琉球岛弧两侧底质中有孔虫的特征

FORAMINIFERA AND THEIR DISTRIBUTION IN THE BOTTOM SEDIMENTS ON BOTH SIDES OF RYUKYU RIDGE

In the Late Miocene,a system of Trench-Arc-Basin evolved along the west-ern border of the Pacific Ocean, as a result of the northwestward subduction of the Pacific plate. Being a part of the system, the area studied includes the Ryu-kyu Island Arc in the upfold and the Ryukyu Trench and Basin of the Okinawa Trough on both sides of it. Geomorphologically they appear to be the Ryukyu Ridge and trench and trough on both east and west sides of the ridge. Forami-niferal faunas in the bottom sediments of the Ryukyu Trench and the Okinawa Trough show apparent difference in composition, distribution and preservation of their thanatocoenoses, due to the distinction of geographical environment of their habitats, developed on different structural background. 1.The Ryukyu Trench on the east side of the Ryukyu Ridge Is in the oce-anic environment, in which both abundance and diversity of the foraminifera are higher than those of the foraminifera in the interarc Okinawa Trough. 2. The ratio between planktonic and benthonic foraminifera in the Ryukyu Trench on the east side of the ridge is higher than that in the Okinawa Trough on the west side. 3. The most important representative planktonic foraminifera are Globorotalia menardii,G.tumida,G.truncatulinoides,Orbulina universa in the Ryukyu Trench, but are Neogloboquadrina dutertrei ,Globigerinita glutinata, Globigerinoides ruber in the Okinawa Trough. 4. Dissolution of carbonate tests in thanatocoenose of foraminifera in the bot-tom sediments of the Okinawa Trough takes place at the depth of 1 500~1 600m, that means the lysocline is reached in this depth, while in the Ryukyu Trench, in samples from the depth of 2 000m foraminifera are very abundant, which exhi-bit no effect of dissolution, the lysocline occurs at greater depth in the Trench than that in the Okinawa Trough.