Novel In-situ pH Measurement Method Constructed for High-Temperature Hydrothermal Systems

Date:May 26, 2023    |  【 A  A  A 】

Alkaline hydrothermal systems are considered ideal environments for the origin of life because they can provide the ideal ion gradient conditions for the formation of early life on Earth. However, alkaline hydrothermal vents have only been found in the Lost City hydrothermal field in the Atlantic Ocean. Are there other alkaline hydrothermal vents in the widely distributed black smoker regions around the world? Besides, accurately obtaining the in situ pH of high-temperature hydrothermal vent fluids is a challenge.

Recently, the research team led by Prof. ZHANG Xin from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) and co-researchers from the University of Science and Technology Beijing successfully constructed an in situ pH measurement method for high-temperature hydrothermal fluids based on the self-developed Raman insertion Probe (RiP) system and applied it to the measurement of arc-back arc (ABA) hydrothermal systems, revealing that the in situ pH of high-temperature fluids in the sediment-host hydrothermal systems is alkaline.

The study was published in Geophysical Research Letters as a cover article on May. 10.

Researchers used a deep-sea extreme environment simulation platform to conduct quantitative analysis of the H2S-HS- ion equilibrium system. They established Raman quantitative analysis models for H2S and HS- under high-temperature and high-pressure conditions, as well as an in situ pH calibration model for hydrothermal fluids.

Researchers focused on a typical hydrothermal system in a back-arc setting and conducted in situ detection using the ROV Faxian equipped with RiP. The observations revealed that the in situ pH value of the high-temperature vent (6.3) exceeded the pH value of neutral fluids at the same vent temperature and pressure (5.6), indicating weak alkalinity. The in situ pH value was approximately 1.5 units higher than the measurements conducted at room temperature.

"This study confirmed that alkaline hydrothermal vents not only exist in hydrothermal areas like the Lost City, which are controlled by serpentinization reactions, but may also be widely distributed in hydrothermal areas near continental margins affected by significant sedimentation," said LI Lianfu, first author of the study.

The research was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the operational teams of the R.V. Kexue and ROV Faxian.

Fig. 1 In situ detection of hydrothermal system based on the Raman insertion Probe system carried by ROV Faxian.

Fig. 2 Comparison of in situ H2S, HS- concentrations and in situ pH values obtained based on Raman insertion Probe system and gas-tight sampling method.

Li, L., Li, Z., Zhong, R., Du, Z., Luan, Z., Xi, S., & Zhang, X*. (2023). Direct H2S, HS- and pH measurements of high-temperature hydrothermal vent fluids with in situ Raman spectroscopy. Geophysical Research Letters, 50, e2023GL103195.

(Text by LI Lianfu)

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