Undersea lava rubble acts as a 'sponge' for carbon dioxide, study finds

Ancient lava rubble on the seafloor can store huge amounts of carbon dioxide like a giant "sponge," a new study finds.

Cores collected from beneath the southern Atlantic Ocean show that this rubble — which formed through volcanic activity and spread across the ocean floor over millions of years — stores between two and 40 times as much carbon dioxide as the upper crust at the bottom of the ocean, according to research published Nov. 24 in the journal Nature Geoscience. The findings may help scientists better understand how Earth's climate changed in the past.

Over millions of years, carbon cycles through the planet's crust and atmosphere. Volcanic activity in mid-ocean ridges — underwater mountain ranges where tectonic plates spread apart — releases carbon dioxide into the ocean and atmosphere, and forms volcanic rocks on the seafloor. But the ocean acts as a sink for carbon dioxide, too.

"Ocean basins are not just a container for seawater," study co-author Rosalind Coggon, an ocean crust researcher at the University of Southampton in the U.K., said in a statement. "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."

That mineral-laden volcanic rubble, known as breccia, could help scientists understand how undersea processes might have impacted how much carbon dioxide was in the atmosphere millions of years ago, and how that carbon dioxide might have affected the global climate.

In the new study, Coggon and her colleagues drilled deep into the Earth's crust on the seafloor in the southern Atlantic Ocean to collect some for further study. "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," Coggon said.

The team collected cores from a chunk of 61 million-year-old crust that included sediments and breccias. The cores were porous and crumbly, and the various rubble pieces had calcium carbonate growths in the open spaces, on their ends and between fragments of the core.

Carbon dioxide that had been converted into carbonate minerals through reactions with seawater made up an average of 7.5% of the weight of the core, the team found. That's anywhere from two to 40 times higher carbon dioxide content than any previously collected samples of the upper ocean crust held. The breccias could store as much as 20% of the carbon dioxide released when the underlying crust formed, the researchers wrote in the study.

The breccias act "as a sponge for carbon in the long-term carbon cycle," Coggon said.

The amount of carbon dioxide that the breccias can store depends on the amount of carbon dioxide present in the ocean, the thickness of the breccia on the seafloor, and how quickly the tectonic plates at mid-ocean ridges are spreading apart, the researchers wrote in the study. Past changes in any of these values could have affected the extent to which the breccias played a role in the carbon cycle and Earth's climate.

The extra carbon stored in the lava rubble hasn't been accounted for previously, the researchers wrote in the study. The new findings could help researchers better understand the role they played in controlling Earth's past thermostat.