Fine-scale chemostratigraphy of cross-sectioned hydrogenous ferromanganese nodules from the western North Pacific |
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| Authors: | Shiki Machida Kentaro Nakamura Tetsu Kogiso Ryo Shimomura Koichi Horinouchi Kyoko Okino Yasuhiro Kato |
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| Affiliation: | 1. Ocean Resources Research Center for Next Generation (ORCeNG), Chiba Institute of Technology, Chiba, Japan;2. Department of Systems Innovation, School of Engineering, The University of Tokyo, Tokyo, Japan;3. Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan;4. Department of Systems Innovation, School of Engineering, The University of Tokyo, Tokyo, Japan Frontier Research Center for Energy and Resources (FRCER), School of Engineering, The University of Tokyo, Tokyo, Japan;5. Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan |
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| Abstract: | A broad area densely covered by ferromanganese nodules was recently discovered around Minamitorishima (Marcus) Island, representing a high-potential metal resource, particularly for Co, Ni, Mo, and W. We studied 16 nodule samples from nodule fields around Minamitorishima Island. To define the fine-scale chemostratigraphy of the nodules, polished cross-sections of the samples were analyzed by microfocus X-ray fluorescence. Our results show that a general pattern of compositional variation was common throughout the growth history of the nodules in all the regions we studied. Chemical mapping clarified changes in the chemical signature and proportion of five lithological components throughout the growth history: Mn represented columnar δ-MnO2; Fe represented layered amorphous FeOOH*xH2O; Ti represented TiO2*2H2O intergrown with an amorphous FeOOH phase; P, Ca and Y represented particles of biogenic calcium phosphate; and Si, Al, K, Cu, and Ni represented pelagic sediment infills. We proposed a method for a creating a multi-dimensional compositional map of the fine-scale chemostratigraphy observed in the ferromanganese oxide layers on the basis of merging the mapped Mn, Fe, Ti, P, Si and Cu intensities. Multi-dimensional compositional mapping of the sampled nodules from the western North Pacific revealed two fundamental findings: (1) previously recognized first-order Fe–Mn layers, L0, L1, and L2, were further divided into two, three, and four sublayers, respectively, and (2) a delayed supply of material to be nuclei of nodule or a growth hiatus of Fe–Mn layer(s), leading to missing sublayers in the layers L0 and L2, regulated the nodule size. In contrast, layer L1, which does not have any missing sublayers, was commonly observed in the samples for this study and has been reported in studies of other regions in the western Pacific. We propose, therefore, that the layer L1 is a key facies for examining chemostratigraphic correlations with other areas of seafloor. |
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| Keywords: | μ-XRF chemostratigraphy ferromanganese nodule Minamitorishima |
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