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Reduced gases released from hydrothermal vents supply energy to local deep-marine ecosystems and play an important role in global biogeochemical cycles of sulfur and carbon. The habitable, lower-temperature diffuse flow sites in a hydrothermal system generally have higher biomass than focused flow sites. However, a scarcity of observational data of diffuse flows limits our understanding of the role of volatile gases in these environments. We deployed in situ Raman spectroscopy in the Iheya North hydrothermal field of the mid–Okinawa Trough (East China Sea). A Raman probe inserted directly into hydrothermal vent orifices with temperatures of 30–302 °C collected Raman spectra of hydrothermal fluids. In situ observation data show that the greater volume of diffuse flows results in a flux of volatile gases one to two orders of magnitude higher than that from focused flow environments. This indicates the great potential of diffuse flow for supplying energy and material to hydrothermal systems. The role played by diffuse flow should be reassessed.

Microorganisms contribute significantly to the formation of deep-sea elemental sulfur, so studies on their growth and dynamic sulfur metabolism are important to understand the deep-sea sulfur cycle. However, near real-time in situ nondestructive metabolic studies of microorganisms remain a great challenge due to the limitations of existing methods.

The chlorinity of deep-sea hydrothermal fluids, representing one of the crucial deep-sea hydrothermal indicators, indicates the degree of deep phase separation of hydrothermal fluids and water/rock reactions. However, accurately measuring the chlorinity of high-temperature hydrothermal fluids is still a significant challenge. In this paper, a piecewise chlorinity model to measure the chlorinity of high-temperature hydrothermal fluids was developed based on the OH stretching band of water, exhibiting an accuracy of 96.20%. The peak position, peak area ratio, and F value were selected to establish the chlorinity piecewise calibration model within the temperature ranges of 0–50 ℃, 50–200 ℃, and 200–300 ℃. Compared with that of the chlorinity calibration model built based on a single parameter, the accuracy of this piecewise model increased by approximately 4.83–12.33%. This chlorinity calibration model was applied to determine the concentrations of Cl for high-temperature hydrothermal fluids in the Okinawa Trough hydrothermal field.

Marine sediment is the primary sink of microplastics and is an indicator of pollution levels. However, although there are well-developed detection methods, detection is rarely focused on low-micrometer-sized particles, mainly due to technique limitations. In this study, a simplified process omitting digestion procedures was developed to pretreat microplastics obtained from marine sediment and was coupled with micro-Raman spectroscopy to identify microplastics. Based on the overall analysis of the characteristic peak assignments, a Raman spectral reference library was constructed for 18 types of plastic. In addition, the effects of the measurement parameters were systematically described. Field research was then conducted to validate the developed process and investigate microplastic contamination in Huiquan Bay, Qingdao, China. This simplified process could retain the original appearance of microparticles and accomplish the detection of

Supercritical CO2 appearing as bubbles in hydrothermal vents was identified in the south part of the Okinawa Trough using in situ Raman spectroscopy. Significantly, the N2 peak in supercritical CO2 is much larger than those in seawater and vent fluids, indicating that supercritical CO2 enriches N2 from the surrounding environment. Considering that the partial pressures of CO2 and N2 in the Earth’s proto-atmosphere were ~10–20 MPa, supercritical CO2 with high N2 was likely the dominant CO2 phase near the water-air interface in the early history of the Earth, which promoted the synthesis, pre-enrichment and preservation of amino acids and other organic matters that are essential to the origin of life.

Zero-valent sulfur (ZVS) has been shown to be a major sulfur intermediate in the deep-sea cold seep of the South China Sea based on our previous work, however, the microbial contribution to the formation of ZVS in cold seep has remained unclear. Here, we describe a novel thiosulfate oxidation pathway discovered in the deep-sea cold seep bacterium Erythrobacter flavus 21–3, which provides a new clue about the formation of ZVS. Electronic microscopy, energy-dispersive, and Raman spectra were used to confirm that E. flavus 21–3 effectively converts thiosulfate to ZVS. We next used a combined proteomic and genetic method to identify thiosulfate dehydrogenase (TsdA) and thiosulfohydrolase (SoxB) playing key roles in the conversion of thiosulfate to ZVS. Stoichiometric results of different sulfur intermediates further clarify the function of TsdA in converting thiosulfate to tetrathionate (−O3S–S–S–SO3−), SoxB in liberating sulfone from tetrathionate to form ZVS and sulfur dioxygenases (SdoA/SdoB) in oxidizing ZVS to sulfite under some conditions. Notably, homologs of TsdA, SoxB, and SdoA/SdoB widely exist across the bacteria including in Erythrobacter species derived from different environments. This strongly indicates that this novel thiosulfate oxidation pathway might be frequently used by microbes and plays an important role in the biogeochemical sulfur cycle in nature.

Seafloor features, seismic amplitude anomaly, geophysical attributes, and chemosynthetic community are different in the methane seepage areas of Taixinan Basin, South China Sea. We integrate the new acquired seafloor images with seismic data to present the differences of three cold seeps which are associated with fluid flows. These three cold seeps include two inactive sites (CS02-8 and CS02-9) and one active site (Site F). At site CS02-8 area, the seafloor carbonate and buried carbonate layer are identified from core samples at Site GMGS2-08. BSR is interrupted by the upward migration of fluids from the base of gas hydrate stability zone (BGHSZ) and gas hydrate-bearing sediments show pull-up seismic reflections below the buried carbonate layer. At site CS02-9 area, near seafloor carbonate was found at hole GMGS2-09B where a 62 m-high pinnacle and gas hydrate were also found from the core samples. The LWD data at hole 09A indicate that free gas is present below the depth of 64 m without gas hydrate. The average saturations of free gas estimated from P-wave velocity are about 3% with a patchy distribution and 0.3% with homogenous distribution respectively, which is accord with the enhanced reflection below BGHSZ. At Site F area, the exposed gas hydrate, carbonate and flourishing chemosynthetic communities are detected using ROV with a high amplitude seismic reflection at seafloor. The BSR is interrupted by a chimney structure from the seismic data which indicates that the fluids are migrating vertically and rise to the seafloor as gas bubbles. The geophysical evidence at these three seep sites show that pore-filling gas hydrates with saturations ranging from 10% to 40% are widely distributed above the BSR. However, massive or vein gas hydrates only occur around cold seeps.

Laser Raman spectroscopy has been successfully applied for the in situ detection of gases and liquids, but has not yet been extensively applied to rocks in the deep sea. Authigenic carbonate rocks are widely distributed at the Formosa Ridge (Site F) and the eastern region of the Pear River Mouth basin (Site P) in the South China Sea (SCS). Considerable Raman spectral information on authigenic carbonates was obtained from laboratory measurements of slices of authigenic carbonate, direct measurements in a simulation environment, and in situ measurements in the deep sea. The dominant band of authigenic carbonate from Site F ranges from 1084 to 1087 cm−1 while the dominant band from Site P ranges from 1088 to 1091 cm−1, suggesting that compositional differences exist between the samples from the two cold seep areas. The results of the point-counting method indicates that the aragonite content in Site F is higher than that at Site P. The wider peak width of the aragonites at Site P shows the aragonites at Site P have lower degree of crystallinity, which suggests that aragonite is likely to be destroyed when the cold seep weakens or dies. The above information obtained via the Raman spectroscopy can guide sample collection in the deep sea and can be used in conjunction with other analysis methods to better understand environmental changes or the geological processes.

Properties of gas hydrate are commonly investigated in laboratory by simulators or studied using the hydrate samples recovered from permafrost and marine sediments. Here synthetic gas hydrate (SGH) is quickly formed for the first time by fluids and bubbles erupted from the active cold seeps at the Formosa Ridge in the South China Sea (SCS). SGH samples are in situ detected by a Raman insertion probe for gas hydrate (RiP-Gh) which is deployed by the remotely operated vehicle (ROV) Faxian. Both spatial and temporal in situ Raman spectra of the SGH samples are acquired in order to determine the structure and the evolution of the synthetic gas hydrate. Authigenic carbonate debris or other debris serving as nucleation particles, which is found in the in situ Raman spectra of SGH samples for the first time, may be one factor that can promote the formation of synthetic gas hydrate. The gas-water interface demonstrated by the observation results, also contributes to the quick formation of SGH samples, which agrees well with the previous study. In situ Raman spectra of three SGH samples which have been placed on the seafloor for 0 h, 4 h and 21 h respectively are acquired. Laboratory Raman spectra of one sample which has been surprisingly found and recovered after 9840 h (410 days) since formed on the seafloor are also acquired. The Raman spectra of the four SGH samples indicate that the methane large-to-small cage occupancy ratios of the hydrates vary from 1.01 to 1.39, and the ratios of methane large-to-gas of the hydrates increase from 0.53 to 1.55. Overall, our work suggests a new explanation for the quick formation of gas hydrate compared to that in laboratory simulation work, and reveals the evolution of synthetic gas hydrate after quick formation, which provides a new insight for the study of natural gas hydrate.

Based on the previously developed deep-sea hybrid Raman insertion probe for cold seeps, the in situ detection of a cold seep vent and geochemistry analysis of fluids in chemosynthetic communities were conducted at the Formosa Ridge in the northern South China Sea. Three different methods were used to measure the components of the fluids erupting from the cold seep vent. The in situ Raman spectra of the cold seep fluids indicated the presence of gaseous CH4, C3H8, and H2S. The results indicate that the gases at this site are of biogenic origin; however, the presence of C3H8 suggests that thermogenic methane should not be excluded. The conclusion is also supported by the results of gas chromatography and stable carbon isotope analysis. More significantly, we found that the concentration of SO42- decreases with increasing depth, while the concentrations of CH4 and S-8 increase in fluids in chemosynthetic communities, but without H2S. This finding indicates that the methane is oxidized by sulfate and that elemental sulfur is formed. This process usually occurs in marine sediments as the anaerobic oxidation of methane. Overall, the findings in this work provide a new insight into the geochemical analysis of cold seep fluids and in situ evidence of the oxidation of methane in the chemosynthetic communities near cold seeps.

In situ Raman detection is an ideal method to determine the concentration of dissolved H-2 in deep-sea high temperature hydrothermal fluids, but studies on in situ Raman qualitative and quantitative analyses of H-2 that are suitable for detection in high temperature hydrothermal fluids are lacking. In this study, the Raman characteristics of gaseous and dissolved H-2 were researched at 0-400 degrees C and 0-40MPa 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 H-2 was also developed with the linear equation , where A (H-2)/A (H2O) is the peak area ratio of H-2 and H2O, and C (H-2) is the concentration of dissolved H-2 in mol/kg. The experimental temperature and pressure conditions did not influence the linear trend between the peak area ratio of A (H-2)/A (H2O) and the concentrations of H-2, which indicated that the calibration model can be applied to high temperature and pressure environments.

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 degrees 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 (59198 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.

Inverted lakes filled with shimmering water are unique environments within hydrothermal systems. Recently, vapor-phase fluids have been detected on the top of an inverted lake in a mushroom cap-shaped (MC) sulfide structure based on in situ Raman spectra at the Yokosuka Site in the Okinawa Trough (OT). However, the effects of vapor-phase fluids on the migration and metallogenic characteristics of elements remain unclear. To characterize the influence of such fluids, a comparative study of the micro-Raman spectroscopy, electron microprobe analyses (EMPA), polarizing microscopic, element distribution, and S-Pb isotopic composition among the hydrothermal sulfides collected from MC area, the middle Okinawa Trough (MOT) and southern Okinawa Trough (SOT), was conducted. Raman spectra show that the dominant band of chalcopyrite in MC sulfide generated a blue shift, while sphalerite generated a red shift, indicating that compositional differences exist in MC sulfide samples compared to the other two areas. This was also confirmed by EMPA data for these two minerals. Moreover, the full width(s) at half maximum (FWHM) of the Raman spectra of minerals in the MC sulfide samples showed a lower degree of crystallinity influence by the vapor-phase fluids. Elemental analyses further demonstrated that the MC samples share a particular elemental composition. That is, MC sulfides featured high Mn, Ba, and Sr and low Fe, Cu, Zn, Pb, Bi, Mo, Cd, and Ga. The same enrichment trends were observed among Ba–Sr, Cu–Mo, Cu–Bi, Zn–Pb, Zn–Cd, and Zn–Ga. Moreover, the δ34S of 5.44–5.85‰ in these deposits suggest that S was contributed by the underlying basalt, sediments, and seawater. The Pb isotopic composition of MC sulfides exhibited the same compositional patterns as the other sulfides and the underlying sediments at the SOT site, indicating that the sedimentary contribution of Pb has increased. The S–Pb isotopic composition suggests that a hydrothermal source may not serve as the primary cause of differences in the MC deposits. Instead, the unique vapor-phase fluids and other gaseous components may have predominantly affected the precipitation and migration of elements at this site. Although the shimmering water regions of inverted lakes accompanied by vapor-phase fluids are difficult to observe in deep-sea hydrothermal systems, here, we describe the geochemical characteristics of sulfides affected by this particular vapor-phase fluid, providing a unique perspective on the migration and precipitation occurring during sulfide deposit formation.

Gas hydrates are usually buried in sediments. Here we report the first discovery of gas hydrates exposed on the seafloor of the South China Sea. The in situ chemical compositions and cage structures of these hydrates were measured at the depth of 1,130 m below sea level using a Raman insertion probe (RiP-Gh) that was carried and controlled by a remotely operated vehicle (ROV) Faxian. This in situ analytical technique can avoid the physical and chemical changes associated with the transport of samples from the deep sea to the surface. Natural gas hydrate samples were analyzed at two sites. The in situ spectra suggest that the newly formed hydrate was Structure I but contains a small amount of C3H8 and H2S. Pure gas spectra of CH4, C3H8, and H2S were also observed at the SCS-SGH02 site. These data represent the first in situ proof that free gas can be trapped within the hydrate fabric during rapid hydrate formation. We provide the first in situ confirmation of the hydrate growth model for the early stages of formation of crystalline hydrates in a methane-rich seafloor environment. Our work demonstrates that natural hydrate deposits, particularly those in the early stages of formation, are not monolithic single structures but instead exhibit significant small-scale heterogeneities due to inclusions of free gas and the surrounding seawater, there inclusions also serve as indicators of the likely hydrate formation mechanism. These data also reinforce the importance of correlating visual and in situ measurements when characterizing a sampling site.

Hydrocarbon seepage in cold seeps provides an appropriate environment for chemosynthetic communities to arise and for authigenic carbonates to form. To study the relationships between the chemosynthetic communities and underlying authigenic carbonates, extensive in situ Raman measurements were conducted on the fluids, chemosynthetic communities, and authigenic carbonates found at the Formosa Ridge (Site F) in the South China Sea. Interestingly, the in situ Raman spectra of the authigenic carbonates indicated the highest crystallinity in the fauna-rich area. Scanning electron microscopy and X-ray diffraction analyses of the samples supported the conclusion that the aragonite in the fauna-rich area is more regular than that in the desert area. The in situ Raman spectra suggested that the fluids with relatively low salinity and low sulfate concentrations in the bottom of the chemosynthetic communities can buffer the erosion of underlying authigenic carbonates via the salt effect. In addition, the aragonite content decreased, while the quartz content increased from the fauna-rich area to the desert area (i.e., with increasing distance from the seepage vent). Authigenic aragonite exhibited a negative correlation with terrigenous quartz in the cold seep. The results indicate that changes in chemosynthetic communities have biogeochemical implications on the evolution of authigenic carbonates. Finally, we built a distribution model of the present chemosynthetic communities and underlying authigenic carbonates at Site F, based on systematic in situ Raman measurements and laboratory analyses. This study provides new insights into the evolution of authigenic carbonates based on differences in the spatial distribution of chemosynthetic communities in the seepage system.

Natural gas hydrate is a great potential energy source in the future, and it also plays a significant roe in the study of global climate change. In past decades, natural gas hydrate is usually studied in laboratory by simulators or studied using the hydrate samples. However, it is a big challenge for the simulating devices to completely recreate field conditions for gas hydrate study. Here we report the first discovery of natural gas hydrates (NGH) exposed on the seafloor in the South China Sea (SCS). Synthetic gas hydrate (SGH) is quickly formed for the first time by the fluids and bubbles erupted from the active cold seeps in the SCS. The NGH and SGH samples are in situ detected by a Raman insertion probe for gas hydrate (RiP-Gh) which is deployed and controlled by the remotely operated vehicle (ROV) Faxian. The in situ Raman spectra of the NGH suggest that the newly formed gas hydrate is Structure I but contains a small amount of gaseous CH 4 , C$_{3}H_{8}$ and H 2 S, which are consistent with the results SHG. Authigenic carbonate debris or other debris serving as nucleation particles, and the gas-water interface were proposed as two factors which can promote the formation of synthetic gas hydrate. Overall, our work demonstrates the components of NGH and SGH near the active cold seep vent in the South China Sea, and suggests a new explanation for the quick formation of gas hydrate compared to that in laboratory simulation work.

The OH stretching band of water is very sensitive to temperature and salinity for the existence of hydrogen bonds between H2O molecules. In this study, the OH stretching band was deconvoluted into two Gaussian peaks, with peak 1 at approximately 3450 cm−1 and peak 2 at approximately 3200 cm−1. The positions of peaks 1 and 2 both shifted to higher wavenumbers with increasing temperature from 50 ℃ to 300 ℃. The effects of salinity in the range of 0–2 mol/kg NaCl on the OH stretching band were also studied. Linearity for the relationship between Raman shift of peak 1 and temperature increased as the salt concentration increased from 0 to 2 mol/kg, while peak 2 displayed an opposing trend. Two temperature calibration models were developed based on the temperature-dependent changes in the Raman frequency shifts of peaks 1 and 2 (precision of 0.9 ℃ and 1.0 ℃, respectively). The calibration models for temperature were successfully applied to determining the temperatures of deep-sea hydrothermal fluids in the Okinawa Trough hydrothermal field. The degree of mixing of hydrothermal fluids and ambient seawater during in situ Raman measurements was estimated by the difference in temperatures determined through these calibration models and those measured through thermocouple sensors.

Raman spectroscopy has been applied to the quantitative analysis of the concentration of bisulfate in acid-sulfate fluids at different temperatures. The quantitative analysis method is based on the peak area ratios of [Formula: see text](ν1) and H2O (ν2), where PA([Formula: see text]/H2O) = [[Formula: see text]] × (0.0066 × T + 1.3070) at a temperature range of 0–100 ℃. We found that the molal scattering coefficient of bisulfate increases slightly at the elevated temperature may be due to the changes of fraction of water molecules that are hydrogen-bonded. The method can also be applied to analyze physicochemical parameters of other acid fluids, such as hydrogen phosphate, bicarbonate, etc., and especially to the in situ detection of deep sea acid-sulfate hydrothermal fluids in the future.

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.