Humans are pouring large quantities of carbon dioxide into the atmosphere every year, which is changing the climate and the chemistry of the oceans. Finding a better place to put all that carbon is a topic of great importance. Although there is some debate over the efficacy of various carbon offset schemes, even motorsports are looking into carbon sequestration. A recent article in the Proceedings of the National Academy of Science looks at the viability of storing all of that carbon in the ocean floor. Most of the ocean floor is made up of basalt, an igneous rock that formed from lava extruded at mid-ocean ridges. This basalt is overlain by a relatively thin layer of extremely fine sediment; it forms an excellent reservoir for CO 2 for a variety of reasons.

To store significant amounts of CO 2 , you need two things: storage space, and a way to keep the CO 2 from escaping. When lava is extruded directly into the ocean, it cools very rapidly, cracking in the process. This creates lots of empty space within the newly formed rock for storage of CO 2 . The ocean floor also has the CO 2 trapping problem solved. The basalt, in combination with the sediments on top, provide three mechanisms to trap the CO 2 : a physical barrier, gravitational containment, and chemical conversion into carbonates.

First, the physical barrier is provided by the fine sediments on the top of the ocean floor. These sediments have extremely low permeability, and thus prevent the CO 2 from escaping. Second, gravity will trap CO 2 at water depths greater than 2700m as, at this depth, the injected CO 2 will be denser than sea water. Finally, CO 2 can combine with calcium and magnesium ions released by the basalt to form carbonate minerals, which are insoluble.





Basalts like this could store carbon at greater depths

The authors estimate that there is around 780km3 of available space to store CO 2 off the coast of northern California and Oregon. This works out to the ability to store 200-250Gt of carbon. Considering that the entire US releases around 1.7Gt of carbon per year, that would provide over 100 years of carbon storage.

The authors propose extensive drilling experiments to better characterize this potential reservoir. For example, it not known just how permeable the basalt is, or how rapidly the CO 2 will be converted to carbonates. If pilot experiments are successful, maybe the environmentalists will be clamoring for their own style of offshore drilling.

Proceedings of the National Academy of Science, 2008. DOI: 10.1073/pnas.0804397105