Technical note: Verification of coastal enhanced weathering by tracking the dissolution of alkaline minerals: Theory and laboratory tests with olivine

Abstract. Coastal Enhanced Weathering (CEW) is a marine Carbon Dioxide Removal (CDR) method that adds ground alkaline minerals to shallow regions of the ocean in order to increase seawater alkalinity, i.e., its capacity for storing atmospheric CO₂ as bicarbonate. While CEW is promising with regard to cost and scalability, it is an uncontained, “open-system” style of CDR, and presents significant challenges to effective measurement, reporting, and verification (MRV) of the process. In particular, quantifying how much alkalinity is released from an added amount of mineral is challenging as the minerals dissolve and release alkalinity over wide spatial and long temporal scales. Such quantification is further complicated by the fact that dissolved alkalinity is rapidly diluted below detectable concentrations. Here, we propose an approach to measure alkalinity formation that relies on solid sediment tracers to track mineral grains underwater and quantify how much they have dissolved over time. The amount of dissolution at any given point in time is proportional to the amount of alkalinity released. Thus, the approach aims to overcome the near impossible detection of alkalinity accumulation in the dissolved phase by tracking the loss of alkaline material in the solid phase. We describe a test of the fundamental aspects of this method including the measurement of the mineral and tracer content of a sediment sample, and the extent to which those measurements correlate with changes in seawater chemistry. We found that olivine dissolution significantly increased alkalinity in seawater, compared to control incubators. X-Ray Diffraction (XRD) was able to quantify the change in the olivine in the sediment (albeit with large relative errors) and automated particle counting methods were able to enumerate tracer particles when illuminated under UV-A light. These results serve as a proof-of-principle (and starting point) to further explore a promising way to measure alkalinity addition to the ocean resulting from CEW deployments.