Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications
Key finding. Reacting CO2-rich power-plant gas with seawater to make carbonic acid, then reacting that on site with limestone to yield calcium and bicarbonate ions for release to the ocean, would greatly expand the ocean's capacity to store anthropogenic carbon while minimizing the impact of that carbon on ocean biota — potentially more cost-effective and less environmentally harmful than capture followed by direct deep-sea injection.
What question did this research address?
Injecting captured CO2 into the deep ocean was a leading disposal proposal, but the carbon stays as dissolved CO2 — acidic, harmful to deep-sea organisms, and destined to degas back to the atmosphere after several hundred years.
Nature disposes of CO2 permanently by dissolving carbonate rocks, but takes millennia to do it. This paper asked what happens chemically if that natural reaction is simply run at the power plant instead.
What did we find?
The overall reaction is the natural one, accelerated. Carbon dioxide plus water plus calcium carbonate yields a calcium ion and two bicarbonate ions.
The chemistry works because flue gas is concentrated. Power-plant exhaust carries about 0.15 atmospheres of CO2, over 400 times ambient; dissolved in seawater it gives a solution at pH 5.7, or 4.8 if pressurized to 1 atmosphere — corrosive enough to dissolve calcite, aragonite, and limestone, particularly when crushed.
What is released to the ocean is a relatively harmless solution of calcium and bicarbonate ions. The added alkalinity keeps the carbon in bicarbonate form, which cannot exchange directly with the atmosphere.
The method short-circuits the natural sequence. CO2 released to the atmosphere equilibrates with the surface ocean in under a year, mixes to depth over about 300 years, and is neutralized by carbonate dissolution over about 6,000 years, with silicate weathering supplying the cations for final burial over about 100,000 years. Direct injection skips the first two steps; carbonate dissolution at the source skips the natural dissolution step as well.
The comparison with direct injection is a matter of both capacity and chemistry: storing carbon as bicarbonate rather than dissolved CO2 avoids the local acidification that makes deep-sea injection biologically damaging.
Why does it matter?
It is one of the earliest quantitative treatments of what is now called ocean alkalinity enhancement, and it framed the idea correctly from the start — as accelerating a natural neutralization reaction rather than inventing a new sink.
The distinction it draws between storing carbon as CO2 and storing it as bicarbonate is the one that decides whether ocean carbon storage is permanent and whether it harms marine life.
Written before ocean acidification was widely discussed, the paper already treats the chemical impact on ocean biota as the criterion a disposal scheme has to meet.
Citation
Ken Caldeira and Greg H. Rau (2000). Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications. Geophysical Research Letters 27, 225-228.
Related
- How long does a carbon dioxide emission go on warming the planet?
- What does adding carbon dioxide to the ocean do to it?
- Anthropogenic carbon and ocean pH (Caldeira and Wickett, 2003)
- Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean (Caldeira and Wickett, 2005)
- Depth, radiocarbon, and the effectiveness of direct CO2 injection as an ocean carbon sequestration strategy (Caldeira et al., 2002)