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1 Ocean Research Institute, University of Tokyo, Minami-dai 1-15-1, Nakano-ku, Tokyo 164-8639, Japan. E-mail: yurika{at}ori.u-tokyo.ac.jp
2 Professor Emeritus, University of the Ryukyus; Izumi-cho 1156-4-338, Tachikawa City, Tokyo 190-0015, Japan. E-mail: ujiie{at}green.ocn.ne.jp
Recent planktonic foraminifera were analyzed in 52 surface sediment samples collected from the Ryukyu Arc region in the northwestern Pacific Ocean and adjacent East China Sea with the goals of establishing relationships between species distributions and oceanographic features with emphasis on the Kuroshio Current. Thirty-nine species of planktonic foraminifera were identified and grouped into 27 species and species groups based upon taxonomic and ecologic criteria. Calculation of a correlation matrix in the form of a similarity index allowed for a comparison of the relative abundances of the 27 species and species groups within and among all sample sites. Q-mode cluster analysis was subsequently applied using the UPGMA method, and five major site clusters A to E were distinguished. The geographic distribution of samples in site cluster A coincides well with the distribution of higher loading values of factor 2, identified by Q-mode factor analysis carried out using the same coefficient matrix. Similar associations were established for site cluster C-1 with those of factor 1, and site clusters C-2, D and E with those of factor 3. These associations suggest that site cluster A reflects the main path of the Kuroshio Current, cluster C-1 represents the open waters of the Pacific Ocean, and clusters C-2, D and E express susceptibility to carbonate dissolution at depths greater than 3000 m on the Ryukyu Trench slope. R-mode factor analysis using different loadings of factors 1 to 3 allows the 27 species and species groups to be classified into four groups A through D as follow: (1) Group A consists of taxa susceptible to carbonate dissolution such as the Globigerinoides ruber species group, G. sacculifer, etc. against group B which includes the Pulleniatina group, Neogloboquadrina dutertrei, Globorotalia tumida and G. menardii. Group B is dissolution resistant and also characterizes the Kuroshio Current. (2) Group C consists of Neogloboquadrina pachyderma (dextral), N. incompta and Turborotalia inflata, all indicative of "relatively cold" water. (3) Group D composed of Globigerina bulloides and Globigerinella calida suggests influence of coastal water invasion.
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