Carbon dioxide emitted from hydrothermal vents, as an important part of the global carbon cycle, can directly affect hydrothermal ecosystems. However, traditional chemical analysis methods cannot directly measure the concentrations of dissolved CO2 in high‐temperature hydrothermal fluids. Although in situ mass spectrometry has been applied to the measurements of deep sea, it cannot be used to detect high‐temperature fluids. In this study, an in situ Raman quantitative method for measuring dissolved CO2 suitable for a hydrothermal environment is established. The Raman relative intensity of CO2 displayed a linear relationship with increasing concentration of CO2 under the investigated conditions (up to 300°C and 40 MPa), allowing this in situ measurement method to be applied to most hydrothermal fields worldwide. Moreover, we find that the quantitative calibration curve for SO42− for high‐temperature and high‐pressure conditions is identical to that of SO42− for room temperature and atmospheric pressure. The concentrations of CO2 in mid‐Okinawa Trough hydrothermal fluids determined by in situ Raman measurement are 188.4–532.3 mmol/kg, which are about 3 times higher than those obtained by traditional sampling methods (59–198 mmol/kg). However, the concentrations of SO42− calculated from in situ Raman spectra were near zero, indicating that the in situ Raman measurement avoids hydrothermal fluids contaminated with seawater.

No abstract