Countering the effect of ocean acidification in coastal sediments through carbonate mineral additions

Abstract. Along with its impact on calcifying plankton, ocean acidification also affects benthic biogeochemistry and organisms. Compared to the overlying water, fluid composition in sediments is altered through the effect of the mineralization of organic matter, which can further lower both pH and the carbonate saturation state. This can potentially be counteracted by the addition of carbonate minerals to the sediment surface. To explore the biogeochemical effects of mineral additions to coastal sediments, we experimentally quantified carbonate mineral dissolution kinetics, and then integrated this data into a reactive transport model that represents early diagenetic cycling of C, O, N, S and Fe, and traces total alkalinity, pH and saturation state of CaCO₃. Model simulations were carried out to delineate the impact of mineral type and amount added, porewater mixing and organic matter mineralization rates on sediment alkalinity and its flux to the overlying water. Model results showed that the added minerals undergo initial rapid dissolution and generate saturated conditions. Aragonite dissolution led to higher alkalinity concentrations than calcite. Simulations of carbonate mineral additions to sediment environments with low rates of organic matter mineralization exhibited a significant increase in mineral saturation state compared to sediments with high CO₂ production rates, highlighting the environment-specific extent of the buffering effect. Our work indicates that carbonate additions have the potential to effectively buffer surficial sediments over multiple years, yielding biogeochemical conditions that counteract the detrimental effect of OA conditions on larval recruitment, and potentially increase benthic alkalinity fluxes to support marine carbon dioxide removal (mCDR) in the overlying water.