Surface area and Ω-aragonite oversaturation as controls of the runaway precipitation process in ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is a strategy for marine carbon dioxide removal that aims to increase the total alkalinity (TA) of seawater to sequester atmospheric CO₂ in the form of dissolved inorganic carbon (DIC). An intense alkalinization of seawater resulting from OAE treatment could trigger a significant runaway carbonate precipitation process, which may lead to a loss of initially added alkalinity, thereby limiting its efficiency. Even under natural background aragonite saturation states, a continuous yet barely detectable loss of alkalinity is theoretically expected to occur in seawater. With the additional increase through OAE, time ranges to initiate an appreciable TA loss process could be reduced significantly. Therefore, predicting the alkalinity stability ranges might be a necessity for application scenarios. The main drivers of the precipitation process are i) the aragonite saturation state of seawater and ii) the available surface area for heterogeneous precipitation.

In this study, we refined the use of logistic functions to describe the temporal evolution of both drivers, with experimental datasets using natural seawater from the Raunefjorden (Bergen, Norway; Temp.: ~11 °C, Sal.: ~32.6). The observed patterns were then used to derive a process-based model for calculating TA-loss rates, focusing on the accelerated precipitation phase of the runaway process while considering saturation levels and available particle surface area. The formation of carbonate phases reduces seawater TA concentrations, inducing a delay or stopping the TA-loss process. In addition, the sinking of precipitated particles decreases the potential for further precipitation by reducing the available surface area in the system. To assess the impact of particle sinking on TA-loss, their shape and size distribution were determined. Under the environmental conditions presented here, TA-loss rates could be reduced by up to 30–40 % due to the sinking of particles, after just one day.

Integrating the proposed concepts into ocean models could enhance the accuracy of predictions regarding the fate of added alkalinity. Gaining insights into the evolution of the identified, seemingly stable TA levels can help prevent accelerated precipitation phases. Additionally, an understanding of particle sinking or dilution processes reducing the available reactive particle surface area is relevant to assess the efficacy and durability of OAE.