Deep-Sea Sediments Could Store CO2 Safely, Scientists Say

Supplying energy without affecting Earth's climate is pressing technical challenge

An innovative solution for storing excess human-generated carbon dioxide in the atmosphere could lie far beneath the ocean's surface, say Harvard University scientists in Massachusetts.

The research was funded by in part by the U.S. Department of Energy, according to an August 7 Harvard press release.

The researchers found that deep-sea sediment could become a virtually unlimited and permanent reservoir for this gas, which has been a primary driver of global climate change in recent decades.

Deep-sea ocean floor sediment is made up of tiny particles of sand, silt, clay and animal skeletons that have settled on the ocean bottom. Over time, the particles become compressed and form stratified layers. The scientists estimate that seafloor sediments in U.S. territory are vast enough to store the nation's carbon dioxide emissions for thousands of years. (See related article.)

Harvard's Kurt Zenz House and Daniel Schrag, with colleagues from the Massachusetts Institute of Technology and Columbia University in New York, detailed the advantages of sequestering (storing) excess carbon dioxide thousands of meters beneath the ocean surface.

"Supplying the energy demanded by world economic growth without affecting the Earth's climate is one of the most pressing technical and economic challenges of our time," said Schrag, professor of earth and planetary sciences and director of Harvard's Center for the Environment.

"Since fossil fuels - particularly coal - are likely to remain the dominant energy source of the 21st century," he added, "stabilizing the concentration of atmospheric carbon dioxide will require permanent storage of enormous quantities of captured carbon dioxide safely away from the atmosphere."

Schrag and colleagues say an ideal storage method could be injecting carbon dioxide into ocean sediments hundreds of meters thick. The combination of low temperature and high pressure at ocean depths of 3,000 meters turns carbon dioxide into a liquid denser than the surrounding water, removing the possibility of escape and ensuring virtually permanent storage.

Injecting carbon dioxide into seafloor sediments rather than putting it directly into the ocean traps the gas, minimizing damage to marine life and ensuring that the gas will not eventually escape to the atmosphere due to the mixing action of ocean currents.

At sufficiently extreme deep-sea temperatures and pressures, carbon dioxide moves beyond its liquid phase to form solid and immobile crystals, further boosting the system's stability. Such stored gas would be secure enough to withstand severe earthquakes and other geomechanical upheavals, the scientists said.

Other researchers are experimenting with storing carbon dioxide in geologic formations such as natural gas fields, but carbon dioxide gas can leak from terrestrial reservoirs.

"Some 22 percent, or 1.3 million square kilometers, of the seafloor within the United States' exclusive economic zone is more than 3,000 meters deep," said House, a graduate student in Harvard's Department of Earth and Planetary Sciences.

Outside the United States' economic zone, the scientists wrote, the total carbon dioxide storage capacity in deep-sea sediments is unlimited.

The scientists said thin or impermeable sediments are inappropriate for carbon dioxide storage, as are areas beneath steep deep-sea slopes, where landslides could free the gas. Further assessment of the mechanical feasibility of delivering carbon dioxide to the seafloor, and study of possible effects on sea levels, is needed, they said.

Source: U.S. Department of State

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