In Situ Monitoring of the Liquid-Gas Phase Transition for CO₂-Rich Fluids Emitted at Deep-Sea Hydrothermal Vent Fields

One of the promising targets for a deep-sea Raman spectrometer, which enables label-free direct analysis for any phases of targets, is the supercritical and liquid carbon dioxide (sc-/liq-CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) rich fluid spontaneously emitted from hydrothermal vent fields. Sc-/liq-CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) creates a unique dry environment distinct from seawater as a non-aquatic solvent, which could play a potentially significant role in ocean geochemical cycles. In addition, the behaviour of the CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-rich fluids in the water column, particularly the water depth of the fluid phase transition from liquid to gas, is important to understand the transport system of the volatiles in the fluids as well as the potential impact on acidification of the water column. However, contamination from the seawater during collection and measurement is unavoidable, and the dynamic phase and compositional changes cannot be measured by sampling-based methods. In this paper, the history of the development of deep-sea Raman spectrometers is first reviewed, and a novel method for sampling and analysing CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-rich fluids in different phases using a deep-sea Raman spectrometer mounted on a remotely operated vehicle (ROV) is reported. In situ monitoring of the phase of liq-CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-rich fluids while the ROV ascended was performed at different hydrothermal ventfields in the Mariana Arc and Okinawa Trough. The in situ Raman measurement revealed that the phase transition occurred at different depths depending on the fluid compositions. The study demonstrates the ability of the Raman spectroscopy to monitor the phase transition of CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-rich. fluids and the chemical composition in real time on site, showing the ability as a novel marine geochemical sensing technique.