Circulation in the Okinawa Trough, the deepest part of the East China Sea, is governed by the prevailing East Asia monsoons and remote forcing from the Kuroshio Current and Taiwan Strait (Lee and Chao, 2003). Besides the Kuroshio Current, the other major input of water to the Okinawa Trough is riverine influx from the Yangtze River (928 km3/yr) (Zhang et al., 1990), Minjiang River (58.4 km3/yr), Qiantangjiang River (35.3 km3/yr) (Lee and Chao, 2003), and Lan-Yang River (2.73 km3/yr) (Hsu et al., 2004). The main ocean current affecting the southern Okinawa Trough is the Kuroshio, which flows northward along the eastern coast of Taiwan with a maximum speed 100 cm/s and a width of 100 km (Liang et al., 2003). The water transported by the Kuroshio Current, estimated by Nitani (1972), Chu (1976), and Liu (1983), varies from 19 to 47 Sv (Sverdrup: 1 Sv = 106 m3/s). The estimated depth of the Kuroshio Current varies from a few hundred meters to 1000 m. The Kuroshio Current enters the Okinawa Trough through Suao-Yonaguni Pass on the I-Lan Ridge and then flows along the shelf break (Fig. F1B) (Liang et al., 2003). Inside the Okinawa Trough the main axis of the Kuroshio Current varies seasonally, moving close to the coast in winter and away from the coast in summer (Sun, 1987). The northeastward-flowing Kuroshio Current follows the curved isobaths and leaves the East China Sea through Tokara Strait, south of Kyushu, Japan (Fig. F1). Upon encroaching the continental shelf, part of the Kuroshio flows southwest and forms permanent cyclonic eddies above Mien-Hua Canyon (MHC in Fig. F1B) (Hsu et al., 1998, 2000; Tang et al., 2000; Liang et al., 2003). These eddies represent important cells of water mixing and a mechanism for trapping and sucking suspended sediments into the canyons, from which they are subsequently transported laterally by contour currents (Hsu et al., 1998).
The southern Okinawa Trough has the highest sedimentation rates in the Okinawa Trough. Documented modern sedimentation rates reach 0.10–0.95 cm/yr (Chung and Chang, 1995), 10 times higher than those observed in the northern and central trough (<0.01–0.08 cm/yr) (Ikehara, 1995). Present sedimentation rates near Site 1202 are estimated to be 0.30–0.50 cm/yr (Lee et al., 2004).
The mouth of the Lan-Yang River flows directly into the southern end of the Okinawa Trough (Figs. F1, F2). The annual sediment discharge of the Lan-Yang River amounts to 6–9 Mt/yr, making it a major source of sediment supply to the trough (Jeng et al., 2003; Kao and Liu, 2000; Liu et al., 2003; Hsu et al., 2004). The distribution of sand, silt, and clay grains in seafloor sediments indicates that the sediment discharged from the Lan-Yang River can be transported up to 70 km offshore. However, the silt and clay portions are entrained in the Kuroshio Current and dispersed farther toward the northeast before being deposited preferentially on the southeast side of the Kuroshio (Chen and Kuo, 1980; Lee et al., 2004). Contour lines of silt and clay contents in percent are semiparallel to the topographic contours (Chen and Kuo, 1980). The silt and clay contents at Site 1202 were estimated to be 35% and 45%, respectively (Chen et al., 1992).
Regarding the dispersal of sediments from East China Sea to the southern trough, Liu et al. (2000) proposed a conceptual model to illustrate the possible routes of sediment transport: the Yangtze River delivers 478 Mt/yr of sediment onto the East China Sea shelf (Milliman and Meade, 1983), and a substantial fraction of the fine-grained sediments from the Yangtze River is transported by the China Coastal Current southward along the coast of mainland China (Beardsley et al., 1985; Milliman et al., 1985; Chao, 1991). In the northernmost section of the Taiwan Strait, the sediments are transported eastward to enter the Okinawa Trough (Liu et al., 2003). As the main stream of the Kuroshio Current encounters the shelf break along the northeast rim of the Okinawa Trough, cyclonic eddies form and the southwest arms of the cyclonic eddies send sediments down to the southern Okinawa Trough. Such sediments, together with the fluvial sediments exported from the rivers of eastern Taiwan, are entrained in the eddy and deposited later around the eddy's center (Hsu et al., 1998). The influence of undercurrents at intermediate depths reaches 550–800 m (Hung et al., 1999). The highest modern sedimentation rates have been observed along the southwestern corner of the southern Okinawa Trough, and Site 1202 is situated very near the depocenter (Huh et al., 2004; Lee et al., 2004).
Observations from sediment traps (Hsu et al., 2004) show that the sediment fluxes to the southern Okinawa Trough are much higher than those in other marginal seas and display great temporal and spatial variability. Spatially, the sediment fluxes exhibit a seaward decline from northeastern Taiwan to the central trough. On the continental slope, however, the flux increases with water depth, implying strong lateral transport of lithogenic particles. The temporal variation has a strong positive correlation with water runoff from the Lan-Yang River, whereas the highest fluxes were associated with typhoon-induced floods and, occasionally, with large earthquakes (Hsu et al., 2004; Huh et al., 2004). The Kuroshio Current may transport a significant portion of the fluvial sediment from the east coast of Taiwan (72 Mt/yr) (Water Resources Bureau, 1997) toward the southern Okinawa Trough. Furthermore, based upon regional hydrography and the spatial distribution pattern of particulate Al (as an indicator of the lithogenic component of total suspended matter), Hsu et al. (1998) considered that terrigenous sediment from the rivers of eastern Taiwan is the main source of the southern Okinawa Trough deposition.