In Situ Measurement of Liquid and Gas CO<sub>2</sub> with High Purity at Deep-Sea Hydrothermal Vents in the Mariana Arc Using Raman Spectroscopy
Tomoko Takahashi, Wataru Takahagi, Eiji Tasumi, Akiko Makabe, Koudai Taguchi, Blair Thornton, Ken Takai
Abstract
Supercritical and liquid CO 2 (sc-/liq-CO 2 ) 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-CO 2 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 CO 2 in different phases in which in situ Raman measurements can be directly performed and applied them to the CO 2 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 CO 2 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 CO 2 ─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 CO 2 emission consisted of the gas phase. The in situ Raman measurement also revealed that gas H 2 S 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 CO 2 emissions and the chemical composition in different phases of CO 2 in the oceans in real time.