Geochemical Characteristics Of The Sediments And Bottom Seawater Near Hydrothermal Fields,Mid-Okinawa Trough

With the application of the in situ sensors(carbon dioxide sensors, methane sensors and CTD) carried by ROV(Remotely Operated Vehicle), properties of bottom seawater were revealed in Iheya North and CLAM hydrothermal fields in mid-Okinawa Trough. Besides, it is also the first time to have a comprehensive analysis of methane and carbon dioxide dissolved in seawater near different activity hydrothermal fields. At the meanwhile, the control factors of hydrothermal faunas assemblage were priliminary discussed combined with video materials and distribution characteristics of seawater temperatures. On the other hand, the surface hydrothermal sediments were first collected by push core mounted on ROV, and the distance between hydrothermal vent and sampled site were only about 35 m. However, previous sampled sediments collected by grab had a long distance to hydrothemral vent. Therefore, unique geochemical characteristics of near hydrothermal vents were indicated through analysis.The results of methane and carbon dioxide concentration dissolved in seawater far from hydrothermal vent( normal seawater) were 0 μmol/l and 400 ppm, respectively. However, the counterparts near black smoker reach up to 8μmol/l and 12000 ppm, which are relative higher than those near low-temperature diffusive oriface, with the concentrations of 2 μmol/l and 4000 ppm. In spite of this, it is still ten times higher than normal seawater in terms of concentrations of carbon dioxide. The other studied hydrothermal field- CLAM, was featured by dead chimneys, and there was no vigourously venting black smoker observed during this research. There was no methane detected, and only carbon dioxide was found near dying hydrothermal fauna assemblage, the maximum concentraion was 2200 ppm. As previous scholars focused more on hydrothermal fluids, and proved the enrichment of carbon dioxide and methane in hydrothermal fluids. However, little attention was paid on seawater near hydrothermal vents. In this research, carbon dioxide and methane gas dissolved in seawater were first detected by in situ sensors, and has at least four magnitude greater than traditional gas chromatograph due to the vicinity to sampled seawater.The geochemical characteristics of surface hydrothermal sediments indicated that major components are SO3, ZnO and Fe2O3, which shows an obvious distinction to normal seafloor sediments. The latter is characterized by Al2O3, SiO2 and CaO. In addition, the related analysis reflects Sr, Hg and Ag are the hydrothermal indicators. Some minor elements like Cu, Pb, Zn, As, Sb, Hg, Se, Ag, Ba, Mo and Cd are also highly enriched in hydrothermal sediments. What is more, the R-mode factor analysis further shows the collected sediments are mainly from hydrothermal input, terrestrial input and biogenous input. It is also suggested weak interactions between sediments/hydrothermal fluids by strontium ratios.