No abstract

Bottom-simulating reflectors were observed beneath the southeastern slope of the Dongsha Islands in the South China Sea, raising the potential for the presence of gas hydrate in the area. We have analyzed the chemical and isotopic compositions of interstitial water, headspace gas, and authigenic siderite concretions from Site 1146. Geochemical anomalies, including a slight decrease of chlorine concentration in interstitial water, substantial increase of methane concentration in headspace gas, and 18 O enrichment in the authigenic siderite concretion below 400 meters below seafloor are probably caused by the decomposition of gas hydrate. The low-chlorine pore fluids contain higher molecular-weight hydrocarbons and probably migrate to Site 1146 along faults or bedded planes.

Gas hydrate formation conditions and its fraction out of pore space are two important issues. Based on the theoretical thermo‐dynamic model of gas hydrate formation and accumulation, the semi‐quantitative analysis on various intrinsic factors (such as temperature, pressure, gas composition, pore‐water salinity, sediment pore size) is conducted to characterize their effects on gas hydrate formation conditions and its fraction out of pore space. The analytical results indicate that gas composition plays a significant role in affecting gas hydrate formation conditions of temperature and pressure; comparatively the effect of the mixture of methane and propane is the largest and that of the mixture of methane and hydrogen sulphide is secondary, and effects of the mixture of methane and ethane and the mixture of methane and carbon dioxide follow in turn with respect to pure methane. Pore‐water salinity also greatly affects gas hydrate formation conditions. Sediment pore size within a certain range (1 × 10–6 ~ 4 × 10–8m) has a small effect on gas hydrate formation conditions; beyond this range when sediment pore size becomes smaller and smaller, its effect gets more and more evident on inhibiting gas hydrate formation and especially when sediment pore size is smaller than 1×10–9m, it seems impossible for pure methane and pure water to form gas hydrate; contrarily when sediment pore size is bigger than 1 × 10–3m, it nearly has no effect on gas hydrate formation conditions. Gas hydrate fraction out of pore space is little affected by pore size and pore‐water salinity, but is strongly dependent on gas supply.

The Qarhan potash‐magnesium salt deposit, which consists from east to west of four playa districts, namely Huobuxun, Qarhan, Dabuxun and Bieletan, may be divided from bottom to top into three salt beds (S1 S2 and S3), with potash salts mainly occurring in S3 The paper lays emphasis on the evolutionary history of the deposition of Salt Bed S3 and the modes of formation of potash salts in the Qarhan playa.The analysis of the data obtained from more than 170 boreholes and the geochemical studies on bromine, magnesium and calcium reveal the existence of four concentration stages and three dilution stages during the formation of the Upper Salt Bed S3. In the process of concentration, evaporation of brine and precipitation of salt minerals dominated, which obeyed the regularity indicated by the phase diagram of the quaternary system Mg‐Na‐K/Cl−H2O. In the dilution stages, however, mixing of brines, dissolution of salts (especially the playa) and mechanical clastic deposition played the leading role. The prerequisite for mixing in the Qarhan salt deposit lies in that chloride type brine of the northeastern part coexists with magnesium sulfate type brine of the southwestern part in the same salt lake. The appearance of bassanite has verified the process of mixing. The dissolution refers to the partial dissolution of the salt beds originally deposited. Particularly in the last dilution process, approximately 200 km2 of the playa north of the Tuanjie Lake had been dissolved before a new‐born lake took form.Solid potash salts were formed in three ways: (1) The surficial brine of the Dabuxun lake coexistent with the playa had an intercompensational relationship with the intercrystalline brine in the playa. As a result, K+ ions were collected into the lake, and layered carnallite precipitated from lake brine during the seasons of evaporation, which was accelerated by the action of wind. (2) Disseminated potash‐magnesium salts were formed by further concentration of intercrystalline brine. (3) Dissolution and reformation of the playa resulted in the formation of a new‐born salt lake, in the drying of which brine collected the potash‐magnesium salts originally dispersed over the playa into local low‐lying areas, forming finally stratoid potash‐magnesium salts.

Gas hydrate, mainly composed of hydrocarbon gas and water, is considered to be a clean energy in the 21st century. Many indicators such as BSRs (Bottom‐Simulating Reflections), which are thought to be related to gas hydrate, are found in the South China Sea (SCS) in recent years. The northeastern part of the SCS is taken as one of the most potentials in the area by many scientists. It is situated in the conjunction of the northern divergent continental margin and the eastern convergent island margin, whose geological settings are much preferable for gas hydrate to occur. Through this study, brightness temperature anomalies recorded by satellite‐based thermal infrared remotely sensed images before or within the imminent earthquake, the high content of hydrocarbon gas acid‐degassed from subsurface sediment and the high radioactive thermoluminescence value of subsurface sediment were found in the region. Sometimes brightness temperature anomalies alone exist in the surrounding of the Dongsha Islands. The highest content of hydrocarbon gas amounts to 393 μL methane per kilogram sediment and the highest radioactive thermoluminescence value is 31752 unit; their geometric averages are 60,5 μL/kg and 2688.9 unit respectively. What is more inspiring is that there are three sites where the methane contents are up to 243, 268 and 359 μL/kg and their radioactive thermoluminescence values are 8430, 9537 and 20826 unit respectively. These three locations are just in the vicinity of one of the highest confident BSRs identified by predecessors. Meanwhile, the anomalies are generally coincident with other results such as headspace gas anomaly in the sediment and chloride anomaly in the interstitial water in the site 1146 of Leg 184. The above‐mentioned anomalies are most possibly to indicate the occurrence of gas hydrate in the northeastern SCS.