When scientists using underwater robots detected chlorophyll — the green pigment that algae need to live — more than 3,000 feet beneath the surface of the Labrador Sea, they could hardly believe their instruments. Chlorophyll normally stays near the ocean surface where sunlight reaches it. Finding it at such enormous depth was a complete surprise.
This puzzling discovery led researchers at the Institute of Marine Sciences in Barcelona and the Barcelona Supercomputing Center to uncover a powerful natural mechanism that moves carbon from the ocean surface down to the deep sea much faster than scientists had realized. Their findings were published in the journal Science Advances.
The process starts with harsh winter winds that cool the surface of the ocean in regions like the North Atlantic. When surface water grows cold and dense, it sinks rapidly in a phenomenon called "deep convection." This sinking creates powerful currents that drag living microalgae and decaying organic material straight down to depths exceeding 1,000 meters — roughly the height of ten Empire State Buildings stacked on top of each other.
"These vertical currents act as an unexpected seasonal feast that nourishes and energizes communities of microbes and small animals living in the deep ocean," said Maria Andrea Orihuela-García, a doctoral researcher at the Institute of Marine Sciences who worked on the study.
The researchers tracked particles sinking from 2014 to 2017 using a fleet of autonomous underwater robots called Biogeochemical-Argo floats. These buoyant machines drift through the deep ocean, measuring water properties and sending data back to scientists on shore. The data revealed that this underwater "cascade" acts like a mechanical shortcut, bypassing the slow, gradual sinking that particles normally follow.
Computer simulations run on the Barcelona Supercomputing Center's powerful computers showed that during the most intense winter storms, this shortcut delivers between 30 and 50 percent of all organic particles found at depths between 500 and 2,000 meters. The researchers also discovered that carbon does not simply stay where it lands — some of it drifts sideways and gets stored far from where it first sank, extending its climate benefits for centuries.
Lead author Martí Galí explained that the discovery helps scientists understand a hidden but important part of how the ocean stores carbon pulled from the atmosphere. "These data allow us to quantify a previously little-known source of carbon within the ocean," Galí said, "and open up a vast range of possibilities for using these robots as the eyes of science in the deep ocean."
The team emphasizes that protecting and expanding ocean observation systems — including the international Argo program that maintains thousands of these underwater robots — will be crucial for tracking how this carbon highway responds as the climate continues to change.