To slow global warming, scientists are exploring ways to pull carbon dioxide from the air and safely lock it away. Trees already do this naturally through photosynthesis; now, in a new report, geologists have mapped large rock formations in the United States that also can absorb CO2, which they say might be artificially harnessed to do the task at a vastly increased pace.
The report, by scientists at Columbia University's Earth
Institute and the U.S. Geological Survey, shows 6,000 square miles of
ultramafic rocks at or near the surface. Originating deep in the earth, these
rocks contain minerals that react naturally with carbon dioxide to form solid
minerals. Earth Institute scientists are experimenting with ways to speed this
natural process, called mineral carbonation. If the technology takes off,
geologic formations around the world could provide a vast sink for
heat-trapping carbon dioxide released by humans.
Lead author Sam Krevor, a graduate student working through
the Earth Institute's Lenfest Center for Sustainable Energy, says the United
States' ultramafic rocks could be enough to stash more than 500 years of U.S.
CO2 production. Conveniently, most of them are clustered in strips along the
East and West Coasts – some near major cities, including New York, Baltimore
and San Francisco. "We're trying to show that anyone within a reasonable
distance of these rock formations could use this process to sequester as much
carbon dioxide as possible," says Krevor.
So-called carbon sequestration has become a hot area of
research, but so far, most work has focused on storing liquid or gaseous CO2
underground where there is room: in saline aquifers, depleted oil wells and
porous coal seams that are not commercially viable. However, concern about
leaks has scientists pursuing natural chemical reactions within the earth to
turn the carbon back into a solid.
Ultramafic rocks generally form in earth's mantle, starting
some 12 miles under the surface and extending down hundreds of miles. Bits of
these rocks – peridotite, dunite, lherzholite and others – may be squeezed to
the surface when continental plates collide with oceanic plates, or, less
often, when the interiors of continents thin and develop rifts. Because of
their chemical makeup, when the rocks are exposed to carbon dioxide, they react
to form common limestone and chalk. A map accompanying the report shows that
most such rocks are found in and around coastal mountain ranges, with the greatest
concentrations in California, Oregon and Washington, and along the Appalachians
from New England to Alabama. Some also occur in the interior, including
Montana. Worldwide, other formations are scattered across Eurasia and
Klaus Lackner, who directs the Lenfest Center, helped
originate the idea of mineral sequestration in the 1990s. The U.S. survey is
the first of what Lackner hopes will become a global mapping effort. "It's
a really big step forward," he says. Krevor produced the map as part of
his PhD. dissertation, with help from another Columbia student, Christopher
Graves, and two USGS researchers, Bradley Van Gosen and Anne McCafferty. By
combining more than a hundred existing maps, the researchers were able to
pinpoint the areas nationally where ultramafic rocks are most abundant.
Another rock, common volcanic basalt, also reacts with CO2,
and efforts are underway to map this in detail as well. The U.S. Department of
Energy has been working on a basalt atlas for the northwestern United States as
part of its Big Sky Carbon Sequestration Partnership; extensive mapping in
Washington, Oregon and Idaho already has been done through Idaho State
The major drawback to natural mineral carbonation is its
slow pace: normally, it takes thousands of years for rocks to react with
sizable quantities of CO2. But scientists are experimenting with ways to speed
the reaction up by dissolving carbon dioxide in water and injecting it into the
rock, as well as capturing heat generated by the reaction to accelerate the
process. "It offers a way to permanently get rid of CO2 emissions,"
says Juerg Matter, a scientist at Columbia's Lamont-Doherty Earth Observatory.
Matter and his colleague Peter Kelemen currently are researching
peridotite formations in Oman, which they say could be used to mineralize as
much as 4 billion tons of CO2 a year, or about 12 percent of the world's annual
output. And in Iceland, Matter is about to participate in the first major pilot
study on CO2 sequestration in a basalt formation. In May, he and three other
Lamont-Doherty scientists will join Reykjavik Energy and others to inject
CO2-saturated water into basalt formations there. Over nine months, the rock is
expected to absorb 1,600 tons of CO2 generated by a nearby geothermal power
plant. Matter and another Lamont-Doherty scientist, David Goldberg, also are
involved in a study by Pacific Northwest National Laboratory, which will
eventually inject 1,000 tons of C02 into formations beneath land owned by a
paper mill near Wallula, Wash.
One model is to capture CO2 directly from power-plant
smokestacks or other industrial facilities, combine it with water and pipe it
into the ground, as in the upcoming Iceland project. Lackner and his colleagues
also are working on a process using "artificial trees" that would
remove CO2 already emitted into the atmosphere.
Combining rocks and carbon dioxide could provide
an added benefit, as Krevor points out. For decades, some large U.S. peridotite
formations were mined for asbestos, used for insulation and other purposes.
After a link between asbestos and cancer was proven, the substance was banned
for most uses, and the mines were closed. Mine tailings left behind, at
Belvidere Mountain in Vermont and various sites in California, provide a ready
supply of crushed rocks. These potentially hazardous tailings would be rendered
harmless during the mineralization process.
Geologists Map Rocks to Capture CO2
March 24, 2009