Polymetallic sulfides deposited in seafloor hydrothermal vents have recently attracted attention as potential deep-sea mineral resources for base, rare, and precious metals such as Cu, Zn, Pb, In, Ga, Ge, Au, and Ag. For future exploitation of this type of deep-sea mineral resources, development of effective methods for exploring seafloor hydrothermal activity is a key to provide the most promising list of fields. However, conventional exploration methods are likely laborious and time-consuming, and a more efficient methods for exploration of seafloor hydrothermal vents are to be further developed. In the last decade, water column observation using multibeam echo souder (MBES) systems have become successfully applied to exploration of seafloor hydrothermal vents. In 2013 and 2014, we conducted extensive water column surveys using MBES systems in the mid-Okinawa Trough. During the surveys, we detected 10 hydrothermal vent sites, including previously known sites, belonging to four relatively large hydrothermal vent fields, located at the Izena Hole, Iheya North Knoll, Iheya Small Ridge, and a seamount 15 km northwest of the Izena Hole. All of the hydrothermal vent sites are in groups of 2–3 vent sites belonging to a hydrothermal field. Morphological features of the acoustic water column anomalies (rising vertically up to ~1000 m from the seafloor without a significant change of width) implied that the acoustic water column anomalies were not caused directly by hydrothermal vent fluid flows. The depth of the top of the acoustic water column anomalies (~500 m) corresponded rather well to the depth of the CO₂ phase transition from liquid/clathrate-hydrate to vapor. This suggests that the acoustic water column anomalies are attributed to water mass including dense liquid CO₂ droplets with clathrate-hydrate crusts, which are originally derived from the seafloor hydrothermal fluid discharges.

View full abstract

A survey method using acoustic equipment has attracted recently interest of researchers for the research of gas bubbles and hydrothermal activity because gas bubbles and hot water exposed from hydrothermal vents produce acoustic impedance anomalies in cold seawater. We tried a new hydrothermal exploration using a hull-mounted Multi Beam Echo Sounder (MBES) and a high-frequency MBES system mounted on AUV URASHIMA as one approach in the first stage of a comprehensive hydrothermal deposit survey process including geophysical exploration. The surveys using the MBES system were conducted at the North Iheya Knoll. First, we discovered heretofore unidentified hydrothermal activity at two areas of the North Iheya Knoll using hull-mounted MBES. Then, MBES data obtained during the AUV dive provided detailed positions of new hydrothermal activity zone. The existence of previously unknown hydrothermal vent was identified through ROV dives using our estimated plume position data for the North Iheya Knoll. These achievements demonstrate clearly that the acoustic survey using shipboard MBES and a vehicle with high-frequency MBES are useful as one approach for hydrothermal exploration.

View full abstract

A deep-sea hydrogen sulfide ion (HS^–) sensor was developed using electrochemical techniques for the direct measurement of hydrogen sulfide ion. In the sensor, linear sweep voltammetry is applied using three simple electrodes, Ag working electrode, Pt counter electrode and Ag/AgCl reference electrode. The measurement times are very small approximately 20 s. The sensor does not require an ion-selective membrane or chemical modification of the electrodes and are suitable for use in the deep-sea environment. In addition, the sensor is encased in a pressure-resistant container, and the electrodes have pressure adjustment functions. As a result, the sensor can withstand the temperature and pressure that exist up to 45°C and at water depths of 5000 m, respectively. With this sensor, it was possible to measure hydrogen sulfide ion levels with a detection limit of 2.2 μmol/L and a quantification range of 2.2–700 μmol/L. Field applications of the hydrothermal plume observations in the Mariana Trough and Okinawa Trough clarified the spatial distribution of hydrogen sulfide ion around the hydrothermal vents.

View full abstract

Development of submarine resource may occur pollution of mercury in the ambient seawater. It is useful if there is a handy mercury-monitoring tool for deep-sea. We developed a deep-sea mercury sensor based on an anodic striping voltammetry method. To enhance the sensitivity of the sensor to mercury in seawater, we used a large gold-ring disc with a surface area of 402 mm² as a working electrode. In addition, a propeller screw was located in front of the working electrode to create water flow and enhance the electrodeposition efficiency of the mercury. The sensor was able to detect peak current, depending on the mercury concentration in a 0.6 M sodium chloride solution in a test water container with 0.94 ng L^–1 (ppt) of the lowest detection limit (deposition time was 20 min), which was lower than 800 ng L^–1 of the lowest detection limit with an electrode surface area of 20 mm². We were then successful in in-situ measurement of mercury at the ppt level in the sea, coordinating with another measurement using the cold vapor atomic fluorescence spectrometry method. Accurate calibration was difficult for the sensor in the labo-scale. It is necessary to build another calibration method without a large volume of standard solution in the future. In addition, the sensor does not fully work in H₂S-rich environments (more than about 1 μmol L^–1) adjacent to hydrothermal fluid discharges. In spite of the weak points, this sensor will be a very useful tool for wide-range monitoring mercury in seawater.

View full abstract

We analyzed 1338 deep-sea sediment samples from 19 Deep Sea Drilling Project/Ocean Drilling Program sites covering a large portion of the Indian Ocean, and constructed a new and comprehensive data set of their bulk chemical compositions, including rare-earth, major, and trace elements. The bulk-sediment rare-earth and yttrium (REY) composition of the REY-enriched samples, characterized by relatively small negative Ce anomalies, almost no Y anomalies, and enrichment in heavy rare-earth elements, can be interpreted as the superposition of the REY compositions of apatite and hydrogenous Fe-Mn oxides. Although the hydrothermal component is a key factor in the formation of REY-rich mud in the Pacific Ocean, it is less important in the Indian Ocean, probably because there is less hydrothermal input of Fe-oxyhydroxides from seafloor hydrothermal vents there. The relationships among Fe₂O₃, MnO, P₂O₅, Co, and total REY contents suggest that a primary factor controlling REY enrichment in deep-sea sediments is the sedimentation rate. A low sedimentation rate allows both fish debris apatite and hydrogenous Fe-Mn (oxyhydr)oxides to accumulate in the surface sediments. On the basis of these results, we identified two potential areas in the Indian Ocean where REY-rich mud may be present in surface sediments: the south-southeastern Wharton Basin and the southern Central Indian Ocean Basin. The resource potential of the latter area might be particularly high if the distributions of REY-rich mud and Fe-Mn nodule fields broadly overlap.

View full abstract

Seafloor sediment rich in rare earth elements and Y (REY-rich mud) has received attention as a new resource for REY. During research cruise KR13-02 of R/V Kairei, mud containing more than 5,000 ppm total REY was collected near Minamitorishima Island, northwestern Pacific Ocean. We conducted a series of chemical leaching experiments on this material, varying the acid concentrations, leaching times, and temperature to determine the optimum conditions of REY leaching from REY-rich mud. The highest extraction efficiency of REY other than Ce was 95.1% using hydrochloric acid and 81.3% using sulfuric acid. Extraction efficiency of REY was highest under conditions of relatively low acid concentrations (0.25–0.5 mol/L), short leaching times (2–5 min), and room temperature (25°C). REY extraction amount decreased with increasing acid concentration, leaching time (only in the case of sulfuric acid), and temperature, apparently because of precipitation of calcium sulfate, rare earth phosphate, and rare earth-Na double sulfate. Although further research is needed, the leaching properties we determined are generally favorable for industrial exploitation of REY-rich mud.

View full abstract

The migration of REEs in pelagic siliceous sediments were studied, especially (i) accumulation of REEs at sea floor to Mn⁴⁺ oxides, (ii) release of REEs from Mn⁴⁺ oxides accompanied with the reductive dissolution of Mn⁴⁺ oxides during early diagenesis, and (iii) incorporation and fixation of REEs released from Mn⁴⁺ oxides to phosphates such as apatite below 0.6 meters below sea floor (mbsf). These processes have been indicated by various geochemical findings: (a) chemical compositions of bulk sediment and pore water, (b) REE patterns of bulk sediment, (c) oxidation states of Ce, Mn, and Fe and host phase of Y by XANES, and (d) chemistry of specific phases such as Mn⁴⁺ oxides and apatite by means of chemical leaching, LA-ICP-MS, and XANES. The roles of Mn⁴⁺ oxides and apatite as host phases of REEs at sea floor and below 0.6 mbsf, respectively, were discussed using the chemical leaching data. Reductive dissolution of Mn⁴⁺ oxides and reduction of Ce⁴⁺ to Ce³⁺ with depth were revealed by direct determination of oxidation states of Mn and Ce by XANES. The transfer of REEs released by the reductive dissolution of Mn⁴⁺ oxides is strongly suggested by the presence of positive Ce anomalies in apatite at 0.80 mbsf (LA-ICP-MS) and at 1.80 mbsf (chemical leaching), which must be inherited from Mn⁴⁺ oxides which can accumulate Ce by oxidizing Ce³⁺ to Ce⁴⁺. This observation shows that apatite fixes the REEs with positive Ce anomaly once dissolved from Mn⁴⁺ oxides during early diagenesis. Consequently, we found that total REEs in the two phases (Mn⁴⁺ oxides and apatite) are preserved even after diagenetic alteration, because apatite fixes the most of the REEs released from Mn⁴⁺ oxides. The results indicate two geochemical implications: (i) REE abundances in apatite in sediment, which has attracted great interests in terms of REE resources, depend on the amount of REEs fixed in Mn (and Fe) oxides initially formed at the sediment surface, and then apatite finally fixes the REEs during early diagenesis; (ii) the reliability of apatite as a proxy of seawater chemistry is affected seriously by the overprint of REE signature by the diagenetic effect. However, if the contribution of REEs in Mn (and Fe) oxide is small, then the REE pattern of apatite can preserve information of the REE pattern of seawater, including the degree of Ce anomaly.

View full abstract

We performed a calibration of a Ge-detector gamma ray spectrometer to measure the activity of ⁴⁰K in natural volcanic glass and minerals. The goal of this experimental study is to demonstrate that non-destructive measurements of ⁴⁰K in rocks and minerals can be obtained with a good precision, comparable at 2σ to that obtained by classical atomic absorption spectroscopy. This is the first step toward an enhanced K-Ar dating method in which both parent (⁴⁰K) and daughter (⁴⁰Ar*) nuclides are measured in the same sample aliquot. This method should minimize the undeterminable internal error produced by the natural chemical heterogeneity of the measured sample. The efficiency of the Ge-detector was calibrated at 1461 keV, corresponding to the emission energy of ⁴⁰K, on five geostandards of biotite, feldspar, trachyte, glauconite and a synthetic glass. Calculated efficiency ranges from 0.0403 to 0.0442 with a mean value of 0.0418 ± 0.0018. Comparison between K₂O contents calculated after measurements on the gamma ray detector (K₂Oγ) of nineteen natural volcanic mineral and groundmass samples and those obtained by classical atomic absorption spectroscopy (K₂O_AAS) are identical within 2σ uncertainties. Best results are obtained by calibrating the natural sample measurements with the efficiency obtained for the synthetic glass geostandard VS-N with a K₂Oγ/K₂O_AAS of 1.040 ± 0.060.

View full abstract

Heretofore the presence of platinum in the hydrogenetic ferromanganese crust has been known yet, the host to platinum has remained poorly understood. Here we first report on the microanalysis of platinum in ferromanganese crust using secondary ion mass spectrometry (SIMS). SIMS microanalysis revealed that Pt concentration of vernadite was between 0.21 and 1.22 ppm, and that of brown matrix was very low (around the detection limit of 0.036 ppm or less) in a ferromanganese crust. The SIMS spatial resolution was 3 μm by 3 μm on the sample surface, and several tens of nanometers along the depth. A novel Pt detection method using SIMS was performed, in which most SIMS depth profiles show homogeneous Pt concentrations (0.21 to 0.55 ppm), however, some profiles detected local enrichment of Pt on the nm level.

View full abstract