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The evolution of gas hydrates influenced by the seawater environment is unknown. We present a model of structural transformation from sI hydrate to sII hydrate due to the influence of seawater environment and vent fluid in nature through in situ experiments of gas hydrate formation in the Haima cold seep area. The in situ experimental results indicate that gas hydrates preferentially form as sI hydrates even in cold seep environments where C2+ hydrocarbons are present. During subsequent evolution, the sI hydrates could restructured at the effect of seawater environment and vent fluid, causing transformation to sII hydrates under the influence of hydrate stability. The supply of gas and direct contact with seawater environment are critical factors for structural transformation. Such structural transformation is the result of gas hydrates seeking thermodynamic stability and may be common in active cold seep areas. Plain Language Summary: At high pressures and low temperatures, methane molecules are able to be captured by water in a solid hydrate state, known as natural gas hydrate. The significance of the abundant reserves of natural gas hydrates in the oceans, both in terms of global climate issues and energy resources, has been widely recognised. Since most natural gas reservoirs are dominated by methane (>90%), it is generally accepted that gas hydrates in the oceans tend to be structure I. However, through a field experiment in an active cold seep area, we found that newly generated gas hydrates under the influence of the same cold seep area were of structure I, whereas naturally exposed gas hydrates that had existed for a long period of time on the seafloor were instead of structure II. Based on this, we propose a model of structural transformation (structure I to structure II) of gas hydrates under the influence of seawater environment and vent fluid. This structural transformation may be a natural phenomenon that is common in active cold seep areas. Key Points: In situ experiments have demonstrated that gas hydrates can undergo structural transformation under the influence of seawater environmentNatural structural transformations of gas hydrates are probably common, especially in active cold seep where C2+ hydrocarbons present [ABSTRACT FROM AUTHOR] more...

Hydrothermal H2S is an important energy source for hydrothermal ecosystems. However, it is difficult to obtain accurate hydrogen sulfide concentrations in high‐temperature hydrothermal fluids because they are highly susceptible to oxidation and compositional variability with mixing. In this study, a new in situ approach for measuring H2S, HS− and pH in hydrothermal fluids was developed and applied to the detections of Okinawa Trough hydrothermal activities. The in situ total H2S concentrations in the Jade and Biwako fluids were determined to be 31.4 and 76.7 mmol/kg, respectively. The in situ measured pH of the Jade fluids was determined to be 6.3, which has exceeded that of a neutral fluid at a specific temperature and pressure, indicating that the pH of Jade fluids is weakly alkaline. The pH transition of hydrothermal fluids from alkaline to acidic may be attributed to the thermal decomposition of organic matter and sulfide precipitation. Plain Language Summary: Hydrogen sulfide is one of the important gases released from hydrothermal systems, but it is difficult to measure its concentration accurately because it is highly susceptible to oxidation and compositional variability with mixing. The in situ pH of hydrothermal fluids is also a challenging parameter to obtain, as it is significantly affected by factors such as temperature variation and mineral precipitation. Traditional methods of sampling followed by laboratory analysis inevitably result in changes in hydrogen sulfide concentration and pH. Furthermore, conventional chemical sensors and pH electrodes cannot withstand the high‐temperature extreme environment of hydrothermal vents, making it impossible to directly obtain these parameters at the high‐temperature vent orifice. Here, we developed a new method for in situ measurement of the H2S‐HS− ionization equilibrium system based on Raman spectroscopy and used it to determine the H2S concentration and in situ pH of hydrothermal fluids in the Okinawa Trough. Key Points: The first in situ measured pH of high‐temperature hydrothermal vent fluids at arc‐back arc basins was reportedA new approach to obtain in situ H2S/HS− concentration and in situ pH of high temperature hydrothermal vent fluids was establishedThe pH transition of hydrothermal fluids from alkaline to acidic should attributes to the precipitation of sulfide minerals [ABSTRACT FROM AUTHOR] more...

In situ Raman detection is an ideal method to determine the concentration of dissolved H2 in deep‐sea high temperature hydrothermal fluids, but studies on in situ Raman qualitative and quantitative analyses of H2 that are suitable for detection in high temperature hydrothermal fluids are lacking. In this study, the Raman characteristics of gaseous and dissolved H2 were researched at 0–400°C and 0–40 MPa in detail, which cover most deep‐sea hydrothermal environments. The strong density and temperature dependences of the wavenumber and bandwidth of gaseous hydrogen vibrational Raman bands were observed. The interactions between the water molecules and hydrogen molecules were affected by temperature and pressure, and the opposite effect on the vibrational band of dissolved hydrogen was observed before and after reaching the critical condition of water. A high temperature and pressure quantitative analysis model suitable for in situ Raman detection of dissolved H2 was also developed with the linear equation AH2/AH2O=1.437or1.262×CH2, where A (H2)/A (H2O) is the peak area ratio of H2 and H2O, and C (H2) is the concentration of dissolved H2 in mol/kg. The experimental temperature and pressure conditions did not influence the linear trend between the peak area ratio of A (H2)/A (H2O) and the concentrations of H2, which indicated that the calibration model can be applied to high temperature and pressure environments. In situ Raman characteristics of gaseous and dissolved H2 in pure water and seawater were systematically studied by using an extreme environment simulator in laboratory. A Raman quantitative model for dissolved H2 suitable for in situ Raman measurements of deep‐sea hydrothermal fluids was established, and it can be applied to determine the concentrations of dissolved H2 in seawater near hydrothermal chimney. [ABSTRACT FROM AUTHOR] more...

The cover image is based on the Research Article I In situ Raman quantitative detection of methane concentrations in deep-sea high-temperature hydrothermal vent fluids i by Lianfu Li et al., https://doi.org/10.1002/jrs.5981. GLO:O8T/01nov20:jrs6038-toc-0001.jpg PHOTO (COLOR): . gl. [Extracted from the article] more...

The minerals in the hydrothermal and cold seep system form at different temperatures and show responses to the laser power to varying degrees. Here, we focus on the heat-induced by laser to study thermal transformations of the chalcopyrite, covellite, pyrite, barite, and aragonite based on Raman spectroscopy. Chalcopyrite mainly transforms into hematite, and covellite mainly transforms into chalcocite with the increase of laser power. Interestingly, comparing with the previous study, the pyrite can transform to the marcasite firstly, and form hematite finally. We also find that high-temperature opaque chalcopyrite is more likely to occur thermal transformations due to the smaller absolute energy difference (|ΔE1|) based on the frontier orbital theory. In contrast, the oxysalt minerals won't transform into new components under high laser power. However, the structure of the barite has been destroyed by the high laser power, while the more transparent aragonite is not affected by the high laser power due to the laser penetrates through the transparent aragonite crystal and causes little heat absorption. Finally, we established the minimum laser power densities for thermal transformations of these minerals formed under different environments. The above study provides a simple way to study the thermal transformations of minerals by the local heat-induced by laser and also enlightens us to identify the minerals phases precisely. [ABSTRACT FROM AUTHOR] more...

Deep-sea carbon dioxide (CO2) plays a significant role in the global carbon cycle and directly affects the living environment of marine organisms. In situ Raman detection technology is an effective approach to study the behavior of deep-sea CO2. However, the Raman spectral characteristics of CO2 can be affected by the environment, thus restricting the phase identification and quantitative analysis of CO2. In order to study the Raman spectral characteristics of CO2 in extreme environments (up to 300 ℃ and 30 MPa), which cover most regions of hydrothermal vents and cold seeps around the world, a deep-sea extreme environment simulator was developed. The Raman spectra of CO2 in different phases were obtained with Raman insertion probe (RiP) system, which was also used in in situ Raman detection in the deep sea carried by remotely operated vehicle (ROV) “Faxian”. The Raman frequency shifts and bandwidths of gaseous, liquid, solid, and supercritical CO2 and the CO2–H2O system were determined with the simulator. In our experiments (0–300 ℃ and 0–30 MPa), the peak positions of the symmetric stretching modes of gaseous CO2, liquid CO2, and supercritical CO2 shift approximately 0.6 cm–1 (1387.8–1388.4 cm–1), 0.7 cm–1 (1385.5–1386.2 cm–1), and 2.5 cm–1 (1385.7–1388.2 cm–1), and those of the bending modes shift about 1.0 cm–1 (1284.7–1285.7 cm–1), 1.9 cm–1 (1280.1–1282.0 cm–1), and 4.4 cm–1 (1281.0–1285.4 cm–1), respectively. The Raman spectral characteristics of the CO2–H2O system were also studied under the same conditions. The peak positions of dissolved CO2 varied approximately 4.5 cm–1 (1282.5–1287.0 cm–1) and 2.4 cm–1 (1274.4–1276.8 cm–1) for each peak. In comparison with our experiment results, the phases of CO2 in extreme conditions (0–3000 m and 0–300 ℃) can be identified with the Raman spectra collected in situ. This qualitative research on CO2 can also support the further quantitative analysis of dissolved CO2 in extreme conditions. [ABSTRACT FROM AUTHOR] more...