Coenzyme F430 is the hydrocorphinoid nickel complex which acts as active site in methyl-coenzyme M reductase (MCR). The MCR-F430 complex catalyzes the last step of methanogenesis: reduction of methyl-coenzyme M to methane. Since F430 is a common enzyme for mathanogens, it can be a function-specific biomarker to estimate biomass and activities of methanogens in environmental samples. A recently developed high sensitive method for coenzyme F430 analysis and a further purification step revealed F430 concentrations at 69 and 88 meters below seafloor (mbsf) off Shimokita Peninsula (529 and 31.3 fmol g-wet^–1) and at 60 and 275 mbsf at Nankai Trough (31.4 and 26 fmol g-wet^–1). Estimated methanogenic biomass and activities based on reported F430 content and activities in methanogens and prokaryotic cell weight, were ranging from 3.9 × 10⁶ to 2.7 × 10⁶ cells g-wet^–1 and from 1.7 to 11 μmol d^–1 g-wet^–1.

The Bayonnaise knoll, an active submarine volcano belonging to an actively rifted part of the Izu-Bonin volcanic arc, exhibits hydrothermal ore deposits on its caldera floor in a region known as the Hakurei Sulfide Deposit (HSD) area. We observed the HSD area using high-resolution acoustic observation equipment consisting of multibeam echo sounder (MBES), sidescan sonar (SSS), and sub-bottom profiler (SBP) systems, on the AUV Urashima. We used visual and acoustic results to examine the consistency of the HSD area extent and to consider possibilities of other ore areas within the caldera. The resultant high-resolution acoustic imageries suggest expansion of the HSD area to the northeastern caldera wall and the southwestern sub-seafloor of the caldera floor. The SBP data show a thick sediment layer on the western part of the caldera floor where many high-backscattering signals were observed. Small chimney-like features were acoustically observed in the HSD area and also at the central cone and along the rim of the caldera. However, most are remnant features of ancient volcanic activity of the knoll, and thus may not indicate current hydrothermal deposits. Acoustic investigations such as this, along with appropriate interpretation, are very useful to determine the detailed distribution of ore on the seafloor and at the shallow subsurface, and should be an effective tool for regional site surveying before seabed mineral mining.

We conducted four dives using the Urashima autonomous underwater vehicle (AUV) over three hydrothermal vent fields (Snail, Archean, and Pika) within the South Mariana Trough in July 2009, and measured the echo intensity (EI) with an acoustic Doppler current profiler (ADCP) mounted on the AUV. The ADCP data produced 3D structures of the EI over the vents, which clearly showed higher EI anomalies over the Pika and Archean vents. The characteristics of the higher EI anomalies were carefully examined and described in terms of their relationships with turbidity that was simultaneously observed with a single point turbidity sensor. It is plausible that the higher EI anomaly indicates the thickness of the plumes at some locations (i.e., 230 m thickness at the Pika site, 140 m at the Archean, and 160 m at the Snail) where both the turbidity and EI have a signal. The results also explain the long-term southwestward advection of the higher anomaly water by the local mean current.

Chemical and isotopic compositions of hydrothermal fluids from Hatoma Knoll in the southern Okinawa Trough were investigated. The hydrothermal fluids were derived from a single pure hydrothermal fluid source, but they underwent phase separation beneath the seafloor prior to venting. Only vapor-like fluids vent at the Hatoma system, and the most active area is around the center of the crater, based on the location of the maximum temperature and the lowest Cl^– concentrations. Compared with other hydrothermal systems in the world, at Hatoma the pH and alkalinity, as well as the B, NH₄⁺, K, Li, CO₂, and CH₄ concentrations, were higher, and the Fe and Al concentrations were lower, suggesting that the characteristics of the Hatoma hydrothermal fluids are comparable to those of the other Okinawa Trough hydrothermal fluids. Helium isotope ratios were lower than those of sediment-starved hydrothermal systems, suggesting that ⁴He derived from the sediment is supplied to the hydrothermal fluids in Hatoma Knoll. The carbon isotope ratios of CO₂ in the hydrothermal fluids indicate an influence of organic carbon decomposition. The carbon isotope ratios of CH₄ in the hydrothermal fluids imply that most of the CH₄ originated from microbial methane produced in a recharge zone of the hydrothermal system. Although sediment influences are a typical feature of Okinawan Trough hydrothermal fluids, the Hatoma hydrothermal system has the lowest carbon isotope ratios of CH₄ among them, which suggests that Hatoma is the most highly influenced by the sediments in the recharge zone. Thus, the degree of the sediment influences has a variable in each hydrothermal field in the Okinawa Trough.

We report major element and trace element compositions of ferromanganese crusts (Fe-Mn crusts) on Takuyo Daigo Seamount, northwestern Pacific Ocean. Highly enriched elements in these crusts, such as Co (6500 ppm), Ni (4000 ppm), Mo (520 ppm), Te (39 ppm), W (92 ppm), Pt (0.19 ppm), and rare earth elements (REEs; ∑REE = 1700 ppm), exhibit strong positive correlations with either Fe or Mn concentrations. Chemical analyses of the crusts show that the Fe/Mn ratio decreases toward shallower water and older deposits. Thus, deeper or younger crust is more promising as a REE (other than Ce) resource, and shallower or older crust has a greater potential as Co, Ni, Ce, and Pt resources. REE profiles of Fe-Mn crusts with less than 0.5 wt% P, normalized to post-Archean average Australian shale, are flat with positive Ce anomalies whereas samples with more than 0.5 wt% P are enriched in heavy REE, and four samples from deep in the crusts with more than 2.2 wt% P do not exhibit positive Ce anomalies. These differences in REE patterns can be explained by mixing of Fe-Mn oxide and Ca-phosphate minerals (apatite). Assuming that Fe-Mn crusts with 5 cm thickness and 1.29 g/cm³ dry density are homogeneously distributed at water depths from 1000 to 3000 m along the 500 km perimeter of Takuyo Daigo Seamount, the possible reserve represented by Fe-Mn crusts is roughly 65 Mt of ore. Based on the average concentrations obtained here, resource amounts at Takuyo Daigo Seamount are 420 × 10³ t Co, 110 × 10³ tREO, 2500 t Dy, and 440 t Tb, comparable to the known reserves of major mineral producing nations. Thus, Fe-Mn crusts in the northwestern Pacific Ocean have a notable potential as a source of Co.

A dense field of ferromanganese (Fe-Mn) nodules was discovered on a seamount approximately 300 km east of Minamitorishima Island, in the Japanese Exclusive Economic Zone. To evaluate its potential as a resource for critical metals, we describe results of a geological survey using the SHINKAI 6500 submersible (dive No. 6K 1207) during cruise YK10-05 of R/V Yokosuka in 2010, and geochemical analyses of collected samples. Submersible observations showed that spherical nodules 5–10 cm in diameter almost fully cover the region of high acoustic reflectivity. The large nodules generally consist of three concentric layers: the outermost mottled (sediment-filled) layer L0, the massive black layer L1, and the innermost porous (sediment-filled) layer L2. Elements including Fe, Ti, Co, As, rare earth elements other than Ce, Th, U, and Pb are concentrated in the nodule rim rather than the center. In contrast, Mn, Al, P, Ca, Ni, Zn, Y, Mo, Ce, and W are concentrated in the center, and decrease toward the rim. Geochemical and structural features indicate that the nodules are compositionally and morphologically similar to Fe-Mn crusts, suggesting that they owe their origin solely to prolonged hydrogenetic precipitation of Fe-Mn-(oxyhydr)oxides. As the nodules include metals of economic interest, especially Co, Ni, Mo, and W, this deposit should have high potential for future mining. The changes in nodule composition from the center to the rim may yield information on paleoceanographic events since early Oligocene time.

We have discovered deep-sea mud that is extremely enriched in rare-earth elements and yttrium (together called REY) in the Japanese Exclusive Economic Zone around Minamitorishima Island, in the western North Pacific Ocean. The maximum total REY concentration reaches approximately 7000 ppm, which is much higher than that reported for conventional REY deposits on land and other known potential REY resources in the ocean. The extremely REY-rich mud is characterized by abundant phillipsite and biogenic calcium phosphate. In addition, the stratigraphic layer with the highest REY concentration occurs just ~3 m beneath the seafloor. The shallow burial of these strata together with the high REY content, especially those of heavy rare-earth elements, suggest that the newly discovered extremely REY-rich mud may be a promising REY resource.

We report detailed lithological and chemical characteristics of deep-sea sediments, including rare-earth elements and yttrium-rich mud (REY-rich mud), in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island. Three research cruises obtained fourteen sediment cores collected by piston coring. Based on the visual descriptions and geochemical analysis of the sediment cores, we confirm the presence of REY-rich mud containing more than 400 ppm total REY (∑REY) in the southern and northwestern areas of the Minamitorishima EEZ. The REY-rich mud layers are characterized by abundant grains of phillipsite, biogenic calcium phosphate, and manganese oxides, and are widely distributed in relatively shallow depths beneath the seafloor. In contrast, relatively thick, non-REY-rich mud lies near the seafloor in the northern areas of the EEZ. In the three cores from the southern part of the EEZ, we also confirm the presence of highly/extremely REY-rich mud layers. Further accumulation of geochemical data from the sediments will be required to constrain the extent of the highly/extremely REY-rich mud layers.

Seven piston cores were collected from the seafloor ~250 km south of Minamitorishima Island in the western North Pacific Ocean during the cruise KR13-02 of R/V Kairei; in some portions of cores PC04 and PC05, the total contents of rare-earth elements and yttrium (∑REY) exceeded 4,000 ppm. Microscopic observations showed that the highly REY-enriched layers (∑REY > ~2,000 ppm) contained significant amounts of calcium phosphate and phillipsite. We conducted microscopic observations and grain size distribution analyses of bulk sediments and distinctive components (calcium phosphate and phillipsite) from cores PC04 and PC05 to elucidate the mechanism of the anomalous REY enrichment. The shapes of the calcium phosphate grains suggest that they were mostly biogenic in origin. The grain size distributions of bulk sediments from the REY-enriched layers of cores PC04 (7.8–8.6 mbsf) and PC05 (2.6–3.6 mbsf) were bimodal, with peaks at fine (~4 μm) and coarse (~40–80 μm) sizes. Calcium phosphate and phillipsite were the major components of the coarse-grained portions of these REY-enriched layers. The bulk ∑REY content was mainly controlled by the amount of biogenic calcium phosphate, which is well known to concentrate REY. Thus, increased accumulation of biogenic calcium phosphate was responsible for the REY enrichment. The volume-based cumulative median diameters of calcium phosphate and phillipsite grains appeared to be proportional to the contents of both calcium phosphate and ∑REY. Increased phillipsite grain size suggests a low sedimentation rate, which may have allowed biogenic calcium phosphate to accumulate without dilution by low-∑REY components. In addition, increased grain sizes of calcium phosphate and phillipsite suggest that sorting by a bottom current allowed coarse-grained biogenic calcium phosphate to become concentrated in sediments by removing fine-grained particles, including low-∑REY components. Multiple factors should be considered to explain the overall features of the highly REY-enriched layers.

Sub-bottom profiling was conducted in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island, western Pacific Ocean, to investigate the features and distribution of mud rich in rare earth elements and yttrium (REY-rich mud). Based on the echogram records, we distinguished three acoustic facies: opaque (O), transparent (T), and layered (L). The O-type facies is acoustically opaque and highly reflective, without visible structures beneath the top surface. The T-type facies is acoustically transparent, with a basal reflector from the acoustic basement. This facies is subdivided into irregular (T_I) and smooth (T_S) types according to the topography of its upper surface. The irregular surface morphology of the T_I-type is generally parallel to the topography of the acoustic basement, whereas the smooth morphology of the T_S-type is independent from the basement topography. The L-type facies is characterized by a layered sequence of multiple reflectors. It always overlies the T-type facies. Correlation of the acoustic facies of T- and L-types with lithological and geochemical characteristics of sediment core samples shows that T-type facies correspond to REY-rich mud and L-type facies correspond to non-REY-rich sediment covering REY-rich mud. Distribution of the O-type facies is restricted to seamounts or their immediate vicinity, suggesting that it corresponds to rocky outcrops. The T-type facies (REY-rich mud) mainly occurs in the southern and southeastern part of the Minamitorishima EEZ, whereas the L-type facies (non-REY-rich sediment) is widespread in the northern and western part of the EEZ. Our results reveal that, in the southern and southeastern part of the Minamitorishima EEZ, REY-rich mud lies at a shallow depth below the seafloor without a cover of non-REY-rich sediment. These areas, therefore, should be a primary target for future mining of REY-rich mud.