New progress in in-situ identification and quantitative research of methane carbon isotopes in the ocean

Recently, Zhang Xin's research team from the Institute of Oceanography, Chinese Academy of Sciences, based on the in-situ laser Raman spectroscopy technology, made new progress in the in-situ recognition and quantification of methane carbon isotopes by using the significant differences in the Raman spectra of methane carbon isotopes (13CH4 and 12CH4). The relevant results were recently published in the international spectroscopy journal Spectra Acta Part A: Molecular and Biomolecular Spectroscopy.

The deep-sea hydrothermal system releases a large amount of reducing gases such as CH4 and H2, providing a unique community of chemosynthetic organisms, which is of great significance for studying the origin of early life. However, there is still great controversy over the source of such high concentration methane, such as the methane concentration in the "Rainbow" ultramafic hydrothermal system reaching up to 2.5mmol/kg, which is much higher than the methane production from water rock reactions in the laboratory.

The carbon isotope composition of CH4 is a powerful means of distinguishing biogenic and abiogenic methane, but existing experimental techniques and carbon isotope value testing methods cannot exclude the influence of background carbon sources, greatly affecting the reliability of the experiment. In recent years, the rapid development of in-situ Raman spectroscopy technology has made it possible to determine gas isotopes in situ. However, there is still a lack of Raman spectroscopy research on methane carbon isotopes in high-temperature and high-pressure hydrothermal systems.

In response to the above issues, the research team systematically studied the Raman spectral characteristics of 13CH4 and 12CH4 under high temperature and high pressure (25-400oC, 50-400 bar) pure CH4 system and CH4-H2O system using a capillary high-pressure transparent cavity. Research has shown that the peak position of the characteristic peak of 13CH4 is between 2907cm-1 and 2912cm-1, moving towards a lower wavenumber with increasing temperature and decreasing pressure; The characteristic peak of 12CH4 ranges from 2912cm-1 to 2917cm-1, consistently 4.6 to 5.1cm-1 higher than 13CH4 at the same temperature and pressure, indicating that the two can be distinguished well 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 (Figure 2). The study showed that the differences in Raman scattering cross-sections between dissolved 13CH4 and 12CH4, rather than changes in water molar density or Raman scattering cross-sections, resulted in differences in their Raman quantitative calibration models. The relevant research results provide strong support for in-situ identification and quantitative analysis of the carbon isotope composition of methane, and have broad application prospects in high-temperature and high-pressure hydrothermal experiments and deep-sea in-situ detection.

The first author of the paper is Ge Yuzhou, a doctoral candidate from the Institute of Oceanography, Chinese Academy of Sciences, and researcher Zhang Xin is the corresponding author of the article. The research was jointly supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences Class A strategic pilot project.

The relevant achievements 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．

Figure 1: Peak positions and full width at half height of characteristic peaks of 13CH4 and 12CH4 at different temperatures and pressures

Figure 2 Raman quantitative calibration models for 13CH4 and 12CH4 based on water OH bending vibration band (a) and stretching vibration band (b)

Source: Institute of Oceanography, Chinese Academy of Sciences