Comparison of Phase Conditions of CO ₂ -Rich Fluids Measured at Different Hydrothermal Fields in the Okinawa Trough Using an In Situ Deep-Sea Raman Spectrometer

In this work, in situ liquid carbon dioxide (CO2)-rich fluid measurements using a deep-sea Raman spectrometer at different hydrothermal fields in the Okinawa Trough were performed. It was found that the water depths where the CO2 phase turned from liquid to gas ranged from 710 to 1060 m, which were deeper than the depth indicated in the phase diagram of the mixture of water (H2O) and CO2 (∼415 m), while our previous observation at the North-West Eifuku site at the Mariana Arc was consistent with the diagram. From observations with in situ Raman measurements and gas chromatography (GC) quantitative analysis of collected fluid samples, it can be assumed that the depth of the phase transition depends predominantly on the concentration of methane (CH4) and nitrogen (N2) in the fluid. The study demonstrates the ability of the Raman spectroscopic technique to be used as a real-time, on-site sensing tool for monitoring the dynamic phase transitions of CO2 and volatiles in fluids in the oceans. In addition, the apparatus reported in this work makes it possible to conduct multimodal liquid CO2-rich-fluid analysis by performing in situ continuous Raman measurements, GC analysis of the sampled fluid, and visual observation. This will lead to a better understanding of the role of CO2-rich fluids emitted from deep-sea hydrothermal fields in biogeochemical cycles and their contribution to acidification in the water column.