PROVENANCE OF DETRITAL DEPOSITS AND PALEOENVIRONMENTAL CHANGES IN THE MIDDLE OKINAWA SINCE 18.5 KA: EVIDENCE FROM MAJOR ELEMENTS

Major elements and grain sizes for 151 detrital samples collected from the core KX12-3 at the middle Okinawa Trough were studied for reconstruction of sediment provenance and paleoenvironment over the time of last 18.5 ka. Upon the variations in major element compositions, the core is divided to two distinct depositional units, Segment 1 (Z1;18.5-9.6 ka) and Segment 2 (Z2; < 9.6 ka). Deposits of the Z1 (18.5-9.6 ka) are dominated by terrigenous detrital sediments from large Chinese rivers (e.g. the Changjiang and the Huanghe). The detrital sediments of the Z2 have same provenances dominated by these Chinese rivers, but mixed with some fine-grained materials from Taiwan, which may have been transported to the study area by the Kuroshio Current. The K-Ah volcanic eruption happened at 7.3 ka is also recorded in the core. Integrated study of correlation analyses, R-type factor, and major-element ratio show that the vertical change in major elements are mainly controlled by inputs of nearby terrestrial materials, whereas the contribution from hydrothermal activity and volcanism is little. In particular, the contribution of nearby terrestrial provenance shows an obvious trend which could be well correlated with the sea level fluctuation and the evolution of the Kuroshio Current. Furthermore, a prominent decline in Fe₂O₃、MgO、Fe/Ti、Mg/Ti values and finer grain size that occurred 5.0-3.5 ka may be linked to a suppression of the Kuroshio Current, or the shifting out of the current from the study area.