In situ Raman spectroscopic measurement of the ¹³C/¹²C ratio of dissolved CO₂ at high temperatures and pressures: Method and implications

Submarine hydrothermal activities release a large amount of greenhouse gases such as CO₂ and CH₄ into the ocean, influencing the global carbon cycle. Carrying out high-temperature and high-pressure hydrothermal experiments to simulate these geochemical processes is a prerequisite for clarifying the source of carbon-containing substances in the hydrothermal fluid. In situ monitoring of the change in carbon isotope composition of CO₂ is essential for high-temperature and high-pressure simulation experiments, but it is also a great technical challenge. In recent years, laser Raman spectroscopy has attracted wide attention as a supplementary means to mass spectrometry for measuring the ¹³C/¹²C value of CO₂. However, the existing research is limited to the Raman spectroscopy study of the carbon isotope composition of supercritical/liquid CO₂, and there is little research on dissolved CO₂ in solution. In this study, we systematically studied the Raman spectral characteristics of dissolved ¹³CO₂ and ¹²CO₂ in the H₂O ± ¹³CO₂ ± ¹²CO₂ system at 25–300 °C and 10–350 bar. The results show that the peak position of the Raman characteristic band of ¹³CO₂ (aq) is 1367–1370 cm⁻¹, which is 14–17 cm⁻¹ lower than that of ¹²CO₂ (aq), and the full width at half maximum is 2–3 cm⁻¹ smaller than that of ¹²CO₂, which indicate the ¹³CO₂ (aq) and ¹²CO₂ (aq) can be identified by Raman spectroscopy. On this basis, we proposed the optimal likelihood curve fitting method (OLCF) for the first time to decompose the overlapping bands and accurately obtain the peak height ratio (H₁₃/H₁₂) of dissolved ¹³CO₂ and ¹²CO₂. It has been shown that the G-factor ratios (G₁₃/G₁₂) are significantly affected by temperature and the relative content of ¹³CO₂ and ¹²CO₂. Obtaining an appropriate G-factor ratio is a prerequisite for accurately determining the ¹³C/¹²C of dissolved CO₂. Based on the functional relationship between the H₁₃/H₁₂ and the ¹³C/¹²C, we established an empirical equation to quickly estimate the ¹³C/¹²C value, thereby assisting in selecting an appropriate G-factor ratio. The calibrated G-factor can be well used to determine the ¹³C/¹²C of dissolved CO₂ in the hydrothermal experiments with ¹³C labelled. In-situ monitoring experiments show that the phase separation of the hydrothermal fluid hardly causes changes in the carbon isotope composition of dissolved CO₂. However, the contamination of organic matter, such as oxalic acid, will result in a higher content and a more negative δ¹³C of CO₂ in the hydrothermal fluids.