Showing papers by "Kentaro Nakamura published in 2016"

Tracking the spatiotemporal variations of statistically independent components involving enrichment of rare-earth elements in deep-sea sediments.

Abstract: Deep-sea sediments have attracted much attention as a promising resource for rare-earth elements and yttrium (REY). In this study, we show statistically independent components characterising REY-enrichment in the abyssal ocean that are decoded by Independent Component Analysis of a multi-elemental dataset of 3,968 bulk sediment samples from 101 sites in the Pacific and Indian oceans. This study for the first time reconstructs the spatiotemporal variations of the geochemical signatures, including hydrothermal, hydrogenous, and biogenic calcium phosphate components that were closely involved in the formation of REY-rich mud over the past 65 million years. An underlying key factor of significant REY-enrichment is a sufficiently low sedimentation rate that enables the mud to accumulate REY from seawater. In the early Cenozoic, a remarkably small supply of aeolian dust, compared with any other time and region, facilitated the deposition of very high-grade REY-rich mud in the South Pacific. This indicates an important link between the genesis of the seafloor mineral resources and Earth's dynamic phenomena such as climate change and plate tectonics.

Discovery of extremely REY-rich mud in the western North Pacific Ocean

Abstract: Copyright © 2016 by The Geochemical Society of Japan. ment of green technologies, including electric and hybrid vehicles, compact and long-life LED lights, and wind power generation, is required to shift human society toward sustainable growth. Rare-earth elements and yttrium, hereinafter together called REY, are essential elements for products with these advanced and green technologies, because of the magnetic and optical properties of REY that result from the characteristic configurations of electrons in these elements. With rising economic growth in Discovery of extremely REY-rich mud in the western North Pacific Ocean

Geology and geochemistry of ferromanganese nodules in the Japanese Exclusive Economic Zone around Minamitorishima Island

Abstract: Copyright © 2016 by The Geochemical Society of Japan. moteness is an obstacle to mining operations. However, large Fe-Mn nodules have also been found to be densely distributed on a seamount in the eastern part of the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima (Marcus) Island, Japan, where their resource potential is more promising because they are located closer to the main islands of Japan than the CCZ. Thus, further exploration in the EEZ can be expected, along with future development followed by the self-sufficiency and the stabilization of the market for critical metals. In this report, we describe results of a geological survey using the submersible SHINKAI 6500 and a multiple narrow-beam echo sounder system during cruise YK10-05 of R/V Yokosuka. We also present the results of chemical analyses to define geochemical features for representative Fe-Mn nodules and adjacent Fe-Mn crusts. Geology and geochemistry of ferromanganese nodules in the Japanese Exclusive Economic Zone around Minamitorishima Island

Geological factors responsible for REY-rich mud in the western North Pacific Ocean: Implications from mineralogy and grain size distributions

Abstract: Copyright © 2016 by The Geochemical Society of Japan. increased access to REY resources are critically needed. Recently, Kato et al. (2011) reported the existence of extensive new REY resources on the abyssal seafloor of many areas of the Pacific Ocean. They referred to pelagic sediment with total REY contents (ÂREY) greater than 400 ppm as “REY-rich mud”. By chemical analyses of sediment samples from two Deep Sea Drilling Project/ Ocean Drilling Program (DSDP/ODP) cores (DSDP Hole 198A and ODP Hole 800A), Kato et al. (2012) identified REY-rich mud within the Japanese Exclusive Economic Zone around Minamitorishima Island (Minamitorishima Geological factors responsible for REY-rich mud in the western North Pacific Ocean: Implications from mineralogy and grain size distributions

Geochemistry of REY-rich mud in the Japanese Exclusive Economic Zone around Minamitorishima Island

Abstract: KOICHIRO FUJINAGA,1,2 KAZUTAKA YASUKAWA,3,1 KENTARO NAKAMURA,3 SHIKI MACHIDA,4 YUTARO TAKAYA,5,2,4 JUNICHIRO OHTA,6 SHUHEI ARAKI,3 HANJIE LIU,3 RYO USAMI,3 RYOTA MAKI,3 SATORU HARAGUCHI,6 YOSHIRO NISHIO,7 YOICHI USUI,4 TATSUO NOZAKI,4,8,2 TOSHITSUGU YAMAZAKI,9,10 YUJI ICHIYAMA,11 AKIRA IJIRI,12,4 FUMIO INAGAKI,12,4 HIDEAKI MACHIYAMA,4 KOICHI IIJIMA,4 KATSUHIKO SUZUKI,4 YASUHIRO KATO2,3,1,4* and KR13-02, MR13-E02 LEG 2 AND KR14-02 CRUISE MEMBERS

Acoustic characterization of pelagic sediments using sub-bottom profiler data: Implications for the distribution of REY-rich mud in the Minamitorishima EEZ, western Pacific

Abstract: Copyright © 2016 by The Geochemical Society of Japan. large resource potential by virtue of its wide distribution, (2) high REY concentrations with significant enrichment in heavy rare earth elements, (3) a stratiform distribution that allows relatively simple and cost-effective exploration, (4) very low concentrations of radioactive elements such as Th and U, and (5) ease of extraction of REY by acid leaching (Nakamura et al., 2015). In 2012, REY-rich mud was also reported in the western Pacific, including the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island (Kato et al., 2012). This discovery was based on chemical analyses of sediment core samples from two Deep-Sea Drilling Acoustic characterization of pelagic sediments using sub-bottom profiler data: Implications for the distribution of REY-rich mud in the Minamitorishima EEZ, western Pacific

Fluid chemistry in the Solitaire and Dodo hydrothermal fields of the Central Indian Ridge

Abstract: Fluid chemistry and microbial community patterns in chimney habitats were investigated in two hydrothermal fields located at the Central Indian Ridge. Endmember hydrothermal fluid of the Solitaire field, located ~3 km away from the spreading center, was characterized by moderately high temperature (307°C), Cl depletion (489 mm), mildly acidic pH (≥4.40), and low metal concentrations (Fe ≤ 105 μm and Mn = 78 μm). Chloride depletion indicates that the subseafloor source fluid had undergone phase separation at temperatures higher than ~390°C while the metal depletion was likely attributable to fluid alteration occurring at a venting temperature of around 307°C. These different temperature conditions suggested from fluid chemistry might be associated with an off-spreading center location of the field that allows subseafloor fluid cooling prior to seafloor discharge. The microbial community in the chimney habitat seemed comparable to previously known patterns in typical basalt-hosted hydrothermal systems. Endmember hydrothermal fluid of the Dodo field, standing on center of the spreading axis, was characterized by high H2 concentration of 2.7 mm. The H2 enrichment was likely attributable to fresh basalt–fluid interaction, as suggested by the nondeformed sheet lava flow expansion around the vents. Thermodynamic calculation of the reducing pyrite–pyrrhotite–magnetite (PPM) redox buffer indeed reproduced the H2 enrichment. The quantitative cultivation test revealed that the microbial community associated with the hydrothermal fluid hosted abundant populations of (hyper)thermophilic hydrogenotrophic chemolithoautotrophs such as methanogens. The function of subseafloor hydrogenotrophic methanogenic populations dwelling around the H2-enriched hydrothermal fluid flows was also inferred from the 13C- and D-depleted signature of CH4 in the collected fluids. It was observed that the hydrothermal activity of the Dodo field had ceased until 2013.

Comparative Analysis of Microbial Communities in Iron-Dominated Flocculent Mats in Deep-Sea Hydrothermal Environments

Abstract: It has been suggested that iron is one of the most important energy sources for photosynthesis-independent microbial ecosystems in the ocean crust. Iron-metabolizing chemolithoautotrophs play a key role as primary producers, but little is known about their distribution and diversity and their ecological role as submarine iron-metabolizing chemolithotrophs, particularly the iron oxidizers. In this study, we investigated the microbial communities in several iron-dominated flocculent mats found in deep-sea hydrothermal fields in the Mariana Volcanic Arc and Trough and the Okinawa Trough by culture-independent molecular techniques and X-ray mineralogical analyses. The abundance and composition of the 16S rRNA gene phylotypes demonstrated the ubiquity of zetaproteobacterial phylotypes in iron-dominated mat communities affected by hydrothermal fluid input. Electron microscopy with energy-dispersive X-ray microanalysis and X-ray absorption fine structure (XAFS) analysis revealed the chemical and mineralogical signatures of biogenic Fe-(oxy)hydroxide species and the potential contribution of Zetaproteobacteria to the in situ generation. These results suggest that putative iron-oxidizing chemolithoautotrophs play a significant ecological role in producing iron-dominated flocculent mats and that they are important for iron and carbon cycles in deep-sea low-temperature hydrothermal environments. IMPORTANCE We report novel aspects of microbiology from iron-dominated flocculent mats in various deep-sea environments. In this study, we examined the relationship between Zetaproteobacteria and iron oxides across several hydrothermally influenced sites in the deep sea. We analyzed iron-dominated mats using culture-independent molecular techniques and X-ray mineralogical analyses. The scanning electron microscopy–energy-dispersive X-ray spectroscopy SEM-EDS analysis and X-ray absorption fine structure (XAFS) analysis revealed chemical and mineralogical signatures of biogenic Fe-(oxy)hydroxide species as well as the potential contribution of the zetaproteobacterial population to the in situ production. These key findings provide important information for understanding the mechanisms of both geomicrobiological iron cycling and the formation of iron-dominated mats in deep-sea hydrothermal fields.

Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization

Abstract: Magnetic properties in serpentinized peridotites are of increasing interest in seafloor mapping and petrologic studies because such data can promote the understanding of serpentinization reactions and hydrogen creation in ultramafic rocks. In order to reveal the magnetic properties and magnetite growth in serpentinized peridotites, we analyzed 30 serpentinized peridotite samples from a nontransform offset massif called the Yokoniwa Rise in the Central Indian Ridge. The results from multiple rock magnetic analyses and petrological observations illustrate the details of the creation and growth of magnetite in serpentinized peridotites that have undergone 17–100% serpentinzation. The magnetic carrier of these samples is pure magnetite, which did not suffer from maghemitization (low-temperature oxidation). The magnetic susceptibility ranged from 0.002 to 0.087 SI and increased nonlinearly with the progression of the serpentinization reaction. The natural remanent magnetization intensities of 0.2–8.4 A/m are comparable to those of basalts, which suggests that the remanence as well as induced magnetization of highly serpentinized peridotite can contribute to magnetization of the oceanic lithosphere. The amount of magnetite estimated from saturation magnetization increased nonlinearly from 0.1 to 5.5 wt % with the progression of the serpentinization. Highly serpentinized peridotites have a well-developed serpentine mesh texture. Pseudo-single-domain (PSD) and multidomain (MD) grains were formed during igneous processes in the mantle and/or during the initial stages of serpentinization. Superparamagnetic (SP) particles were formed during the initial stages of serpentinization. Single-domain (SD) magnetite was formed during the later stage of serpentinization, and it is assembled inside of mesh structures with strong magnetostatic interactions.

Holocene eruption history of the Motoshirane Pyroclastic Cone Group, Kusatsu-Shirane Volcano

Abstract: 1.Graduate School of Science and Engineering, University of Toyama , 2.Graduate School of Science and Engineering for Education, University of Toyama , 3.Mount Fuji Research Institute, Yamanashi Prefectural Government, 4.Volcanic Fluid Research Center, Tokyo Institute of Technology, 5.Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry, 6.Paleo Labo Co.,Ltd.