<国立科博専報>小笠原・父島西方海域の底質

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Sampling of surface sediments were tried at 65 stations in the shallow water west off Chichi-jima, Bonin Islands (Figure 1). The island is located at about 1,000 km south of Tokyo and at the middle of the west Pacific central water. Because of oceanic and subtropical circumstance Chichi-jima is almost fringed with coral reef. Grain-size analysis was carried out on 51 samples utilizing Emery tube and the pipette method. The results are summarized in the relationship between their Mdφ- and αφ-values (Figure 2), where three groups, α, β and γ, can be recognized. Significance of their separation is also ascertained by two ellipses of rejection with 95 percent confidence for α group and β group. α group with very coarse- to coarse-grained Mdφ and with fine skewed mode seems to be located at places under strong turbulence of water so that scouring fractions around 2φ might be expected there. On the other hand, β group is composed of majority of the samples, having Mdφ of medium- to fine-grained sand size and not biased mode of size-frequency distribution. Mdφ of γ group is restricted in very fine-grained sand to silt, and fine skewed mode gives us more muddy impression compared with values of Mdφ. The muddy sediments are exclusively distributed in deep of such an inlet incised as bay of Futami Port. When we treat β and γ groups as a whole, a positive correlation is observed between Mdφ and αφ as suggested by a line drawn through the least square method in Figure 2 or by such a correlation coefficient as 0.719. Sand-grain analysis on 25 samples is based upon the determination and counting of 200-odd total number of grains for each sample under microscope. The grains are separated into six components, i.e. molluscan fragments (m in Figure 3), foraminiferal shells (f), octocoral spicules (c), echinoid spines and fragments (e), ostracod carapaces (o), and rock and mineral fragments (r). The results are summarized as a cumulative percentage diagram and a dendrogram in Figure 3. The dendrogram is obtained by the cluster analysis of the correlation coefficient matrix composed of 300 coefficients between station and station based upon percentages of the above-mentioned six components. Four clusters, A through D, can objectively be identified. As shown in Figure 2 D cluster corresponds to γ group of grain-size analysis, A cluster approximately to β group, and B cluster also approximately to α group, although these corerlations should be ascertained by augmented data of sand-grain analysis. We tentatively conclude that the sediments represented by β group and A cluster are most broadly distributed and most typical in shallow water of this area, whereas the inletdeep sediments in Futami Port bay characterized by γ group and D cluster may have been produced by derivation of non-biogenic clastics from the surrounding land areas and by displacement of molluscan fragments from the surrounding shallower waters. Another type of sediments belonged t γ group and B cluster will sufficiently be interpreted in the near future after analysis of the remained samples, more detailed analysis of size-frequency mode for this group, and benthonic foraminiferal analysis are completed.
国立科学博物館の論文
1978-12-20

<国立科博専報>小笠原・父島西方海域の底質

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