Ocean alkalinity enhancement (OAE) aims to reduce atmospheric CO₂ by shifting the speciation of the seawater carbonate system away from CO₂ toward carbonate and bicarbonate ions. This shift in carbonate chemistry elevates the saturation state (Ω) of calcium carbonate (CaCO₃), which can trigger a negative feedback through enhanced carbonate mineral precipitation, reducing both seawater alkalinity and CO₂ uptake capacity. Similarly, elevated Ω can suppress natural alkalinity generation via the buffering of natural CaCO₃ dissolution in marine sediment. Together, these feedbacks have direct implications for the additionality of OAE, that is the achieved CO₂ sequestration beyond the natural baseline, as both secondary precipitation and a reduction in the natural alkalinity source can offset a fraction of the intended carbon dioxide removal (CDR) benefit. Such OAE-induced feedbacks constitute a significant and poorly constrained source of uncertainty in assessing its net CDR potential.

To address these knowledge gaps, we conducted a set of in situ benthic flux chamber deployments across three coastal sites in southeastern Tasmania (Australia) to assess (1) the variability in natural alkalinity release from coastal sediments and (2) the impact of both dissolved (NaOH) and mineral-based (slag, aglime, olivine) alkalinity additions on alkalinity generation and CO₂ sequestration across varying sediment properties and environmental regimes. Natural alkalinity release from sediments varied more than threefold across sites and was closely linked to metabolic activity, with sites and periods with higher rates of organic matter breakdown showing correspondingly higher natural alkalinity fluxes. Alkalinity generation during OAE treatments, in turn, depended on both alkaline feedstock and local biogeochemical conditions. For example, NaOH treatments ranged from near-target addition (+400 μmol kg^-1) at one site to substantial alkalinity loss at a second site, with a loss in efficiency likely linked to elevated particle load triggering secondary precipitation. Aglime treatments ranged from a net alkalinity sink to a net alkalinity source relative to baseline conditions, tracking site-specific carbonate chemistry conditions, with a shift toward net dissolution at sites with lower Ω. Collectively, these findings demonstrate that both baseline alkalinity fluxes and net alkalinity generation vary with environmental conditions, and that OAE additionality cannot be assumed constant across coastal deployments.