In Situ Quantitative Raman Detection of Dissolved Carbon Dioxide and Sulfate in Deep‐Sea High‐Temperature Hydrothermal Vent Fluids

Abstract

Abstract 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 CO 2 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 CO 2 suitable for a hydrothermal environment is established. The Raman relative intensity of CO 2 displayed a linear relationship with increasing concentration of CO 2 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 for high‐temperature and high‐pressure conditions is identical to that of for room temperature and atmospheric pressure. The concentrations of CO 2 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 calculated from in situ Raman spectra were near zero, indicating that the in situ Raman measurement avoids hydrothermal fluids contaminated with seawater.