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The subsurface sediment–pore water system in the Jade hydrothermal field was investigated along with pore water chemistry and X-ray absorption fine structure analysis for sediment samples. Sediments were collected by coring near the TBS vent (Core 1186MBL) and from near the Biwako vent (Core 1188MB), which are active hydrothermal vents with high (320 °C) and low (90 °C) temperature fluids, respectively. Core 1186MBL is characterized by occurrences of chimney fragments in the shallow part (2–9 cmbsf) and native sulfur in the deep part (6–19 cmbsf). The results of chemical analyses of the pore water suggest a seawater recharge from the seafloor into Core 1186MBL. This hydraulic characteristic of the pore water, which is commonly observed near active submarine hydrothermal vents, leads to oxidation of sulfide minerals in chimney fragments in the shallow part by the oxic recharged seawater. The resulting acidic and suboxic pore water is transported downward, and can form native sulfur in the deeper part of Core 1186MBL. Core 1188MB shows a wide distribution of native sulfur, and its pore water chemistry indicates anaerobic oxidation of methane below 8 cmbsf and a mixing of seawater and hydrothermal fluid below 10 cmbsf. The mixing of acidic and anoxic hydrothermal fluid and seawater allows the precipitation of native sulfur in Core 1188MB, and the uniform circumneutral pH condition despite the input of acidic hydrothermal fluid. The native sulfur deposits in the arc–back-arc hydrothermal fields provide important geochemical information that is useful to understand the subsurface sediment–pore water system involving hydrothermal fluids

In the Okinawa Trough, microbial communities in deep-sea hydrothermal vent chimneys have been extensively studied in the Iheya North (Iheya North Knoll) and the Yonaguni Knoll IV (Daiyon-Yonaguni Knoll) fields. In comparison of the microbial community patterns in geographically and geologically diverse deep-sea hydrothermal systems all the world, the Okinawa Trough deep-sea vent microbial communities have been known to be distinctive and to be associated with the unique hydrothermal fluid chemistry commonly observed in the Okinawa Trough systems. However, this implication is based only on the data from the above two hydrothermal systems. Here, we further show the microbial community patterns of Minami-Ensei (Minami-Ensei Knoll), Izena Hole Jade and Hakurei fields in the Okinawa Trough, using data by culture-dependent techniques. The comparison of the microbial community patterns determined in typical chimney habitats of representative Okinawa Trough hydrothermal systems including new data points that the diversity and abundance of cultivated microbial populations are significantly relevant with intra-field variation of hydrothermal fluid chemistry induced by the subseafloor phase-separation and -partition processes rather than with inter-fields variability in the endmember fluid chemistry. Inter-fields variability in potential microbial community development in the Okinawa Trough hydrothermal systems may be caused by other possible bases in the microbial community development such as the physical mode of hydrothermal fluid discharges, the age of hydrothermal system and the hydrogeological complexity of subseafloor hydrothermal fluid flow structures. Nevertheless, the correlation between the H concentration in hydrothermal fluid and the culturable population size of H-trophic methanogens is reinforced by the new results obtained from the Izena Hole Hakurei chimney habitat hosting the H-enriched hydrothermal fluid. It seems very likely that the H concentration in the hydrothermal fluid is still a very important chemical factor to control the chemolithotrophic microbial community development essentially in the global deep-sea hydrothermal systems.

The doubling time of indigenous bacteria in mixing-zone of hydrothermal fluid and seawater was determined using a diffusion chamber unit deployed on the field of Hatoma Knoll (24° 51.50′N, 123° 50.50′ E), which is a submarine volcano located on southern Okinawa Trough. The diffusion chamber is a reliable tool to incubate and to directly measure the microbial growth under in situ condition of deep-sea, although an operation of submersible and a complicated preparation of seed water became the technical constraints. The doubling time at non-vent site distant from active vent site was estimated from 86 to 110 h, while at active vent sites more rapid doubling time, 21–32 h, were estimated. A potential sulfur-oxidizing bacteria belonging to dominated the population grew in the chambers, which were incubated using the plume water obtained from the mixing zone between the vent fluid and seawater, and Bathymodiolus colony, while no detection of . The methane-oxidizing bacteria were detected only from gill and digestive tract of , and could not be detected from the chamber, although the chamber was placed on Bathymodiolus colony. The results of this study suggested that chemolithoautotrophic growth near by the hydrothermal vent is sustained by the rapid doubling time of using chemical species dissolved in fluid and provides the chemoautotrophic product to deep-sea benthopelagic community, as well as a microbial products in hydrothermal vent plume.

In deep-sea hydrothermal vent fields, faunal distribution is associated with geochemical environments generated by hydrothermal vent activity. The Okinawa Trough is located between the Eurasian Continent and the Ryukyu Arc, and is characterized by sediment-associated fauna associated with vents. In this chapter, the faunal distribution in hydrothermal vent fields in the Okinawa Trough is summarized at inter- and intra-field levels, and its relationship with geochemical environments and species-specific ecologies is discussed. Although the zonation of sediment-associated fauna is not distinct, the fauna on rocky seabed can be categorized into four zones based on thermal conditions. Genetic connectivity among conspecific populations in the Okinawa Trough appears to be common; further, a few faunal groups, such as tubeworms, show connectivity to regions outside of the Okinawa Trough. The faunal composition of vent communities in the Okinawa Trough has been categorized into three groups: the Minami-Ensei Knoll, Yoron Hole, and middle-southern Okinawa Trough. To determine more precisely the relationships between faunal composition and environmental factors in the Okinawa Trough vent fields, both generalized and vent-specific environmental factors should be measured simultaneously with quantitative faunal sampling for analyses.

We conducted the side-scan sonar observations of the Yokoniwa Rise, a non-transform offset massif at the southern Central Indian Ridge using the autonomous underwater vehicle AUV- fitted with a 100 kHz sidescan sonar system. We identified two terrain types with high backscattering signals; one terrain type exhibited typical volcanic features, while the other appeared to correspond to peridotite outcrops. The orientation of linear features identified in the survey area was highly variable, but appeared to be affected by local bathymetry. The standard deviations of the orientation and average length of these linear features were larger and smaller, respectively, than those of similar features observed along the East Pacific Rise. These observations showed that the linear features on the sonar image were likely to be flow channels or areas of the seafloor that had experienced gravitational collapse. A few small chimney-like structures were also detected.

A high-resolution acoustic investigation using the AUV has revealed detailed volcanic and tectonic features along the neo-volcanic zone of the intermediate-rate spreading Southern Mariana Trough, where the high magma flux forms fast-spreading type axial high morphology. Side-scan sonar imagery suggests that the survey area mainly consists of two types of terrain: high-backscattering lumpy terrain occupies the majority of the neo-volcanic zone, and low-backscattering terrain is scattered over the entire area to form various bathymetric features. Visual observations by the submersible show that the former corresponds to bulbous pillow lava and the latter to jumbled or wrinkled sheet lavas. The estimated proportion of sheet lava with respect to study area is approximately 10 %. Pillow lavas are flatly distributed and do not form the pillow mounds that are common in the slow-spreading Mid-Atlantic Ridge. Furthermore, we did not observe any pillars, collapse features, or axial summit troughs, all of which are frequently reported in the fast-spreading East Pacific Rise.

A high-resolution acoustic investigation using AUV- reveals the geological features of three off-axis hydrothermal sites at the Southern Mariana Trough. They are developed ~5 km in distance from backarc spreading axis. The site is developed at the foot of axial horst and forms 60 m-high mound. Hydrothermal chimneys in the site are arrayed along the ridge of the mound. Our acoustic observations detect small structures along the ridge which could be interpreted as the chimneys. To the south of the mound, the seafloor is characterized by rough and elongated fabrics approximately trending in NE–SW direction, which coincides with the strike of background seafloor slope. Visual observation indicates that the area consists of lava tube slightly covered by sediment. The and the sites are developed on top and at foot of a ~1,800 m-high off-axis knoll. Unprocessed sidescan sonar imagery above the and the sites shows anomalous backscattering signatures in water column. A series of hills with convex shape develops on the southwestern slope of the off-axis knoll. It shows unique facies that is rough surface with high-backscattering intensity on the sonar imagery. Tube lavas are recognized in corresponding seafloor by visual observation.

During NT10-16 cruise with R/V NATSUSHIMA and ROV HYPER-DOLPHIN in 2010, a new hydrothermal vent community was discovered at the Yoron Hole, where is the shallowest hydrothermal system in the known hydrothermal site found in the Okinawa Trough. The maximum temperature of clear smoker fluid was 247 °C, which was slightly lower than the boiling temperature (275 °C) of the seawater at the water pressure of 560-m water depth (5.6 MPa). Hydrothermal chimneys are composed of barite, sphalerite, galena, pyrite, chalcopyrite, and tetrahedrite. Although crustaceans and polychaetes were found around the hydrothermal vent, the diversity and population density were lower than that of the Iheya North Knoll and the Izena Hole hydrothermal sites.

The Irabu Knoll hydrothermal vents were first discovered in 2000 during YK00-06 cruise using R/V YOKOSUKA and manned submersible SHINKAI 6500. The Irabu Knoll consists of three seamounts from 1,680 to 1,970-m in water depth. The Irabu Knoll is located in the Southern Okinawa Trough and is constructed from basalt as the host rock. Two hydrothermal venting sites have been found at East and West seamount. Sulfide deposit of the Irabu Knoll hydrothermal system consists of barite, sphalerite, pyrite, and chalcopyrite. Major taxa in the vent ecosystem are sp., and sp.

Tarama Knoll is located about 60 km north of Tarama Island, Sakishima Islands, southwestern Japan. The knoll has an almost conical shape, with foot and summit depths of 2,000 and 1,490 m (total relief = 510 m) from the sea surface, respectively. This area has been identified as a possible active submerged volcano called “Tarama Knoll” (Otani et al. 2004). However, there are actually two separate knolls in the area. This knoll is located northeast of the other, which is named Tarama Hill. During the KT05-26 cruise on the R/V , a methane anomaly was detected near the seafloor around the area and was considered to be of possible hydrothermal origin. Based on visual observation of the seafloor and its bathymetry, this knoll is considered a pumice cone. Dense turbid water is often observed around summit of the knoll, and a methane anomaly was detected in the water. These observations suggest that the turbid water is a hydrothermal plume. An iron-rich, red-brown sediment-covered area was discovered at a depth of 1,510–1,540 m on the southwestern slope near the summit. At the red-brown sediment area, a weak shimmering of clear fluid could be observed, and the fluid temperature reached 20 °C. Sampled shimmering fluid showed a high silica concentration (≥1 mM), indicating an interaction between the fluid and the surrounding rock. These chemical data support the occurrence of active hydrothermal circulation at Tarama Knoll.