The Concept:
The ocean contains 50 times as much carbon as the atmosphere and acts as a natural thermostat. Based on natural weathering that occurs on geological time scales, ocean alkalinity enhancement is intended to speed the process of removing CO2 from the atmosphere and reducing ocean acidification by increasing seawater alkalinity, the capacity of a solution to neutralize acid. This approach transforms CO2 into bicarbonate (HCO3-), carbonate (CO32-) and to a much smaller extent hydroxide (OH-) anions. The former are charge balanced by cations other than H+ (GESAMP, 2019), increasing pH and causing more drawdown of CO2 from the atmosphere (Gagern et al., 2019; Fig. 2; NASEM, 2021; Fig. 1). Ocean alkalinity enhancement aims to increase the alkalinity of the oceans by either:
- adding calcium carbonate (CaCO3) to the ocean from limestone rocks (Renforth and Henderson, 2017); calcium silicates (Ca₂O₄Si) from rocks, construction waste or desalination waste, slaked lime (calcium hydroxide Ca(OH)2; e.g., Caserini et al., 2021; Butenschön et al., 2021) as well as magnesium hydroxide (Mg(OH)2)) (Ocean Visions Road Map – https://www2.oceanvisions.org/roadmaps/ocean-alkalinity-enhancement/) or
- using electrochemistry – technologies for carbon dioxide removal from seawater, sometimes called “direct ocean capture” (House et al., 2007; Rau, 2008; Rau et al., 2013; Lu et al., 2015; La Plante et al., 2021). These techniques capture and remove dissolved inorganic carbon from seawater (either as CO2 gas or as calcium carbonate), and/or produce a CO2-reactive chemical base, e.g., sodium hydroxide (NaOH), that can be distributed in the surface ocean to ultimately consume atmospheric CO2 and convert it to long-lived, dissolved, alkaline bicarbonate (Ocean Visions Road Map –https://www2.oceanvisions.org/roadmaps/electrochemical-cdr/).
Alkalinity enhancement approaches will likely start in coastal areas more affected with ocean acidification, and will capture and store carbon dioxide predominantly in the form of bicarbonate. This will result in increases in pH and alkalinity as well as the aragonite saturation state.
Key Points
- Using silicate or carbonate minerals to achieve gigatonne scale CO2 removal would require very large quantities of these materials to be mined, crushed and distributed across the ocean.
- While mineral-induced changes in the form and flux of surface production might be reflected at the deep-sea floor, effects on the deep sea would mainly be in the long-term due to the ocean over turning circulation unless materials were directly placed in the deep sea. However, deep sea biota that have near-surface-dwelling larval stages could be adversely affected.
- The environmental effects of electrochemical alkalinization techniques on the deep sea is unclear except where acid material would be discharged directly into the deep sea. This could result in potential lethal and sub-lethal effects on organisms close to the discharge zone.
- Deposition of alkaline material into the ocean could be governed by the London Protocol.
….
DOSI, 15 March 2022. Policy brief.
