In Situ Raman Spectral Characteristics of Carbon Dioxide in a Deep-Sea Simulator of Extreme Environments Reaching 300 ℃ and 30 MPa

Abstract

Deep-sea carbon dioxide (CO₂) plays a significant role in the global carbon cycle and directly affects the living environment of marine organisms. In situ Raman detection technology is an effective approach to study the behavior of deep-sea CO₂. However, the Raman spectral characteristics of CO₂ can be affected by the environment, thus restricting the phase identification and quantitative analysis of CO₂. In order to study the Raman spectral characteristics of CO₂ in extreme environments (up to 300 ℃ and 30 MPa), which cover most regions of hydrothermal vents and cold seeps around the world, a deep-sea extreme environment simulator was developed. The Raman spectra of CO₂ in different phases were obtained with Raman insertion probe (RiP) system, which was also used in in situ Raman detection in the deep sea carried by remotely operated vehicle (ROV) “Faxian”. The Raman frequency shifts and bandwidths of gaseous, liquid, solid, and supercritical CO₂ and the CO₂–H₂O system were determined with the simulator. In our experiments (0–300 ℃ and 0–30 MPa), the peak positions of the symmetric stretching modes of gaseous CO_2, liquid CO₂, and supercritical CO₂ shift approximately 0.6 cm^–1 (1387.8–1388.4 cm^–1), 0.7 cm^–1 (1385.5–1386.2 cm^–1), and 2.5 cm^–1 (1385.7–1388.2 cm^–1), and those of the bending modes shift about 1.0 cm^–1 (1284.7–1285.7 cm^–1), 1.9 cm^–1 (1280.1–1282.0 cm^–1), and 4.4 cm^–1 (1281.0–1285.4 cm^–1), respectively. The Raman spectral characteristics of the CO₂–H₂O system were also studied under the same conditions. The peak positions of dissolved CO₂ varied approximately 4.5 cm^–1 (1282.5–1287.0 cm^–1) and 2.4 cm^–1 (1274.4–1276.8 cm^–1) for each peak. In comparison with our experiment results, the phases of CO₂ in extreme conditions (0–3000 m and 0–300 ℃) can be identified with the Raman spectra collected in situ. This qualitative research on CO₂ can also support the further quantitative analysis of dissolved CO₂ in extreme conditions.