Understanding the evolution of chemosynthetic communities and environmental changes near fluid seepages in the deep sea requires in-situ long-term observation data. However, in-situ detection or sampling for the investigation of cold seeps, hydrothermal vents, and nearby chemosynthetic ecosystems, by manned submersibles and remotely operated vehicles (ROVs), has great restrictions in terms of time. The observation parameters of a free-fall mode Lander cannot be adjusted in real time because there is no communication channel once the Lander is separated from the vessel. A long-term ocean observation platform (LOOP), that uses a new controllable mode for launching and recovery with the aid of a research vessel and submarine vehicles, has been developed and used in the cold seep area of the South China Sea. The LOOP can be operated in an online real-time control mode allowing landing site selection and adjustment of observation parameters during the launching process, with subsequent switched to an offline stand-alone operation mode for long-term, continuous observation. The effective observation times were 375 days and 414 days, respectively, during the 2016 and 2018 deployments in the cold seep area in the South China Sea. Results of these deployments show that salinity and dissolved oxygen parameters have strong spatial heterogeneity in both the horizontal and vertical directions within the cold seep vent. The spatial heterogeneity of environmental parameters may be one of the main driving factors for the uneven spatial distribution of chemosynthetic communities in cold seep areas. Overall, the LOOP provides an innovative and controllable launching and recovery mode and is expected to become a universal underwater observation platform for in-situ, long-term, and continuous data acquisition.
Abstract Hydrothermal H 2 S 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 H 2 S, HS − and pH in hydrothermal fluids was developed and applied to the detections of Okinawa Trough hydrothermal activities. The in situ total H 2 S 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.
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.
Abstract 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 CH 4 , C 3 H 8 , and H 2 S. The results indicate that the gases at this site are of biogenic origin; however, the presence of C 3 H 8 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 SO 4 2− decreases with increasing depth, while the concentrations of CH 4 and S 8 increase in fluids in chemosynthetic communities, but without H 2 S. 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.
Abstract 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.
Deep-sea carbon dioxide (CO 2 ) 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 CO 2 . However, the Raman spectral characteristics of CO 2 can be affected by the environment, thus restricting the phase identification and quantitative analysis of CO 2 . In order to study the Raman spectral characteristics of CO 2 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 CO 2 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 CO 2 and the CO 2 –H 2 O 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 CO 2, liquid CO 2 , and supercritical CO 2 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 CO 2 –H 2 O system were also studied under the same conditions. The peak positions of dissolved CO 2 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 CO 2 in extreme conditions (0–3000 m and 0–300 ℃) can be identified with the Raman spectra collected in situ. This qualitative research on CO 2 can also support the further quantitative analysis of dissolved CO 2 in extreme conditions.
Methane hydrate (MH) is widely distributed in active deep-sea cold-seep areas. The complex environment of cold seeps influences MH formation. Investigating the effects of cold-seep environments on MH formation can provide an in-depth understanding of the formation mechanism of MH and kinetic process of cold-seep development and evolution. In this study, multiple MH formation experiments were conducted in the laboratory to investigate the effects of a cold-seep environment on the kinetics of hydrate formation. Considering the cold-seep fluid and bottom seawater as the samples, MH was formed in the capillary under the same pressure-temperature conditions as those in cold-seep environments. The entire process of MH formation was recorded by time-series Raman spectra and video, and the results demonstrated the promotion of MH formation by solid particles, especially authigenic carbonates. Compared to 20 min in seawater, the induction time of 2 min in cold-seep fluids indicated promotion of hydrate formation by tiny particles and by its lower salinity. Notably, the reduction in salinity in the cold-seep fluid may be attributed to the dilution effect of pure water produced by hydrate decomposition, indicating the presence of a hydrate memory effect. Overall, our results are in agreement with those of the in situ hydrate formation experiment in a cold-seep environment. A model of hydrate formation in a cold-seep environment has been proposed, which can aid in understanding the kinetics of hydrate formation.
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