DISTRIBUTION AND ENVIRONMENTAL RELATIONSHIPS OF RECENT BATHYAL FORAMINIFERA IN THE RYUKYU ISLAND-ARC REGION, NORTHWEST PACIFIC-OCEAN

Forty-six surface sediment samples (from the upper 2 cm) were recovere d from water depths between 694 and 3,363 m in the Ryukyu Island Are r egion, northwestern margin of the Pacific Ocean. Based on the percenta ge abundances of the 87 most commonly occurring species, benthic foram iniferal assemblages were delineated by cluster analysis and divided i nto eight site clusters. The geographic distribution of these clusters indicates the following trends: 1) alpha-1 characterizes the lower pa rt of the Ryukyu Trench slope, whereas alpha-2 and -3 characterize the upper slope; 2) beta-1 and -2 characterize sites just beneath the mai n track of the Kuroshio Current overlying the Okinawa Trough, whereas beta-3 is limited to the northeastern exit of the Current on the trenc h slope; 3) gamma occupies a site under the depositional influence of the Huanghe River; and 4) delta occurs at two sites in the northern Ok inawa Trough where hydrothermal vents have been reported. R-mode clust er analysis defined nine faunal association clusters (A-1 through Fl a nd indicated some correspondences with the site clusters. Noteworthy t rends include 1) cluster D, composed of agglutinated species such as R habdammina abyssorum, Reophax scorpiurus, R. minimus, Hyperammina squa mosa, Adercotryma sp. A, Hormosinella guttifera, Saccorhiza ramosa and Testulosiphon indivisus, shows a distinctly higher occurrence in site cluster delta; and 2) site cluster gamma is characterized by gross-cl uster A, particularly A-1 composed of Rutherfordoides rotundiformis, H yalinea balthica, Quinqueloculina stalkeri, Bulimina marginata, B. str iata and Chilostomella oolina. High loadings on factors 2, 5, 6 and 8 of Q-mode factor analysis characterize faunal association clusters B-1 (typified by Evolvocassidulina brevis, Ioanella tumidula, Eilohedra l evicula, Osangularia bengalensis, Osangularielloides rugosus, Gyroidin oides kawagatai and Bolivina pusilla), F (by Globocassidulina elegans and Cibicidoides mundulus and its variety), D (by the agglutinated spe cies mentioned above) and A-1, respectively. Loadings on factors 1 and 2 are sharply different between site clusters alpha-1 and alpha-2, li kely because of a bottom water mass boundary along the central Ryukyu Trench slope. From comparison between nine faunal association clusters and the loadings of R-mode factors 1 to 5, we recognize the following : 1) factor 1 reflects water depth changes; 2) factor 3 characterizes site clusters under the influence of hydrothermal activity; 3) factor 4 indicates a suboxic environment; and 4) high loading on factor 5 cha racterizes ''parasitic'' species such as Homalohedra costata, Parafiss urina lata, Fissurina marginata, Fissurina cucullata and Glandulonodos aria calomorpha.