Another strategy proposes to store CO2 in the world's oceans. Several methods have been suggested. One option is "direct injection." In this method, CO2 is pumped directly into the water at extreme depths of 1,148-11,811 feet (350-3,600 m). The CO2 forms a mass on the seabed and transforms into solid CO2 clathrate hydrates, which gradually dissolve into the water around it. There are problems associated with this method, however. It could contribute to ocean acidity and could lead to increased levels of submerged methane gas, which if it reached the surface and escaped into the atmosphere, would also cause a serious greenhouse gas problem.
Another suggested method of long-term sequestration is to bale large bundles of agricultural crop waste biomass (such as corn stalks, hay, and grass) and deposit it into alluvial fans in deep ocean basins, where it would eventually get buried under thick layers of sediment. Critics of this method caution that there could be an increase in aerobic bacteria growth, which would lower the available oxygen levels to ocean life-forms. It has also been suggested to convert CO2 to bicarbonates, using limestone or to store CO2 in solid clathrate hydrates already residing on the ocean floor.
Ocean sequestration is still in its infancy, and much more needs to be learned about it until it can become a proven environmentally sound method of sequestration. To date, pumping CO2 into deep seafloor geological formations appears to be the best type of "ocean sequestration" such as that proposed in the Pacific Northwest, discussed earlier.
According to a report in the Seattle Times, the CO2 would be pumped into porous basalt layers up to a half-mile below the ocean bottom. Dave Goldberg, when talking about the CSS storage in the Pacific Northwest mentioned previously, stated, "This is the first good example of a site that is of the scale that can potentially make a dent on the problem of CO2 storage."
The minerals in the bedrock would serve to form stable carbonates. The deposits would also be covered by sediments 1,000 feet (305 m) thick to eliminate the chance of leaks. It is also a convenient location: close enough to the shoreline that the CO2 could be piped directly from power plants to injection sites.
According to Taro Takahashi, senior scholar of Lamont-Doherty, "Undersea storage has been largely ignored, even though there's much more capacity, and repositories would be much less prone to leak. In principle, the type of reservoir we propose at the ocean floor is one of the safest—if not the safest—way of storing liquefied CO2 for a long, long time."
Some argue, however, that there may be unknown ecological impacts and seismic hazards associated with the technology.
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