[S-CG48] Ocean Floor Geoscience

[SCG48-19] In Situ Measurement of Liquid and Gas CO₂ at Deep-sea Hydrothermal Vents in the Mariana Arc and Okinawa Trough using Raman Spectroscopy

*Tomoko Takahashi^1,5, Eiji Tasumi¹, Wataru Takahagi^2,1, Akiko Makabe¹, Koudai Taguchi³, Shotaro Tagawa^3,1, Blair Thornton^4,5, Ken Takai¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Rensselaer Polytechnic Institute, 3.Tokyo Institute of Technology, 4.University of Southampton, 5.The University of Tokyo)

Keywords:Mariana Arc, Okinawa Trough, Raman spectrometry, Supercritical/liquid carbon dioxide

Deep-sea hydrothermal vent fields where the supercritical/liquid/gas carbon dioxide (sc-/liq-/gas CO₂) are spontaneously emitted have been found in the Mariana Arc and the Okinawa Trough [1]. Particularly, sc-/liq-CO₂ creates a unique dry environment distinct from seawater as a non-aquatic solvent, which could play a potentially significant role in ocean geochemical cycles and even in the prebiotic chemical evolution of the early Earth. While previous studies attempted to sample and analyse sc-/liq-CO₂ in several hydrothermal fields, the sampling and analysis without seawater contamination have been unsuccessful [2, 3]. In this study, we developed a method for sampling and analysing CO₂ in different phases using Raman spectroscopy and applied it to the CO₂ emissions in different hydrothermal vent fields in the Mariana Arc and Okinawa Trough. The in situ Raman spectra taken at the hydrothermal fields indicate the high purity of CO₂ emissions without seawater contamination was successfully sampled and measured. In the Mariana Arc, in situ Raman measurements confirmed that CO₂ emission in the hydrothermal fluid at the North-West Eifuku seamount (approx. 1600 m water depth) consisted of a hydrate of liquid CO₂, whereas CO₂ emission at the Daikoku seamount (approx. 400 m water depth) consisted of the gas phase. The in situ Raman measurement also revealed that gas H₂S was abundant in the emissions at both fields. The phase transitions of CO₂ at the North-West Eifuku seamount were monitored; hydrate (seafloor), liq- (from seafloor to 415 m), and gas (from 415 m to the surface) were successfully observed in the Raman spectra, which follow the theoretical CO₂-water phase diagram [4]. At the hydrothermal vents in the Okinawa Trough, however, the CO₂ phase transitions from liquid to gas were observed at different depths from 1000 to 600 m, depending on locations. From the in situ Raman spectra and quantitative analysis of collected fluid samples, it is assumed to be because of other components in the hydrothermal fluids, such as methane and nitrogen. The study demonstrates the ability of the Raman spectroscopic technique to monitor the phase transition of hydrothermal CO₂ emissions and the chemical composition of the fluids in real time on site, showing the ability as a novel geochemical sensing technique at the deep-sea seafloor.

[1] Shibuya, T.; Takai, K. Liquid and supercritical CO₂ as an organic solvent in Hadean seafloor hydrothermal systems: implications for prebiotic chemical evolution. Progress in Earth and Planetary Science 2022, 9, 1–15.
[2] Lupton, J.; Butterfield, D.; Lilley, M. et al. Submarine venting of liquid carbon dioxide on a Mariana Arc volcano. Geochemistry, Geophysics, Geosystems 2006, 7.
[3] Zhang, X.; Li, L.; Du, Z., et al. Discovery of supercritical carbon dioxide in a hydrothermal system. Science Bulletin 2020, 65, 958–964.
[4] Takahashi, T., Takahagi, W., Tasumi, E. et al. In Situ Measurement of Liquid and Gas CO₂ with High Purity at Deep-Sea Hydrothermal Vents in the Mariana Arc Using Raman Spectroscopy. ACS Earth and Space Chemistry 2023 7, 2489-2497.