Saltmarsh plays a crucial role in mitigating global CO₂ increase through carbon storage in sediments and in the ocean following lateral carbon export (outwelling). Inorganic carbon outwelling in the form of alkalinity (TA) has received limited attention. This study was carried out on the Parker River at the Plum Island estuary, the largest macro tidal saltmarsh system in Northeastern US. We found the average TA outwelling flux is 64.47 mmol d^-1 m^-2, and the average DIC outwelling flux is 85.90 mmol d^-1 m^-2. Porewater-derived flux contributes to 98% of the TA outwelling flux and 82% of the DIC outwelling flux. The TA and DIC fluxes are modulated by temperature diurnally and are controlled by marsh growing pace and hydrological factors seasonally.  The TA: DIC ratio fluctuates below and above 1, and is determined by the relative intensity among primary production, aerobic respiration, and anaerobic respiration. The relative abundance of TA and DIC determines the water buffer capacity. In most cases, the saltmarsh exports more DIC, which improves the water buffer capacity even though acidifies the surrounding water. The TA: DIC fluctuation patterns also show spatial differences, which coincide with porewater flux spatial heterogeneity; downstream porewater inorganic carbon flux is five times higher than upstream porewater flux.