Recently, the research team of Xin Zhang from Institute of Oceanography, Chinese Academy of Sciences (IOCS) has made new progress in the in situ identification and quantification of methane carbon isotopes based on in situ laser Raman spectroscopy, utilizing the significant difference in Raman spectra of methane carbon isotopes (13CH4 and 12CH4), and the related results have been recently published in the international spectroscopy journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.
Deep-sea hydrothermal systems release large amounts of CH4, H2 and other reducing gases that feed unique chemosynthetic biomes, which are important for studying the origin of early life. However, the source of such high methane concentration is still controversial, e.g., the methane concentration in the super-MgFe hydrothermal system of Rainbow is as high as 2.5 mmol/kg, which is much higher than the methane yield of the water-rock reaction in the laboratory.Carbon isotope composition of CH4 is a powerful means to differentiate between biogenic and abiogenic methane, but the existing experimental techniques and carbon isotope value testing methods cannot be used to exclude the methane yield from the laboratory. However, the existing experimental techniques and carbon isotope value testing methods are unable to exclude the influence of background carbon sources, which greatly affects the experimental reliability. In recent years, the rapid development of in situ Raman spectroscopy has made it possible to determine gas isotopes in situ, but there is still a lack of Raman spectroscopic studies of methane carbon isotopes under high-temperature and high-pressure hydrothermal systems.
To address the above problems, the research team systematically investigated the Raman spectral characteristics of 13CH4 and 12CH4 in the pure CH4 system and CH4-H2O system at high temperature and high pressure (25-400oC, 50-400 bar) by using a capillary high-pressure transparent cavity. It was shown that the peaks of the characteristic peaks of 13CH4 were in the range of 2907cm-1-2912cm-1, moving to lower wave numbers with the increase of temperature and decrease of pressure, while the peaks of the characteristic peaks of 12CH4 were in the range of 2912cm-1 -2917 cm-1, which is always higher than 13CH4 by 4.6-5.1 cm-1 under the same temperature and pressure, indicating that the two can be well distinguished by Raman spectroscopy (Figure 1). In addition, the research team also established Raman quantitative calibration models for the concentrations of 13CH4 and 12CH4 in aqueous solutions (Fig. 2), which showed that the differences in Raman quantitative calibration models for dissolved 13CH4 and 12CH4 were due to the differences in the Raman scattering cross sections of the dissolved 13CH4 and 12CH4 themselves, rather than the changes in the molar density of the water or the Raman scattering cross sections of the dissolved 13CH4 and 12CH4. The related research results provide a strong support for the in situ identification and quantitative analysis of the carbon isotope composition of methane, which has a broad application prospect in high temperature and high pressure hydrothermal experiments and deep sea in situ exploration.
The first author of the paper is Yuzhou Ge, a doctoral student at the Institute of Oceanography, Chinese Academy of Sciences, and Xin Zhang, a researcher, is the corresponding author of the article. This research was jointly funded by the National Natural Science Foundation of China and the Class A Strategic Pilot Program of the Chinese Academy of Sciences.
The related results and links are as follows:
Ge, Y., Li, L., Xi, S., Zhang, Y., Luan, Z., and Zhang, X., 2023, Comparison of Raman spectral characteristics and quantitative methods between 13CH4 and 12CH4 from 25 to 400 °C and 50 to 400 bar: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, p. 123380.
Fig. 1 Peak positions and half-height and full width of the characteristic peaks of 13CH4 and 12CH4 at different temperatures and pressures.
Fig. 2 Raman quantitative calibration models of 13CH4 and 12CH4 based on OH bending vibrational bands (a) and stretching vibrational bands (b) of water
