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© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Koopmans, D., Meyer, V., Schaap, A., Dewar, M., Farber, P., Long, M., Gros, J., Connelly, D., & Holtappels, M. Detection and quantification of a release of carbon dioxide gas at the seafloor using pH eddy covariance and measurements of plume advection. International Journal of Greenhouse Gas Control, 112, (2021): 103476, https://doi.org/10.1016/j.ijggc.2021.103476. ; We detected a controlled release of CO2 (g) with pH eddy covariance. We quantified CO2 emission using measurements of water velocity and pH in the plume of aqueous CO2 generated by the bubble streams, and using model predictions of vertical CO2 dissolution and its dispersion downstream. CO2 (g) was injected 3 m below the floor of the North Sea at rates of 5.7–143 kg d − 1. Instruments were 2.6 m from the center of the bubble streams. In the absence of injected CO2, pH eddy covariance quantified the proton flux due to naturally-occurring benthic organic matter mineralization (equivalent to a dissolved inorganic carbon flux of 7.6 ± 3.3 mmol m − 2 d − 1, s.e., n = 33). At the lowest injection rate, the proton flux due to CO2 dissolution was 20-fold greater than this. To accurately quantify emission, the kinetics of the carbonate system had to be accounted for. At the peak injection rate, 73 ± 13% (s.d.) of the injected CO2 was emitted, but when kinetics were neglected, the calculated CO2 emission was one-fifth of this. Our results demonstrate that geochemical techniques can detect and quantify very small seafloor sources of CO2 and attribute them to natural or abiotic origins. ; This project received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 654462 (STEMM-CCS), it also received funding from the Max Planck Society and the Helmholtz Society. MHL was supported by US NSF grant # OCE-1657727.