[Department of Ocean Science] MPhil Thesis Defense Seminar: Assessment of Coastal Mesocosm Ocean Alkalinity Enhancement: From Metal Dynamics to Potential Alkalinity Loss
 

Abstract: 

This thesis evaluates mineral-based ocean alkalinity enhancement (OAE) in coastal waters from a risk-centred perspective. Rather than treating alkalinity addition as automatically durable or environmentally safe, it assesses how different feedstocks alter carbonate chemistry, trace metal dynamics, particle transformation, dissolved organic matter (DOM) molecular composition, biological response, and potential alkalinity loss. Outdoor mesocosm experiments showed that feedstock identity controlled both benefit and risk. In a 50,000 L system, Brucite generated stronger total alkalinity and pH enhancement with weaker sustained trace metal enrichment. Steel slag generated weaker measured alkalinity enhancement during the observation period, but caused sustained dissolved Mn, Cu, Zn, and As enrichment, redistributed metals into particulate reservoirs, and developed a late particle state dominated by coccolithophore cells and coccoliths. FTICR-MS further showed treatment-specific DOM molecular trajectories during this biological transition.

The thesis then examined alkalinity retention and authigenic clay formation in diatom-rich coastal waters. In an Mg(OH)₂ mesocosm, the observed total alkalinity increase reached only about 60% of the calculated addition. SEM-EDS observations showed Ca-rich crystals associated with suspended and settled biogenic aggregates, indicating that carbonate precipitation can create a hidden pathway of alkalinity loss. Brucite treatments also contained Mg-rich rosette phases, while Steel slag showed localized clay-like textures on mineral surfaces. These observations suggest that mineral OAE may enhance reverse weathering and authigenic clay formation.
Controlled diatom culture experiments showed that Al effects on growth were limited and species- or nutrient-dependent, while photosynthetic responses were generally weak. SEM-EDS Al/Si analysis supported surface-scale Al association with diatom frustules under higher Al exposure, but did not show a simple concentration-dependent increase. Nutrient-replete high-Al cultures unexpectedly formed Mg-rich rosette phases within diatom-rich aggregates over a short growth cycle. Overall, coastal OAE assessment must be feedstock-specific: Brucite-like materials require attention to alkalinity retention, carbonate precipitation, and authigenic clay formation, whereas Steel slag-like materials require stronger evaluation of trace metal release, particle redistribution, DOM change, and biological succession.