The plan is for the carbon dioxide to react with the rock to form carbonates, permanently removing it from the carbon cycle in a process known as geochemical trapping. Landlocked basalt is already under investigation for carbon storage but subsea basalt has the advantage that if any of the carbon dioxide doesn’t react, the ocean environment provides additional safety mechanisms to prevent it from returning to the atmosphere. There’s also a large volume of undersea basalt available.

"By being under the ocean you add a number of trapping mechanisms as security – insurance on your insurance on your insurance, if you will," David Goldberg of Lamont-Doherty Earth Observatory told environmentalresearchweb. "There are four trapping mechanisms that are effectively in play – geochemical, sediment, gravitational and hydrate formation. If you have some leakage of carbon dioxide or there are some losses during injection, hydrates will form because it’s cold, it’s physically trapped by sediments above the rock, and carbon dioxide is more dense than seawater and will tend to sink – the gravitational trap – even if it hasn’t turned to carbonate in the geochemical trapping."

Goldberg and colleagues from Lamont-Doherty Earth Observatory, US, found that sediment-covered basalt aquifers on the Juan de Fuca plate off the western coast of the US are suitable for storing around 780 cubic km of carbon dioxide. That’s equivalent to 208 gigatonnes of carbon – enough capacity for 122–147 years of US emissions at current rates (1.7 gigatonnes of carbon per year). What’s more, the midpoint of the plate is only around 150 km away from the coast, making transport and injection of the gas relatively easy.

To begin with, the researchers focused on the US as the country is a major emitter of carbon dioxide and there’s a suitable subsea basalt reservoir close to its shores. They’re currently evaluating the rest of the globe.

"All the reservoirs are very large, a number of them are viable," said Goldberg. The hundreds of gigatonnes of carbon dioxide storage offered by the Juan de Fuca plate is ten times the volume another single reservoir might typically provide, he adds. "Globally, the problem is huge. We’ve got to look at all the reservoirs we possibly can, and this is a huge one that’s not really been considered."

In the laboratory, it takes days for carbon dioxide to react with basalt and form carbonates. But it’s not yet known how long the process would take in situ underneath the ocean. "There are a number of scientific questions still out there to do with reaction rates and flows that we would answer in a pilot experiment of some modest size," said Goldberg. "That would take some years of experimentation, unless it fast-tracks in some way." The researchers are currently seeking funding for such a pilot.

"Then there’s a market process that will take implementation forward at whatever rate the market moves at, which is sometimes incredibly slow and sometimes at lightning speed,"said Goldberg. "I don’t see any technical reason why it couldn’t happen. It’s answering the science questions first and then it’s the political and economic will."

One of the challenges of using undersea basalt, rather than disused oil and gas reservoirs, to store carbon dioxide is the current lack of knowledge.

"It’s a less well known and less well studied area of the globe," said Goldberg. "We know a lot more about oil and gas reservoirs than we do about subocean basalt reservoirs so there’s a research curve that we need to go up pretty quickly. That takes commitment and resources."

Empty oil and gas wells also have the advantage of having "plumbing and piping" infrastructure in place. Goldberg likens the process to "just refilling the can with liquidized carbon dioxide".

"[Using undersea basalt] would take investment in infrastructure which is not there at present, but 40 years ago the offshore oil and gas industry didn’t have that infrastructure either," he said. "The cost would not be unlike the costs of building an offshore industry for the production of hydrocarbons, which to some extent caused the problem." According to Goldberg, like any sequestration option using geology, it will be more difficult to store carbon dioxide in basalt offshore than on. "It will be more expensive than onshore but the two big benefits [volume and security] are such grand prizes that they can’t be ignored," he said. "And doing the research first is the right step."

The researchers reported their work in PNAS.