Chao Lian

In Situ Raman Detection of Gas Hydrates Exposed on the Seafloor of the South China Sea

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.

EXPRESS: In situ Raman spectral characteristics of carbon dioxide in a deep-sea simulator of extreme environments reaching 300°C and 30 MPa

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 300u2009℃ and 30u2009MPa), 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 (0–300u2009℃ and 0–30u2009MPa), the peak positions of the symmetric stretching modes of gaseous CO2, liquid CO2, and supercritical CO2 shift approximately 0.6u2009cm–1 (1387.8–1388.4u2009cm–1), 0.7u2009cm–1 (1385.5–1386.2u2009cm–1), and 2.5u2009cm–1 (1385.7–1388.2u2009cm–1), and those of the bending modes shift about 1.0u2009cm–1 (1284.7–1285.7u2009cm–1), 1.9u2009cm–1 (1280.1–1282.0u2009cm–1), and 4.4u2009cm–1 (1281.0–1285.4u2009cm–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 4.5u2009cm–1 (1282.5–1287.0u2009cm–1) and 2.4u2009cm–1 (1274.4–1276.8u2009cm–1) for each peak. In comparison with our experiment results, the phases of CO2 in extreme conditions (0–3000u2009m and 0–300u2009℃) 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.

Raman vibrational spectral characteristics and quantitative analysis of H2 up to 400°C and 40 MPa

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.

In Situ Quantitative Raman Detection of Dissolved Carbon Dioxide and Sulfate in Deep‐Sea High‐Temperature Hydrothermal Vent Fluids

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.

In situ Raman Quantitative Detection of the Cold Seep Vents and Fluids in the Chemosynthetic Communities in the South China Sea

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.

A New Approach to Measuring the Temperature of Fluids Reaching 300 ℃ and 2 mol/kg NaCl Based on the Raman Shift of Water

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 3450u2009cm−1 and peak 2 at approximately 3200u2009cm−1. The positions of peaks 1 and 2 both shifted to higher wavenumbers with increasing temperature from 50u2009℃ to 300u2009℃. The effects of salinity in the range of 0–2u2009mol/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 2u2009mol/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 0.9u2009℃ and 1.0u2009℃, 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.

A Direct Quantitative Raman Method for the Measurement of Dissolved Bisulfate in Acid-Sulfate Fluids:

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 HSO 4 - (ν1) and H2O (ν2), where PA( HSO 4 - /H2O)u2009=u2009[ HSO 4 - ]u2009×u2009(0.0066u2009×u2009Tu2009+u20091.3070) at a temperature range of 0–100u2009℃. 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.