Clay mineral and grain size studies of sediment provenances and paleoenvironment evolution in the middle Okinawa Trough since 17 ka.

Okinawa Trough receives a large amount of fluvial sediment transported by complex oceanographic circulations, and is an ideal location for the study of paleoceanography and paleoenvironment changes. However, our knowledge of the sediment provenance and paleoenvironment evolution in the Trough during the past 17 ka is still limited. Based on high-resolution grain size, clay minerals and AMS 14 C data of the Core OKI04, we present new evidences of the provenances and paleoenvironment evolution in the middle Okinawa Trough during the last 17 ka. Our results indicate that clay-sized terrigenous sediment deposited in the middle Okinawa Trough is mainly from those rivers flowing into East China Sea and Yellow Sea (e.g., Yangtze and Yellow Rivers) and Taiwanese Rivers. Their contributions varied greatly in space and time during the past 17 ka. From 16.5 to 13.8 ka, sediment was mainly derived from the Yangtze and Yellow Rivers, with little contribution from Taiwan. From 13.8 to 5.4 ka, the contributions from the Yangtze and Yellow Rivers oscillated multiple times: low in 13.8–11.6 ka, high in 11.6–7.6 ka, and back to low in 7.6–5.4 ka, whereas the pattern of Taiwanese contribution was opposite. After 5.4 ka when the sea level was high and shoreline was far away from the Okinawa Trough, the Yangtze River and eastern Taiwanese Rivers became the main provenances. The contribution from the Yellow River decreased, which is probably because Yellow River mouth was far away from the middle Trough. Sediment grain size data show that two environmentally sensitive populations are 71–7 μm and 7–0.5 μm. The variation of the populations' mean grain size and content shows that the strength of Kuroshio Current was weak during 16.5–11.6 ka, and became strong in 11.6–5.4 ka, in which Tsushima Warm Current and Yellow Sea Warm Current formed around 8.5 and 6.5 ka, respectively. The strength of Kuroshio Current decreased in 5.4 ka and increased since 3.2 ka. Based on clay mineralogical analysis, we find that the kaolinite/chlorite could be used as a new effective indicator for the East Asian Winter Monsoon's evolution in the Trough since 17 ka. [ABSTRACT FROM AUTHOR]