In situ detection of CO2/CH4 dissolved in vent-associated seawater at the CLAM and Iheya North hydrothermal vents area, Okinawa Trough

In situ CO₂ and CH₄ concentrations dissolved in vent-associated seawater (within ten meters from the seafloor) in mid-Okinawa Trough had been measured by novelty CO₂ sensor and CH₄ sensor based the Remotely Operated Vehicle (ROV) mounted on R/V KEXUE. The results showed that the maximum values of CO₂ and CH₄ both occur near active hydrothermal vents in Iheya North area. The maximum detection concentration of CO₂ was as high as 12000 ppm, which is four orders of magnitude higher than previous onboard analyses. At the meantime, the maximum concentration of CH₄ was 8 μmol/l, which is two orders of magnitude higher than reported before. On the other hand, only small quantities of CO₂ and CH₄ were reported near diffusive fluids as the CLAM hydrothermal area has become more and more inactive. However, based on the measurement principle of novelty sensors and their adaptability, the in situ real gas concentrations may be even higher than the present values. Therefore, it is still necessary to develop new high precision sensors, which are designed to adapt to more severe deep-sea environment like high temperature hydrothermal vents, to have a comprehensive understanding of gas geochemistry in hydrothermal fields in the future.