Submarine groundwater discharge (SGD) is an important pathway for the transport of dissolved chemical substances to the ocean. In this study, we performed a time-series measurement of ²²²Rn, the concentrations and isotopic compositions of the different carbon species isotopes (particulate organic carbon, POC, dissolved organic carbon, DOC, dissolved inorganic carbon, DIC), physico-chemical parameters during a tidal cycle in a mangrove tidal creek without upstream freshwater inputs in northeastern Hainan Island, China. Our results show mangrove detritus provide the major sources of POC and DOC for the mangrove tidal creek water. Organic matter degradation and terrestrial groundwater discharge are two main processes of providing DIC for the pore water of the mangrove tidal flat sediments. A quantitative evaluation indicates that the amount of DIC production from the organic matter degradation process via sulfate reduction accounts for 59.8% of the total pore water DIC addition, while the proportion of DIC derived from terrestrial groundwater discharge is 40.2%. Using ²²²Rn mass balance model, we quantify the rates of SGD and its associated dissolved carbon: the average SGD rate in the mangrove tidal creek is approximately 165 ($\text{σ}$=68.73) cm d⁻¹, and the SGD-derived DIC and DOC are approximately 107 and 2.18 g C m⁻² d⁻¹, respectively. DIC is the dominant form of dissolved carbon export (∼98%) from mangroves tidal creek to the adjacent Qinglan Bay. The high DIC loading via SGD potentially represents the mangrove missing carbon, and has significant implications for the coastal ocean carbon budget.