How the ocean locks away carbon for millions of years

Sixty-million-year-old rock samples drilled from deep beneath the South Atlantic have clarified a quiet but powerful climate process. Vast heaps of broken lava on the seafloor can lock away carbon dioxide for geological timescales.

In a study led by the University of Southampton, researchers analyzed ancient oceanic lavas.

The results showed that the loose, highly porous rubble accumulating along the flanks of underwater volcanoes becomes a long-lived reservoir for carbon carried by seawater.

How lava rubble traps carbon

According to the researchers, these deposits – technically volcanic breccias – behave like geological sponges.

Over tens of millions of years, seawater percolates through the rubble, chemical reactions begin, and carbonate minerals slowly cement the fragments, trapping carbon in stone.

Study lead author Rosalind Coggon is a research fellow at the University of Southampton.

“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,” said Coggon.

“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.”

The deep carbon cycle

The slow cycling of carbon among Earth’s interior, oceans, and atmosphere governs the planet’s climate over deep time. Mid-ocean ridges create new ocean crust as plates pull apart, releasing CO2 from the mantle into seawater and air.

But that same crust, once it moves away from the ridge and cools, starts to act as a chemical filter. Seawater circulates through cracks and voids, reacts with volcanic minerals, and precipitates carbonates that lock carbon within the rock.

Drilling into the South Atlantic seafloor, the team hit pockets of that filter working at full capacity. Compared with intact lavas, the rubble held far more carbonate, showing its open structure boosts CO2 uptake.

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

Ocean basins act as more than simple containers. Seawater circulates through cracks in cooling lava for millions of years, reacts with the rock, transfers elements, and ultimately removes CO2 from the water by storing it in minerals like calcium carbonate.

Lava rubble outperforms solid rock

Intact basalt – the solid, coherent rock formed when lava cools – does take up carbon. But it does so slowly, limited by how quickly fluids can reach fresh surfaces.

Breccias are different. They form when steep volcanic slopes shed debris or when seamounts break apart into thick piles of blocks and ash.

That architecture creates vast internal surface area and broad pathways for seawater to circulate. The result is more reaction, more carbonate, and more carbon sequestered.

The new cores put numbers on that intuition. “While drilling deep into the seafloor of the South Atlantic, we discovered lava rubble that contained between two and 40 times more carbon than previously sampled lavas,” said Coggon.

“This study revealed the importance of such breccia, which forms due to the erosion of seafloor mountains along mid-ocean ridges.”

The crust’s hidden climate role

Scientists have long recognized oceanic crust as a carbon sink, but they hadn’t fully appreciated the standout performance of breccia layers.

Because these deposits form wherever submarine volcanoes grow and crumble, they may add a significant, undercounted term to Earth’s long-term carbon budget.

That matters for reconstructing ancient climates, testing models of atmospheric CO2 through time, and understanding how the solid Earth helps stabilize climate over millions of years.

Crucially, this is not a quick, engineered fix for today’s emissions. The processes unfold over immense timescales, paced by plate motions, seafloor weathering, and mineral formation.

But the findings sharpen the picture of how the planet has naturally balanced carbon inputs and outputs in the past. They also highlight a previously overlooked, high-capacity “shelf” in the global warehouse where carbon ends up.

A window beneath the waves

Recovering these cores was a feat in itself. The team drilled into deposits that formed at an ancient mid-ocean ridge and have since drifted across the basin atop a migrating plate.

That journey, and the rubble’s endurance, gave scientists a unique laboratory – an intact record of fluid-rock interaction playing out over tens of millions of years.

By tying together seafloor geology, carbonate chemistry, and the physics of fluid flow through porous media, the study adds an elegant new piece to the deep time climate puzzle.

Lava rubble may look chaotic, but it organizes itself remarkably well when it locks away carbon.

The study is published in the journal Nature Geoscience.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–