VARIATION OF GEOCHEMICAL RECORDS IN CORE A7 SEDIMENTS FROM MIDDLE OKINAWA TROUGH DURING THE PAST 18 kaBP AND ITS RESPONSE TO PALEO-ENVIRONMENTAL CHANGES

A piston core retrieved from the Middle Okinawa Trough was studied of the geochemical variation since the last 18 cal. kaBP on the basis of the high-resolution geochemical analysis as well as detailed AMS ¹⁴C dating. R-mode Factor analysis method was used to reunite the whole analyzed 20 element contents into four factors, in which Factor I mainly composing of Al, Ti, Rb, Th, Mg, U, Zn, etc. was regarded as the index of terrigenous detrital components, while Factor Ⅱ with high positive loadings of Ca, Ba, P and Sr was suggested to be the paleo-productivity proxy. Based on variation pattern of the geochemical records, the whole strata were divided into 4 sections, namely, 18~15 kaBP (I), 15~8 kaBP(Ⅱ), 8~2 kaBP (Ⅲ) and 2 kaBP to present (IV). They are all characterized by the distinct geochemical compositions, terrrigeous or biogenetic. The volcanic sedimentation event that occurred during 7.3 cal kaBP played an important role in the fluctuation of the geochemical records in Core A7 sediments, in which the terrigenous and biogenetic contributions were very smaller compared with counterparts from other intervals. We relate these geochemical variations to the coupling effect of temperature change, sea-level change, and the shifts of Kuroshio Currents, which were the main factors controlling the variations of the geochemical composition in Middle Okinawa Trough. However, there are also some exceptions, such as the lowered terrigneous input centering near 15 cal. kaBP. and 2 cal. kaBP. The reason is uncertain because they all coincided with the lowered temperature. Moreover, we also believe that there are tight tele-connections between the Okinawa Trough region and the global climatic change, when comparing the geochemical records of this study with the climatic records from North Atlantic and Greenland.