Keywords

1 Location and Bathymetric Features of Tarama Knoll

Tarama Knoll is located at 25°05.50′N, 124°32.50′E (Fig. 40.1). The knoll lies almost at the foot of the south fault scarp in the rift graben of the southern Okinawa Trough (Letouzey and Kimura 1986). The knoll has an almost conical shape and an additional small peak on the northern slope (Fig. 40.2). The depths of the main summit and foot are 1,490 and about 2,000 m from the sea surface, respectively, and the total relief from the foot to summit is 510 m. The average steepness is c. 18°, and the steepness does not change significantly from the foot to the summit. Another smaller knoll named Tarama Hill is located about 4 km southwest of Tarama Knoll.

Fig. 40.1
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Locations of Tarama Knoll and Tarama Hill and other hydrothermal fields in the Okinawa Trough

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Fig. 40.2
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Bathymetric map of Tarama Knoll and Tarama Hill obtained by the SeaBat 8160 multibeam echosounder on the R/V Natsushima during NT09-10 Leg. 2 and NT10-06 Leg. 2 cruises. The X indicates the sampling point during the KT05-26 cruise

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2 Observation History of Tarama Knoll

Tarama Knoll was originally described as a single-summit active volcano based on low-resolution seismic and geomagnetic observations carried by the Hydrographic Surveys Division of Japan Coast Guard (Otani et al. 2004). Tarama Knoll and Tarama Hill were then identified as one elongated knoll due to insufficient resolution (Fig. 40.2). This area was visited by the R/V Tanseimaru during the KT05-26 cruise, and a methane anomaly was detected near the seafloor that was considered of possible hydrothermal origin (Fig. 40.3). During the NT09-10 Leg. 2 cruise in 2009, a high-resolution seismic survey was done by the R/V Natsushima (which belongs to JAMSTEC) using a SeaBat 8160 multibeam echosounder, and the results showed two independent knolls. A methane anomaly was detected near the Tarama Knoll, and therefore dive surveys using the ROV/HyperDolphin were carried out at the knoll in 2009, 2010, and 2011.

Fig. 40.3
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Vertical profiles of the methane concentrations and δ13C values of methane dissolved in seawater. Samples were obtained during the KT05-26 cruise

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3 Dive Observations at Tarama Knoll

Since 2009, we have done seven dives using the ROV/HyperDolphin (which belongs to JAMSTEC) at the Tarama Knoll (Table 40.1). All dive tracks are shown in Fig. 40.4. During the dives, we often observed dense turbid water resembling hydrothermal plumes, and many planktonic tunicates were observed in the water. As such planktonic tunicates have often been observed in hydrothermal plumes emitted by active hydrothermal vents in the Okinawa Trough (Miyake et al. 2009), hydrothermal venting from a nearby area was expected. However, no active venting site has yet been found. Visual observation of the seafloor indicated rubble and angular gravel covering the slope (Fig. 40.5a). Although benthic animals were rarely found on the slope, dense sponge colonies were found in the area where dense turbid water covered the seafloor (Fig. 40.5b).

Table 40.1 Results of chemical analyses of seawater and shimmering fluid obtained from Tarama Knoll
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Fig. 40.4
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Dive tracks of ROV/HyperDolphin surveys of this area are shown as red dotted lines. Seven dives (Dive Nos. 1032–1034, 1107–1108, 1132) were done

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Fig. 40.5
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Photographs of the seafloor on the slope of Tarama Knoll. (a) Rubble and angular gravel covered the slope. (b) Dense sponge colonies were observed in some areas on the slope. (c) Red-brown sediment widely covered the southwestern slope near the summit. (d) Many “finger” chimneys were found

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Weak shimmering of clear warm fluid (up to 23 °C) was found on the southwestern slope near the summit at a depth of 1,510–1,540 m in an area covered with red-brown sediment. Many finger-like chimneys consisting of the red-brown sediment were observed in this area (Fig. 40.5c, d), which was named the “Fox site” (Makita et al., in prep.). The shimmering fluid was sampled.

4 Fluid, Rock, and Sediment Chemistry

Turbid water, shimmering fluid, rocks, and red-brown sediments were sampled during dive studies and subjected to chemical analyses. The results of the analyses are summarized in Tables 40.1 and 40.2. Analysis of the turbid water showed that the concentrations of major cations (sodium, magnesium, calcium, and potassium) and anions (chloride and sulfate) were similar to those in ambient seawater, but the methane concentration in the turbid water was one or two orders of magnitude higher than that of the ambient seawater (Table 40.1). These observations suggest that the turbid water may be from a hydrothermal plume. The shimmering fluid showed high concentrations of silica, ammonium, and alkalinity relative to seawater, whereas concentrations of magnesium, sodium, and potassium were slightly lower than those of the ambient seawater. Although the data are insufficient for accurate hydrothermal endmember estimation, the estimated endmember concentration of silica reached around 11 mM. The endmember silica concentration corresponds to a reservoir temperature of 260 °C based on the silica geothermometer, assuming equilibrium with quartz (Fournier 1977). The chemical signature enriched in ammonium, methane, and alkalinity suggests interaction with sedimentary organic matter during hydrothermal circulation, similar to other hydrothermal fluids in the Okinawa Trough (Ishibashi and Urabe 1995). The decreased levels of major cations in the fluid may suggest phase separation or segregation in the subseafloor. Hydrogen sulfide was not detected in the fluid.

Table 40.2 Bulk chemical composition of a rock sample obtained from Tarama Knoll
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Most sampled rocks seemed to be pumice and lava fragments based on their visual texture and whitish color (Fig. 40.6). Bulk chemical composition of one lava fragment sample collected during HyperDolphin dive #1109 (Table 40.2) showed a high silica content (>75 %) suggestive of rhyolitic magma activity.

Fig. 40.6
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Photographs of rock samples. (a) Pumice-like porous and whitish rock. (b) Lava fragment

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The red-brown sediment was rich in iron oxide and was composed mainly of ferrihydrite or possibly microcrystalline goethite (Makita et al., in prep.).

5 Biological Observation

Some animal samples were collected during the dive studies, and details were reported by Yamanaka et al. (Chap. 10). Although typical vent endemic species, such as vesicomyid clams, mytilid mussels, and vestimentiferan tubeworms, were not found during the dive studies, one liparid snailfish specimen (Fig. 40.7) obtained during HyperDolphin dive #1322 contained remains of undigested Alvinocaris shrimp and shell of Provanna snail in its gut. These shrimp and snail species are endemic to hydrothermal vents and methane seeps (e.g., Fujikura et al. 2012) and represent feed of snailfish in hydrothermal vent areas (Takemura et al. 2010). The migrating ability of liparid snailfish is not known, and the possible habitat area may be less than several kilometers square. Isotopic signatures obtained from the sponges had significantly low δ34S values (about +2.5 to +5.0 ‰), indicating that they rely on thioautotrophic products (e.g., sulfur-oxidizing bacteria) that utilize reduced sulfur compounds (e.g., hydrogen sulfide), possibly of hydrothermal origin, as energy sources (Yamanaka et al., Chap. 10).

Fig. 40.7
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Photograph of liparid snailfish specimen that contained Alvinocaris shrimp and shell of Provanna snail in its gut. The specimen was sampled during HyperDolphin dive #1322 in 2011. This photograph was provided by Mr. M. Miyazaki

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Microbial communities in turbid water (plume) were investigated and documented by Sunamura and Yanagawa (Chap. 3). The microbial community structures based on 16S rRNA genes revealed that potential thiotrophic and methanotrophic microbial phylotypes were predominant in the plume. The results were comparable to the microbial communities associated with the active hydrothermal areas in the Okinawa Trough.

The observed dense turbid water (possibly hydrothermal plume water) and biological studies suggest that active hydrothermal venting is occurring in this area, and the venting fluid is expected to contain hydrogen sulfide and venting as a black smoker.