In situ measurement of liquid and gas CO₂ with high purity at deep-sea hydrothermal vents in the Mariana Arc using Raman spectroscopy

Abstract

Supercritical and liquid CO₂ (sc-/liq-CO₂) emitted from deep-sea hydrothermal vents create a unique dry environment distinct from seawater and hydrothermal fluids, whose physicochemical characteristics could play an important role in the ocean biogeochemical cycles of the present Earth and even in the prebiotic chemical evolution of the early Earth. While previous studies attempted to sample and analyze sc-/liq-CO₂ in several hydrothermal fields, the sampling and analysis without seawater contamination have been unsuccessful. In this study, we developed the method and apparatus for sampling and analyzing CO₂ in different phases in which in situ Raman measurements can be directly performed and applied them to the CO₂ emissions in two hydrothermal vent fields in the Mariana Arc. The in situ Raman spectra taken at both fields indicate that the high purity of CO₂ emissions without seawater contamination was successfully sampled and measured. In the North-West Eifuku seamount, the collected hydrothermal fluid was monitored from the seafloor (approximately 1600  m) to the surface. The phase transitions of CO₂─hydrate, liquid, and gas─were successfully observed in the Raman spectra. At the Daikoku seamount, the in situ Raman spectra taken at the seafloor (approximately 400 m) identified that the CO₂ emission consisted of the gas phase. The in situ Raman measurement also revealed that gas H₂S was abundant in the emissions at both the fields. This study demonstrates the ability of the Raman spectroscopic technique to monitor the phase transition of hydrothermal CO₂ emissions and the chemical composition in different phases of CO₂ in the oceans in real time.

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