Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ¹³C_DIC and ²²⁴Ra–²²⁸Th disequilibria

Abstract. Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO₂ accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO₃) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO₃ dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the ²²⁴Ra–²²⁸Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ¹³C_DIC) were employed to trace DIC sources in porewater. The results showed that CaCO₃ dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO₃ dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO₂ accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO₂ content, the TA supplied by the CaCO₃ dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO₃ dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO₂ elevation.