Whilst natural blue carbon sinks can be highly efficient, they are simply not large enough to balance more than a few percent of annual anthropogenic emissions. There is therefore growing interest in novel Marine Negative Emission Technology solutions such as Ocean Alkalinity Enhancement (OAE) to assist carbon neutrality. OAE proposes to artificially raise alkalinity in natural waters to stabilize higher concentrations of dissolved inorganic carbon and induce drawdown of CO₂ from the atmosphere to the ocean. But how safe is this method with respect to ecological or biogeochemical side-effects and what magnitude of CO₂ sequestration could be realistically obtained? In 2023 we hosted two large 50,000 L scale incubation experiments in Daya Bay to test the basic theory of OAE using Mg(OH)₂ additions to seawater under contrasting wet/dry season conditions. Mg(OH)₂ additions were efficiently converted to dissolved total alkalinity (TA) resulting in pCO₂ undersaturation and higher pH conditions. Whilst monitoring of macronutrients and basic biological parameters suggested minimal changes to productivity or community composition at TA additions of up to +250 µmol L^-1, evidence of calcite precipitation was found in most experiments over timescales of >20-30 days. Therefore, carbon budgets for TA additions may need to constrain inefficiencies associated with unequilibrated TA stability before the effectiveness of OAE can be properly evaluated
 

Ocean Alkalinity Enhancement,Carbon sink,Mesocosm