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Recently, researchers led by Prof. Jun Yan and Prof. Chaolun Li of the Center for Ocean Mega-Science, Chinese Academy of Sciences have made progress in in-situ Raman spectroscopy quantitative detection of deep-ocean hydrothermal fluid system. With the homemade Raman insertion probe (RiP), they conduct the in-situ quantitative Raman spectroscopy detection of high-temperature hydrothermal fluids in the hydrothermal field in the middle Okinawa trough. They obtained the in-situ concentrations of dissolved CO2 and SO42- in the high-temperature hydrothermal vent fluids for the first time in the world. Relevant research findings are published as the cover paper in Geochemistry, Geophysics, Geosystems.

As a major discovery of earth science in 20th century, the deep-ocean hydrothermal fluid system achieves the matter and energy exchanges between different circles.  In recent years, the physical and chemical properties of high-temperature hydrothermal vent fluid and its influences on ocean have become the new research highlights. Temperature, pressure changes, and seawater mixing can significantly alter the chemical composition or concentration of hydrothermal vent fluids. Although scientists have obtained relatively close data by laboratory analysis with gas-tight sampler, they cannot obtain correct samples of the high-temperature hydrothermal vent fluid due to the restriction of sampling methods, which cause obvious differences between analysis result and the reality. The research team has successfully developed the first high-temperature (450℃) resistant RiP (Xin Zhang et al., DSR-I, 2017) in the world, by solving the effect of heat-resistant and high-concentration particles of optical lens on the optical system. By means of research vessel (RV) Kexue and Remotely Operated Vehicle (ROV) Faxian, the RiP has detected lots high-temperature hydrothermal vent fluids in the hydrothermal fields in the Manus and the Okinawa Trough since 2015 and collected a number of in-situ Raman spectra.

Depending on the in-situ Raman spectra (up to 273℃) of three high-temperature hydrothermal vent fluids in hydrothermal fields in middle Okinawa Trough obtained by RV Kexue comprehensive voyage for hydrothermal fluids and cold seep in 2016, and the numerous Raman spectroscopy quantitative analysis models of CO2 and SO42- (Lianfu Li, Xin Zhang*, et al., Applied Spectroscopy, 2018; Shichuan Xi, Xin Zhang*, et al., Applied Spectroscopy, 2018) set up by high-temperature simulation experiments, the researchers have determined the concentrations of CO2 and SO42- (Lianfu Li, Xin Zhang*, et al., G-cubed, 2018) of hydrothermal vent fluids in the middle Okinawa Trough. According to the research, the content of SO42- as seawater mixing index in measured high-temperature hydrothermal fluids is nearly zero. It proves that the hydrothermal fluids sampled by the RiP contain no seawater, that is to say, the measured sample can be seemed as the primitive hydrothermal vent fluids. By comparing the CO2-concentration in the same hydrothermal vent measured by ROV with gas-tight sampling, it can be found that the concentration determined by in-situ measurement is three times higher than that by gas-tight sampling in laboratory. Based on the result, we can say that the influences of hydrothermal fluids on global carbon cycle and climate change may be much greater than people thought. The findings are of great significance in promoting the in-situ spectroscopy detection in extreme deep ocean environment, and may help people re-understand the influences of hydrothermal fluids on global oceans.

The research was funded by National Natural Science Foundation of China, the Ocean Pilot Project of the Chinese Academy of Sciences and the Frontier Science Key Programs of Chinese Academy of Sciences. PhD student Lianfu Li is the first author of the paper, and Prof. Xin Zhang is the corresponding author.