Article  

The concentration of carbon dioxide (CO2) in the atmosphere remained in a relatively narrow band from ∼180 to ∼280 ppmv for 800 000 years but has risen rapidly over the last 250 years to approximately 420 ppmv today (Lüthi et al., 2008; Monnin et al., 2001; Siegenthaler et al., 2005). This is the result of increasing utilisation of fossil fuels, cement production and land-use change, driving subsequent global climate change (IPCC, 2021). While about 42 % of CO2 emissions remain in the atmosphere and are mainly responsible for global warming, about 26 % are currently absorbed by the oceans, leading to ocean acidification (Friedlingstein et al., 2022; IPCC, 2021). To mitigate the effects of ocean acidification locally and slow down the increase in the Earth's global temperature, CO2 reduction efforts are not sufficient, and carbon dioxide removal (CDR) strategies have become necessary as a supplement to emission reduction (Hoegh-Guldberg et al., 2019).

One emerging marine CDR approach is ocean alkalinity enhancement (OAE). Over long timescales, the natural CO2-facilitated weathering of alkaline rocks supplies alkalinity to the oceans, influencing their CO2 uptake potential and storage. OAE builds upon this weathering feedback in the Earth system and can be accomplished by actively spreading pulverised alkaline minerals in and around marine environments or by electrochemically removing acidity from seawater (Eisaman et al., 2023). In both cases, the seawater total alkalinity (AT) is increased, thereby increasing the atmospheric CO2storage capacity of seawater (GESAMP, 2019; Kheshgi, 1995). On local scales around areas where the OAE perturbation is performed, the increases in alkalinity and pH may also mitigate ocean acidification (Hartmann et al., 2013).

Recent studies have investigated the carbonate chemistry changes following OAE, and a major outcome was the risk of runaway calcium carbonate (CaCO3) precipitation (Fuhr et al., 2022; Hartmann et al., 2023; Moras et al., 2022). There are several inorganic CaCO3 precipitation mechanisms that have been described in the literature (Morse et al., 2007; Pytkowicz, 1965). CaCO3 can precipitate homogeneously in the absence of solid or soluble organic and inorganic particles, pseudo-homogeneously in the presence of organic surfaces, and heterogeneously in the presence of mineral solids (Marion et al., 2009). The key parameter that governs whether precipitation occurs is the calcium carbonate saturation state (Ω), which is calculated from seawater Ca2+ and CO32- concentrations as

where Ca2+ and CO32- are the concentrations of calcium and carbonate in solution, respectively, and Ksp the solubility product of CaCO3 in the solution. Ω is therefore closely related to the composition of the solution and its salinity but is also highly temperature dependent (Zeebe and Wolf-Gladrow, 2001). For aragonite, the CaCO3 morphotype that inorganically precipitates in modern seawater, the saturation state (ΩA) has to be higher than 12.3 for pseudo-homogeneous precipitation to occur in water with a salinity of 35 and at 25 °C (Marion et al., 2009). Homogeneous precipitation will occur at much higher ΩA values, while heterogeneous precipitation will occur at much lower ΩA but depends on the actual lattice compatibility of CaCO3 with the mineral particles present (Morse et al., 2007; Zhong and Mucci, 1989). Another important aspect is that once precipitation becomes measurable, it will continue in a “runaway” fashion, i.e. quickly ramping up until it slows down once ΩA approaches 1 again.

Several studies have reported such behaviour after the addition of alkaline minerals (Fuhr et al., 2022; Hartmann et al., 2023; Moras et al., 2022), with a critical threshold of ΩA = ∼7.0 for the two calcium-based OAE minerals - calcium oxide (CaO) and calcium hydroxide (Ca(OH)2) - and report precipitation stopping at ΩA values of 1.8-2.0 (Moras et al., 2022). Precipitation has also been observed for magnesium-based minerals such as brucite or reagent-grade magnesium hydroxide - Mg(OH)2 - but actual thresholds have not been determined (Hartmann et al., 2023). Furthermore, the effect of grain size, the determining factor for the surface area available for mineral dissolution and CaCO3 precipitation, has not been studied. Similarly, the effect of potential CaCO3 precipitation inhibitors, such as seawater magnesium (Mg) concentrations governed by salinity and dissolved organic carbon (DOC), are relatively unknown (Chave and Suess, 1970; Millero et al., 2001; Pan et al., 2021; Zhong and Mucci, 1989). This study focuses on the impact of Mg(OH)2 grain size on its dissolution kinetics in natural seawater as well as the impact of salinity. Furthermore, the subsequent runaway CaCO3 precipitation that is triggered, along with its kinetics, are reported. Finally, the effects of increased Mg and DOC in seawater on the CaCO3 precipitation process is explored.