Could a rock formation help reduce global warming?

Nature Steps Up: There’s a Rock Formation That Stands Ready to Sequester Our CO2

Under the crust that wraps Earth is an 1,800-mile-deep mantle of rock. The most common rock in the mantle is called peridotite. In some spots around the globe — Oman, New Guinea, the Aegean coast, some Pacific islands, and the coast of California — the peridotite layer has found its way to the surface.
Peridotite has a curious property: when exposed to the atmosphere, it rapidly extracts and absorbs the air’s carbon dioxide. The chemical reaction locks in the CO2 and transforms the peridotite into carbonate rock. “If a leaf or a pebble falls on these rocks and you come back a few days later, it’s all covered up with carbonate,” Peter Kelemen, a geologist with Columbia University’s Lamont-Doherty Earth Observatory, told the Natural Resources Defense Council.

Working on volcanism in Oman, Kelemen had experienced something of a metamorphosis in his thinking when he began contemplating absorption instead of eruption. Could peridotite play a role in helping to reduce global warming? Together with colleague Juerg Matter, the Columbia team calculated that the Oman rocks lock in as much as 100,000 tons of carbon dioxide annually. That’s hardly noticeable compared to the 30 billion tons that human activity pumps into the atmosphere every year, but it led to the question: What might be done to boost the natural process?

Flood the Zone
The answer that the Columbia duo came up with is not to mine surface peridotite and cart it off to power plants in America to soak up our CO2 omissions. They propose an entirely different approach. The outcrops at the various spots around the world are at the oceans’ edges, where tectonic plates perform the bump and grind that have forced peridotite to the surface. Sea water is an enormous natural sink for carbon dioxide, but a number of reports state that mankind’s increasing burning of fossil fuels is overtaxing the oceans’ ability to store it for us (as reported here). Kellemen and Matter reason, why not use standard oil drilling technology to make a sponge of the peridotite below the surface, and then pump sea water into it?

The sea water would first be heated. Heat speeds up the carbon take-up of the peridontite, a reaction which itself generates heat and which, once primed, will thereafter take on the role of speeding of the process. The heat will also fracture the rock, opening fissures that will expose ever more peridotite.

Leached of its CO2, the sea water would be re-cycled to the surface to be replaced with new sea water in a continuous cycle. It is of no consequence that some of these exchanges would be made in places like Oman, far from where most of man’s CO2 is released. The winds and the oceans carry CO2 the world over, and its removal from the sea anywhere counts as a credit on the planet’s CO2 balance sheet. Sea water maintains a CO2 equilibrium with the air; what is removed from the water makes room for the sea to take in more from the air.

The Columbia pair believe that their technique of forced injection into drill holes could boost peridotite’s absorption rate 100,000-fold. They say that if all the peridotite in Oman could be utlilized for CO2 sequestration, it “could take up all the [current level of] human output for a thousand years”.
That sounds overstated, except that patch of bare, exposed peridotite in Oman alone is the size of Massachusetts and it extends five kilometers deep. Kelemen and Matter say that of man’s annual CO2 output of 30 billion tons Oman alone could swallow up 4 billion tons a year.

Wally Broeker, also at Columbia, is a geophysicist whose specialty is employing natural processes to reduce greenhouse gases. He told On Earth magazine that the Kelemen/Matter proposition seems sound and added, “When we’ve done everything we can with the other stuff, with alternative energy and carbon taxes, we’re going to find that we’re still dumping CO2 into the air at an alarming rate. It’s a desperate situation, and we’ve got to push everything we can.”
- Stephen Wilson from Planetwatch.org (reprinted with permission)