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Increasing public awareness of anthropogenic CO2 emissions and consequent global change has stimulated the development of pragmatic approaches for the study of shallow-water CO2 vents and seeps as natural laboratories of CO2 perturbations. How CO2 propagates from the emission sites into surrounding environments (ocean and atmosphere), and its effects on seawater carbonate chemistry, have never been studied from a mechanistic perspective. Here, we combine experimental and modeling approaches to investigate the carbonate chemistry of a shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan. A simple Si-based mixing model is used to trace hydrothermal fluid mixing with seawater along convection pathways. The estimated vent fluid component in the near-vent region is generally < 1%. We further employed a modified bubble-plume model to examine gas bubble-aqueous phase interaction. We explain the dissolved inorganic carbon characteristics of the vertical plume as a synergistic interaction between CO2 gas dissolution and fluid entrainment. The bubble-plume model provides a conservative estimate of the flushing time (tens of minutes) for water in the near-vent region. The acidic, dissolved inorganic carbon-rich water in the lateral buoyant plume readily releases CO2, but mixing with seawater rapidly quenches its degassing potential, so that hydrothermal carbon is retained in the ocean. Ebullition, governed by initial bubble size distribution, is the key mechanism for vent CO2 to exit the seawater carbonate system.