Submarine sediment distribution and its controlling factors around the Tokara Islands

Abstract

Surface sediment sampling was conducted at 102 sites during two marine geological survey cruises (GB21-2 and GB21-3) around the Tokara Islands. Muddy sediments are generally distributed on the flat seafloor below 800 m depth, and are strongly bioturbated in many sites. The distribution of sandy sediments, gravels, and outcrops is not necessarily limited to shallow water depths, but coarsegrained materials of biogenic and non-biogenic sources were observed in and around islands as well as shallow water areas. This distribution of bottom sediments is believed to be influenced by the presence of submarine volcanoes which are a potential source of sediments beneath the seafloor, in addition to islands and shallow-water areas where biological production occurs. The distribution of bedforms such as ripples, outcrops, and gravelly sediments, commonly found around islands, suggests a strong influence of the meandering Kuroshio Current in this area. Analyses of bryozoans, coral distribution, assemblages, size distribution, and preservation of planktic foraminifera suggest that the Kuroshio Current exerts both mechanical effects, through bottom currents, and biological effects, such as on community composition and the body size of organisms, on the sediments.