Liquid CO₂ venting on the seafloor: Yonaguni Knoll IV hydrothermal system, Okinawa Trough

[1] We determined the chemical and isotopic compositions of the liquid CO₂ found on Yonaguni IV knoll hydrothermal site, as well as those in hydrothermal fluid venting from the surrounding chimneys. The δ¹³C of both CO₂ and CH₄ in the liquid CO₂ almost coincide with those in the hydrothermal fluid, suggesting that the liquid CO₂ must be derived from the hydrothermal fluid. While showing homogeneous δ¹³C, the hydrothermal fluids exhibit wide variation in gas contents. Active phase separation must be taking place within the conduits. Besides, H₂-depletion in the liquid CO ₂ suggests formation of solid CO₂-hydrate must also precede the venting of liquid CO₂. In conclusion, liquid CO ₂ must be produced through following subseafloor processes: phase separation of hydrothermal fluid due to boiling, formation of solid CO ₂-hydrate due to cooling of vapor phase, and melting of the solid CO₂-hydrate to liquid CO₂ due to a temperature increase within the sedimentary layer.