Abstract High hydrostatic pressure in deep‐sea environments potentially impacts microbial community diversity, the structure of cellular components and functions. The specific characteristics of aerobic methanotrophs originating from deep‐sea environments and their responses to local pressure fluctuations in terms of community diversity and methane oxidation potential remain unexplored. This study investigates subsurface sediments rich in aerobic methanotrophs from the natural gas hydrate‐bearing region in the Shenhu area, Northern South China Sea. By conducting aerobic oxidation of methane (AeOM) incubation experiments under various environmental pressures up to 10 MPa, the study aims to elucidate differences in microbial community diversity and AeOM rates. The results show a profound impact of pressure on both the taxonomic composition of bacterial and methanotrophic communities and their capacity for methane consumption. The key aerobic methanotrophs, that is, Methylococcales, exhibit a gradual decrease in composition as pressure rises. Accordingly, their AeOM rates also show a significant negative correlation with pressure ( r = 0.986, P < 0.01). The composition of three dominant methanotrophic genera, that is, unclassified_Methylococcaceae , Methylobacter , and Methylocaldum , exhibited irregular fluctuations under varying pressure conditions, with the lowest abundance observed at 2 MPa. Our study also shows that unclassified_Methylococcaceae is the primary methanotroph that exhibits the main response to pressure changes in marine environments.
In this study, a high pressure differential scanning calorimetry, based upon a thermo-analytical technique, was applied in order to investigate the kinetic and thermodynamic characteristics of nitrogen hydrates.The phase equilibrium conditions and the dissociation heat were determined by analyzing the endothermic heat flow curves during the hydrate dissociation process.In the nucleation stage, exothermic heat flow curves provided a lot of information about the kinetics, such as sub-cooling, nucleation time and the growth speed of hydrates crystals.In the bulk phase system, sub-cooling played an important part in the formation process of hydrates.However, other factors, such as the gas supply method, as well as the limitations that are inherent to the experimental apparatus also affected the reaction.It seems that, unlike the relatively stable behaviour in dissociation, hydrate nucleation and formation rely upon a probabilistic scheme.
For the first time, we present the rare earth element (REE) and sulfur isotopic composition of hydrothermal precipitates recovered from the Tangyin hydrothermal field (THF), Okinawa Trough at a water depth of 1206 m. The natural sulfur samples exhibit the lowest ΣREE concentrations (ΣREE=0.65×10–6–4.580×10–6) followed by metal sulfides (ΣREE=1.71×10–6–11.63×10–6). By contrast, the natural sulfur-sediment samples have maximum ΣREE concentrations (ΣREE=11.54×10–6–33.06×10–6), significantly lower than those of the volcanic and sediment samples. Nevertheless, the δEu, δCe, (La/Yb)N, La/Sm, (Gd/Yb)N and normalized patterns of the natural sulfur and metal sulfide show the most similarity to the sediment. Most hydrothermal precipitate samples are characterized by enrichments of LREE (LREE/HREE=10.09–24.53) and slightly negative Eu anomalies or no anomaly (δEu=0.48–0.99), which are different from the hydrothermal fluid from sediment-free mid-oceanic ridges and back-arc basins, but identical to the sulfides from the Jade hydrothermal field. The lower temperature and more oxidizing conditions produced by the mixing between seawater and hydrothermal fluids further attenuate the leaching ability of hydrothermal fluid, inducing lower REE concentrations for natural sulfur compared with metal sulfide; meanwhile, the negative Eu anomaly is also weakened or almost absent. The sulfur isotopic compositions of the natural sulfur (δ34S=3.20‰–5.01‰, mean 4.23‰) and metal sulfide samples (δ34S=0.82‰–0.89‰, mean 0.85‰) reveal that the sulfur of the chimney is sourced from magmatic degassing.
The genetic type classification of the bauxite deposits in China was oversimplified in the past and the author tries to make a relatively detailed division based on their practical conditions. First, the occurrences of bauxite deposits are divided into platform and geosyncline terrains according to their geotectonic setting. Then, the lateritic, accumulated, sedimentary, solution precipitated diagenetitc-epigenetic, regional metamorphic categories and two metacategories are established based on their main metallogenetic processes. Finally 21 types and 9 metatypes are set up according to their formation features (including genesis, sedimentary environment or major mineral features), of which several types are newly established. The classification is quite different from that of the world's bauxite deposits, the metacategories and metarypes refer to the deposits o only scientific significance at present.
Summary Acoustic characteristics of hydrate-bearing sediment has been investigated as it is important for gas hydrate geophysical exploration and resource evaluation, however, although gas migration play an important role in gas hydrate accumulation system, there are rarely reports on acoustic response of hydrate formation in gas migration system. In this paper, a high pressure apparatus was developed to allow gas continuously migrating from bottom to top vertically. Hydrate saturation (Sh) and acoustic velocities (Vp & Vs) were measured in one system by time domain reflectometry and ultrasonic methods respectively during gas hydrate formation in the gas migration system. The results were compared with the previous data obtained in the closed system, which showed that the acoustic velocities of hydrate-bearing sediment in vertical gas migration system are slightly smaller than that in the closed system during hydrate formation process. The acoustic velocities increase at a constant speed as hydrate saturation increases in closed system, while in vertical gas migration system, the increase of acoustic velocities show a fast-slow-fast process with the increase of hydrate saturation.
This paper provides an overview of the developments in analytical and testing methods and experimental simulations on gas hydrate in China. In the laboratory, the analyses and experiments of gas hydrate can provide useful parameters for hydrate exploration and exploitation. In recent years, modern analytical instruments and techniques, including Laser Raman spectroscopy (Raman), X-ray diffraction (XRD), X-ray computed tomography (X-CT), scanning electron microscope (SEM), nuclear magnetic resonance (NMR) and high pressure differential scanning calorimetry (DSC), were applied in the study of structure, formation mechanisms, phase equilibrium, thermal physical properties and so forth of gas hydrates . The detection technology and time-domain reflectometry (TDR) technique are integrated to the experimental devices to study the physical parameters of gas hydrates, such as the acoustics, resistivity, thermal and mechanical properties. It is believed that the various analytical techniques together with the experimental simulations from large-scale to micro-scale on gas hydrate will play a significant role and provide a powerful support for future gas hydrate researches.
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