciding with the strong convection episodes that occurred between 2014 and 2017.
"Finding these chlorophyll concentrations, typical of the sunlit surface, at such depths was a great surprise, because the most normal thing would be for it to degrade long before reaching so far down," explains Martí Galí, researcher at the ICM-CSIC and lead author of the study. "These data allow us to quantify a previously poorly-known source of carbon in the ocean interior and open up a huge range of possibilities for using these robots as the eyes of science in the deep ocean."
To calculate the magnitude of this shortcut on a global scale, the researchers used a computer model that recreates marine physics and biogeochemistry. The simulations were run thanks to the supercomputing resources of the BSC-CNS.
"The story began precisely at the BSC-CNS, where we saw that we had to better connect these large water movements with carbon cycles," recalls Raffaele Bernardello, researcher at the BSC-CNS and co-author of the study. "The simulations have allowed us to estimate that transport doubles during the harshest winters and that carbon does not remain stagnant; instead, a portion travels laterally and is stored far from the area where it sank, extending its positive effect on the climate."
A feast for the deep ocean
On the other hand, the study indicates that, in years of strong winter mixing, this mechanism contributes between 30% and 50% of all organic particles reaching the layers between 500 and 2,000 meters. Furthermore, by analyzing the properties of the particles at 1,000 meters and comparing them with those observed by satellites at the surface, it has been shown that the injected material is very rich and full of energy.
Mª Andrea Orihuela-García, who is finishing her doctoral thesis at the ICM-CSIC linked to the BSC-CNS, highlights the consequences for deep ecosystems:
"This process not only serves to capture carbon from the atmosphere, but it sends energy directly downwards. These vertical currents reach depths where there is practically no food. Therefore, we suspect they act as a genuine unexpected seasonal feast that nourishes and activates the communities of microbes and small animals living in the deep ocean."
In light of this scenario, the team insists on the need to pursue this research line to improve observations of this phenomenon and its representation in numerical models, including those of the IPCC. Furthermore, the team stresses the need to maintain and strengthen ocean observation systems based on satellites and autonomous robots — especially the Argo program, an international, cooperative effort based on the principles of open science.
Finally, at a time when human activity is altering the planet's climate and threatening to weaken winter mixing in the North Atlantic (a very important process), monitoring the ocean constantly and globally -from the surface to the depths- becomes more important than ever to understand and predict climate evolution.