Since the first discovery in 1989 , discharges of liquid and hydrate-coated CO2 has been observed in various submarine hydrothermal systems . Isotopic and compositional analyses have revealed that the venting CO2 derives from marine carbonate and mantle carbon, with a minor contribution from sedimentary organic matter . In the deep reaction zones of submarine hydrothermal systems, elevated temperature and pressure allows even molar-level dissolution of CO2 into hydrothermal fluids, whereas their upward flow and mixing with seawater reduce CO2 solubility, facilitating the phase separation of CO2 . The phase separation of CO2 in subseafloor upwelling regions has also been suggested from fluid inclusions in ~3.5-Gyr-old hydrothermal precipitates from the Pilbara craton, Australia . The Archean oceanic crust has recorded extensive carbonatization under CO2-rich seawater . Considering theoretical and laboratory simulations suggesting comparably, or even further, CO2-enriched surface environments on the Hadean Earth , widespread occurrence of CO2 vents and pools is a likely feature of the Hadean Ocean floor .