Ocean alkalinity enhancement approaches and the predictability of runaway precipitation processes – Results of an experimental study to determine critical alkalinity ranges for safe and sustainable application scenarios

Abstract. To ensure the safe and efficient application of Ocean Alkalinity Enhancement (OAE), it is crucial to investigate its impacts on the carbonate system. While various modeling studies showed promising results in the past, there has been a lack of empirical data to support the applicability of this technology in natural environments. Recent studies have described the effect of runaway precipitation in the context of OAE, showing that calcium carbonate formation was triggered if certain Ω_aragonite saturation threshold levels were exceeded. This precipitation can adversely affect the carbon storage capacity and may in some cases result in CO₂ emissions. Experiments at the Espeland Marine Biological Station (Bergen, Norway) were conducted to systematically study the chemical consequences of OAE deployment. The experiments lasted for 20–25 days to monitor the temporal development of carbonate chemistry parameters after alkalinity addition and the eventually triggered carbonate precipitation process. Identified uniform patterns before and during the triggered runaway process can be described by empirical functional relationships. For the CO₂-equilibrated approaches, total alkalinity levels (TA) of up to 6500 µeq kgsw^-1 remained stable without loss of total alkalinity (TA) for up to 20 days. Higher implemented TA levels, up to 11200 µeq kgsw^-1, triggered runaway carbonate formation. Ones triggered, the loss of alkalinity continued until Ω_aragonite values leveled out at 5.8–6.0, resulting still in a net gain of 3600–4850 µeq kgsw^-1 in TA. The CO₂-non-equilibrated approaches, however, remained only stable for TA additions of up to 1000 µeq kgsw^-1. The systematic behavior of treatments exceeding this level allows to predict the duration of transient stability and the quantity of TA loss after this period. Once triggered, the TA-loss continued in the CO₂-non-equilibrated approaches until Ω_aragonite values of 2.5–5.0 were reached, in this case resulting in a net loss of TA. To prevent a net loss of TA, treated water must be diluted below the time-dependent critical levels of TA and Ω_aragonite within the identified transient stability duration. Identified stability and loss patterns of added TA depend on local environmental conditions impacting the carbonate system, like salinity, temperature, biological activity, and particle abundance. Implementation of such identified stability and loss patterns into ocean biogeochemical models, capable of resolving mixing patterns of treated and untreated water parcels, would allow to predict, from the geochemical perspective, safe local application levels of TA, as well as the fate of added alkalinity, and therefore a more realistic carbon storage potential as if neglecting observed carbonate system response to OAE.