Once previously sequestered elements, such as fossil fuels, mercury, and sulfur, are released into the environment, they are recycled countless times before finding the right conditions in which to become locked away geologically.
The majority of the mercury in the air and water can be traced back to the Industrial Revolution and World War II. Both events relied heavily on burning coal, which is a significant source of atmospheric mercury as well as sulfur and carbon compounds.
For these compounds to be taken out of play, they must be rendered chemically inert and then settle in a place where sediments bury them. Sequestration primarily occurs at the bottom of the ocean or other bodies of water, but can happen on land under certain conditions. For example, there are vast reserves of “methane-ice,” a.k.a. methane hydrate, trapped beneath the seafloor.
Anaerobic bacteria in the seabeds produce methane. Due to low temperatures and the pressure of the water, the methane is sequestered in the crystalline structure of water, forming an ice-like compound. It will remain there for eons unless pressure or temperature changes dramatically—or until people start futzing with it.
If humanity was dead-set on locking away carbon, how could that be accomplished? Some researchers have suggested seeding the ocean with either iron or nitrogen-rich urea to promote the growth of carbon dioxide-consuming phytoplankton. Others have proposed a long-term approach by restoring forests, peat bogs, and wetlands as natural carbon sinks. Some think we should repurpose agricultural land for the production of biomass, which would then be buried deep underground.
Lately, scientists have bandied about the idea of using an aquatic fern that saved the planet from a runaway greenhouse scenario 50 million years ago, known as the “Azolla event” for the genus of fern responsible. Taking advantage of a shallow inland sea that is now the Arctic Ocean, Azolla (a.k.a., the mosquito fern) doubled its biomass every ten days or so and lay down a thick carpet of carbon-rich sediment.
The downside is that, much like algae blooms, Azolla can deplete available resources in its local environment, killing off other organisms. It is considered a nuisance, if not invasive, species by many municipalities.
A technique for injecting CO₂ into layers of basaltic rock deep underground has gained some attention in the last decade. Known as carbon mineralization, it utilizes the chemistry of the “alkaline earth cation” minerals within the basalt to react with carbon dioxide and water to form carbonates like limestone or baking soda.
Basalts are formed as a result of the rapid cooling of magma as it is discharged at or near the earth’s surface. The cooling process traps gas bubbles in the rock, giving it the structure of a solidified sponge.
Basalts are categorized by their mineral composition, which varies from those rich in calcium, magnesium, or sodium silicates, with a healthy dose of iron thrown in here and there. The chemical reaction whereby CO₂ binds to magnesium silicates that make up the basaltic mineral olivine, for example is Mg₂SiO₄ + 4CO₂ + 2H₂O -> 2MgC0₃ + SiO₂ + 2CO₂ + 2H₂O
The chemistry is pretty much the same for calcium, sodium, and iron-based minerals, though some formulations are better at scrubbing CO₂ from the results.
A joint Icelandic/American project known as CarbFix has been investigating the merits of such a process for the past six years. Borrowing technological know-how from the fracking industry, they have been drilling 3,200 feet into seams of basalt, of which volcanic Iceland has plenty. They then inject carbonated water, cap and seal the wellhead, and allow chemistry to take over.
CarbFix researchers have found that it takes approximately 1.5 years for complete mineralization. Their 2012 test injections of 220 tons of CO₂ was so successful that monitoring equipment in adjacent wells became clogged with calcium carbonate. CarbFix currently sequesters 10,000 tons of CO₂ per year, which is equal to the annual output of about 2,000 automobiles.
One drawback to this method of carbon sequestration is the amount of water required (a 25:1 ratio). In water-poor but basalt-rich countries like India, the water requirement is a deal breaker. CO₂ leakage at the well site or within the CO₂ transport chain is also a concern.
As with fracking, tremors, earthquakes, and possible aquifer contamination can result as a side effect, leading some to suggest that this technology be used in remote areas such as the ocean floor.
Land-based areas with high concentrations of basaltic rock occur in Siberia, India, the Pacific Northwest, and the Mid-Continental Rift, of which the Duluth Complex is a small part. The Minnesota Geological Survey examined carbon mineralization as a method of sequestration in 2011. Study author H. Thorleifson suggested that such efforts should be concentrated in the southern portion of Minnesota due to thicker sedimentary layers that would act as a natural capstone. The Iron Range is so riddled with past mining endeavors that CO₂ leakage would make carbon injection impractical.
Admittedly, no technique is ideally positioned to clean up our fossil fuel mess and make the world safe for democracy, kittens, and Mom’s apple pie. The real solution will always be to reduce consumption of those materials that add carbon to the atmosphere.