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 modeling studies reported a sequestration potential of 3–30 Gt carbon dioxide (CO 2 ) per year (Oschlies et al., 2023), 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 carbonate precipitation in the context of OAE, showing that calcium carbonate (CaCO 3 ) formation was triggered if certain Ω aragonite saturation thresholds were exceeded. This effect could potentially lead to a net loss of the initially added alkalinity, counteracting the whole concept of OAE. The related precipitation can adversely affect the carbon storage capacity and may in some cases result in CO 2 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 d to monitor the temporal development of carbonate chemistry parameters after alkalinity addition and the subsequent triggered carbonate precipitation process. Identified uniform patterns before and during the triggered runaway process can be described by empirical functional relationships. For approaches equilibrated to the CO 2 concentration of the atmosphere, total alkalinity (TA) levels of up to 6500 µmol kg −1 remained stable without loss of total alkalinity (TA) for up to 20 d. Higher implemented TA levels, up to 11 200 µmol kg −1 , triggered runaway carbonate formation. Once triggered, the loss of alkalinity continued until the Ω aragonite values leveled out at 5.8–6.0, still resulting in a net gain of 3600–4850 µmol kg −1 in TA. The non-CO 2 -equilibrated approaches, however, only remained stable for TA additions of up to 1000 µmol kg −1 . The systematic behavior of treatments exceeding this level allows us to predict the duration of transient stability and the quantity of TA loss after this period. Once triggered, the TA loss continued in the non-CO 2 -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, such as salinity, temperature, biological activity, and particle abundance. Incorporating such stability and loss patterns into ocean biogeochemical models, which are capable of resolving dilution processes of treated and untreated water parcels, would, from a geochemical perspective, facilitate the prediction of safe local application levels of OAE. This approach would also allow an accurate determination of the fate of added alkalinity and a more realistic carbon storage potential estimation compared to the assessments that neglect carbonate system responses to OAE.