Potential of various minerals and their biogeochemical implications for ocean alkalinity enhancement in the southeastern Arabian Sea

Abstract. Ocean alkalinity enhancement (OAE) is emerging as a promising yet largely untested marine carbon dioxide CO2 removal approach. It involves the addition of alkaline substances such as powdered minerals and aqueous hydroxide solutions to seawater, shifting the carbonate chemistry speciation towards carbonate ions so as to store more CO2. Contemporaneous studies are being carried out to evaluate the efficacy, durability, and risks associated with these substances. Given the heterogeneity in a natural ecosystem, each substance will have distinct implications on carbonate chemistry as well as biogeochemistry of a given ecosystem. Our study contributes to ongoing research by examining the response of the ocean's carbonate chemistry to the addition of various alkalinity feedstocks — including both naturally occurring and anthropogenically (industrially) produced minerals, along the southeastern coastal Arabian Sea. We tested the alkalinity (AT) generation potential and traced the associated changes in the carbonate chemistry speciations for three naturally occurring minerals: (i) olivine ((MgFe)2SiO4), (ii) kaolinite (Al2Si2O5(OH)4), and (iii) dolomite (CaMg(CO3)2), and for two anthropogenically produced minerals: (i) periclase (MgO) and (ii) hydrated lime (Ca(OH)2) of two different mineral compositions, using 300 L mesocosms. Overall, no significant changes in AT, pH and dissolved inorganic carbon (DIC) were observed for the naturally occurring minerals, suggesting the lower efficiency of these minerals to increase AT,. In contrast, the dissolution of periclase and hydrated lime increased AT (up to 16 %, which corresponds to 80 % of the total added AT) and pH by up to 0.6 units. We further demonstrate that the temporal changes in the carbon isotopic composition (δ13C) of DIC as well as the changes in the DIC concentration occurring within the mesocosms can serve as an effective and reliable proxy for tracing secondary carbonate precipitation. As loss of alkalinity via secondary precipitation diminishes the overall efficiency of the OAE approach, accurate determination of the threshold at which secondary precipitation is triggered is critical for maximizing the effectiveness of this method. We determine these thresholds and provide an assessment of various alkalinity feedstocks that could work best for OAE.