Ocean alkalinity enhancement (OAE) is a potential carbon dioxide removal strategy, but its efficiency and environmental impacts remain uncertain. A commonly invoked hypothesis is that alkaline minerals can be used to increase the alkalinity of seawater, and thus sequester atmospheric CO₂ as Dissolved Inorganic Carbon in seawater. The alkalinity efficiency would decrease by generating CaCO3 precipitation if the aragonite precipitation thresholds (Ω_aragonite) is too high. However, here we show that aragonite (CaCO3) precipitation is consistently achieved in coastal seawater at lower precipitation states.
Using brucite (Mg(OH)₂) as a source of alkalinity, we conducted one indoor (55 L) and two outdoor mesocosm experiments (50,000 L and 120,000 L) in Daya Bay (southern China). Brucite addition increased total alkalinity by up to 130 µmol kg⁻¹. Whilst initial biomass varied between the different experiments, particulate organic carbon showed similar trends to chlorophyll-a, with no significant effect of brucite addition compared to control treatments throughout, indicating that brucite addition had limited, or no, effect on phytoplankton biomass.
Despite these limited biological responses, electron microscopy revealed the presence of aragonite particles in the suspended matter of all brucite treatments within 24 hours of brucite addition. X-ray diffraction analysis further showed that CaCO₃ was the main inorganic component of sediment collected from treatments with brucite after 1 month of incubation, whereas sediments from the control remained dominated by SiO₂ from diatom detritus. Notably, aragonite formation consistently occurred in all brucite treatments despite low carbonate saturation states (Ω_aragonite< 5). This is likely due to the high particle loads in coastal seawater which facilitate heterogeneous calcium carbonate precipitation.
These results demonstrate that modest brucite addition can induce substantial secondary CaCO₃ precipitation without causing detectable impacts on phytoplankton biomass. Our findings suggest that particle density plays an important role in promoting secondary mineral formation during OAE, thereby influencing net alkalinity retention and the overall carbon dioxide removal efficiency.
 

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