AbstractThe residual hydrothermalism associated with submarine volcanoes, following an eruption event, plays an important role in the supply of CO₂ to the ocean. The emitted CO₂ increases the acidity of seawater. The submarine volcano of El Hierro, in its degasification stage, provided an excellent opportunity to study the effect of volcanic CO₂ on the seawater carbonate system, the global carbon flux and local ocean acidification. A detailed survey of the volcanic edifice was carried out using seven CTD-pH-ORP tow-yo studies, localizing the redox and acidic changes, which were used to obtain surface maps of anomalies. In order to investigate the temporal variability of the system, two CTD-pH-ORP yo-yo studies were conducted that included discrete sampling for carbonate system parameters. Meridional tow-yos were used to calculate the amount of volcanic CO₂ added to the water column for each surveyed section. The inputs of CO₂ along multiple sections combined with measurements of oceanic currents produced an estimated volcanic CO₂ flux = 6.0 10⁵ ± 1.1 10⁵ kg d⁻¹ which is ~0.1% of global volcanic CO₂ flux. Finally, the CO₂ emitted by El Hierro increases the acidity above the volcano by ~20%.