DEEP-SEA OSTRACODS AND PALEOENVIRONMENTAL CHANGES OF THE SOUTHWESTERN OKINAWA TROUGH DURING THE LAST 20 KA

Ostracods from 114 sediment samples of core 255 (water depth 1 575 m, core length 6.55 m, southwestern Okinawa Trough) were quantitatively studied for interpreting the paleoenviromental changes of the East China Sea during the last 20 ka, in combination with the data of stable isotopes, CaCO_(3) content and foraminiferal fauna. Tree Ostracod assemblages are identified, associated with glacial, deglacial and postglacial stages, respectively. The glacial assemblage is marked by the dominant species Palmoconcha parapontica and Loxoconchidea aff. dolgoiensis, high abundance and species diversity, and lack of cosmopolitan oceanic species, probably indicating the high-nutrient and low-oxygen bottom water which was less ventilation with the Pacific. Both deglacial and postglacial assemblages are predominated by Krithe spp. and Legitimocythere acanthoderma, in which abundance of P. parapontica and L. aff. dolgoiensis decreased while that of cosmopolitan oceanic species (L. acanthoderma and Henryhowella asperrima) increase remarkably. The abundance decreased especially in the postglacial period. It is suggested that the nutrient level in deep-sea decreased gradually with the increasing deepwater ventilation of the Okinawa Trough with the Pacific during the deglacial and postglacial periods. These changes are thought to be associated with the sea-level rise and concomitant retreat of the shoreline, and the enhancement of the Kuroshio Current since the deglaciation.