Scientists find a massive hidden CO2 sponge beneath the ocean floor

Researchers examined lava material drilled from deep below the South Atlantic Ocean to measure how much CO₂ becomes incorporated into these rocks through interactions between seawater and the cooling volcanic material.

Work led by the University of Southampton demonstrates that these accumulations of broken lava, created as underwater mountains erode, act as natural reservoirs for CO₂. This study marks the first time their role as extensive carbon-holding structures has been clearly recognized, offering fresh insight into how Earth manages carbon over millions of years.

Lava Rubble as a Long-Term Geological “Sponge”

Lead author Dr. Rosalind Coggon, Royal Society Research Fellow at the University of Southampton, explained: “We’ve known for a long time that erosion on the slopes of underwater mountains produces large volumes of volcanic rubble, known as breccia — much like scree slopes on continental mountains.

“However, our drilling efforts recovered the first cores of this material after it has spent tens of millions of years being rafted across the seafloor as Earth’s tectonic plates spread apart.

“Excitingly, the cores revealed that these porous, permeable deposits have the capacity to store large volumes of seawater CO₂ as they are gradually cemented by calcium carbonate minerals that form from seawater as it flows through them.”

How Carbon Moves Through Earth Over Geological Time

The amount of carbon dioxide in the atmosphere is influenced by the slow exchange of carbon among Earth’s interior, the oceans, and the air over many millions of years. Understanding this long-term carbon cycle requires studying where and how carbon is added or removed from different parts of the planet.

Dr. Coggon noted: “The oceans are paved with volcanic rocks that form at mid-ocean ridges, as the tectonic plates move apart creating new ocean crust. This volcanic activity releases CO₂ from deep inside the Earth into the ocean and atmosphere.

“However, ocean basins are not just a container for seawater. Seawater flows through the cracks in the cooling lavas for millions of years and reacts with the rocks, transferring elements between the ocean and rock. This process removes CO₂ from the water and stores it in minerals like calcium carbonate in the rock.”

As part of the project, the team quantified how much CO₂ becomes incorporated into ocean crust through these chemical reactions.

Discovering Far Greater CO₂ Storage in Breccia

“While drilling deep into the seafloor of the South Atlantic, we discovered lava rubble that contained between two and 40 times more CO₂ than previously sampled lavas,” said Dr. Coggon.

“This study revealed the importance of such breccia, which forms due to the erosion of seafloor mountains along mid-ocean ridges, as a sponge for carbon in the long-term carbon cycle.”

The findings come from Expedition 390/393 of the International Ocean Discovery Program.