In-ground carbon dioxide capture

As concern over global warming continues to grow, pressure and funding are increasing to find ways to reduce the growth and, in time, the actual levels of atmospheric carbon dioxide (see PHYSICS TODAY, August 2008, page 26). Peter Kelemen and Jürg Matter of Columbia University's Lamont-Doherty Earth Observatory have proposed a new approach for CO₂ sequestration: accelerating the natural carbonation of exposed mantle rock. In many places around the globe—perhaps most dramatically in Oman—sections of the upper mantle have been raised through subduction or tectonic spreading. The resulting outcrops, termed ophiolites, are rich in peridotite, a rock primarily composed of the minerals olivine and pyroxene. (For more on the Oman ophiolite, see PHYSICS TODAY, January 2005, page 21.) Strongly out of chemical equilibrium with the atmosphere, the mantle rock naturally reacts with water and CO₂ to form silicates, carbonates, and iron oxides. Kelemen and Matter find that atmospheric CO₂ reacts with peridotite surprisingly quickly, at a rate of about 4 × 10⁷ kg/yr for the 500-km-long Oman ophiolite. The researchers suggest several options for boosting that reaction rate even higher, starting with increasing the interaction volume by drilling and fracturing the peridotite. Some fracturing will happen spontaneously as the hydration and carbonation reactions expand the rock volume and give off heat. When the two scientists incorporate into their model the effects of raising the CO₂ concentration near the rock and elevating the peridotite temperature, they estimate a potential increase of 10⁹ in the reaction rate, or 2 × 10⁹ tons of CO₂ captured and sequestered each year per cubic kilometer of ophiolite. The researchers call for further modeling and field testing of what could be a permanent storage solution. (P. B. Kelemen, J. Matter, Proc. Natl. Acad. Sci. USA 105, 17295, 2008.) — Richard J. Fitzgerald