The nature of runaway precipitation and consequences for the safe applicability of OAE

To ensure a safe and efficient application of Ocean Alkalinity Enhancement (OAE) it is crucial to investigate its impact on biogeochemical systems. While various theoretical studies have shown promising results, there has been a lack of practical research to test the applicability of this technology in natural environments. Recent studies by Moras et al. (2022) and Hartmann et al. (2022) described the effect of runaway precipitation in the context of OAE. During this process Ca-carbonate formation is triggered, leading to a loss of the initially added alkalinity and counteracting the whole idea of OAE.

At a field campaign at the Espeland Marine Biological Station (Bergen, Norway) we examined the characteristics of runaway precipitation by using local natural seawater and storing the reactor bottles in a flow-through incubation chamber, mimicking the real-time temperature and light conditions of the Raunefjord. Conducted lab experiments lasted between 20-25 days, and tested CO₂-equilibrated and non-CO₂-equilibrated addition of alkalinity. The temporal development of the carbonate chemistry parameters was monitored after alkalinity addition and the triggered Ca-carbonate precipitation process was described in detail. We found that above upper critical limits of alkalinity addition in natural seawater, immediate precipitation prohibited an enhancement to higher alkalinity levels.  Our results could be helpful to guide the definition of upper limits of alkalinity for the safe and efficient application of OAE in an open sea scenario. In addition, the precipitates were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, to characterize the formed particles and follow their growth patterns.

Hartmann, J., Suitner, N., Lim, C., Schneider, J., Marín-Samper, L., Arístegui, J., Renforth, P., Taucher, J., and Riebesell, U. (2022). Stability of alkalinity in Ocean Alkalinity Enhancement (OAE) approaches – consequences for durability of CO2 storage, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2022-126

Moras, C. A., Bach, L. T., Cyronak, T., Joannes-Boyau, R., & Schulz, K. G. (2022). Ocean alkalinity enhancement–avoiding runaway CaCO 3 precipitation during quick and hydrated lime dissolution. Biogeosciences, 19(15), 3537-3557, https://doi.org/10.5194/bg-19-3537-2022