-34.8
(Hwang and Tang, 1993).Site 1202 was proposed to obtain a high-resolution record of the history of the Kuroshio Current during the Quaternary from the southern slope of the southernmost Okinawa Trough (Fig. F1). The Kuroshio (Black) Current is the biggest western boundary surface current in the western Pacific. It plays an important role in the meridional transport of heat, mass, momentum, and moisture from the western Pacific warm pool to high latitudes in the North Pacific. Although its role in the Pacific is as important as the Gulf Stream in the North Atlantic, virtually nothing has been learned about its evolution during the past 32 yr of drilling by DSDP and ODP. The lack of a deep-sea sedimentary record for the Kuroshio Current is due to the shallow carbonate compensation depth (<3500 m) and the great depth of the ocean (generally >4000 m) in the western Pacific. The only possible areas where adequate sequences could be obtained for such a study would be the marginal seas or shallow aseismic ridges located underneath the path of the Kuroshio Current. Site 1202, located in the southern end of the Okinawa Trough, represents one such ideal target. The selection of Site 1202 was based on excellent data sets collected recently during several international cooperative programs and integrated multidisciplinary studies. Of particular relevance are multiple lines of geophysical data accumulated over the past decade, including lines shot by the Maurice Ewing during 1995 (Fig. F2) and the swath-mapping survey of the French L'Atlante during the Active Collision in Taiwan (ACT) cruise in 1996. Recent imaging of volcanic seamounts and dredging of volcanic rocks and deep-sea biota by the southernmost part of the Okinawa Trough (SPOT) program (Lee et al., 1998a, 1998b) have added new insights into the tectonomagmatic evolution of the Okinawa Trough.
The Kuroshio Current is
characterized by its high speed (1.5-2 kt), narrow width (150-200 km), and great
depth (~1000 m) (Fan, 1985; Yuan et al., 1998). It brings a great quantity (~22
sverdrup) (Yuan et al., 1998) of warm equatorial seawater northward while
flowing along the east coast of the island of Taiwan (Nitani, 1972). It passes
between the island of Taiwan and the southernmost part of the Ryukyu Island arc
and then flows northeastward along the Okinawa Trough (Ono et al., 1987) (Fig. F1).
In the area immediately south of Site 1202, the Kuroshio Current appears to be
deflected upward, causing upwelling when it approaches the Ilan Ridge (Chen et
al., 1992b) (Fig. F1).
The sea-surface temperatures of the Kuroshio Current off the eastern side of the
island of Taiwan are ~28°-29°C in the summer and 22°-25°C in the winter. The
salinity of the surface water is 33.6-34.8
(Hwang and Tang, 1993).
The Southern Okinawa Trough is an area of high sedimentation because of the enormous terrigenous sediment supply from the East China Sea shelf and from the island of Taiwan (Boggs et al., 1979; Lin and Chen, 1983; Chen et al., 1992b). Modern sediments in this area consist mainly of terrigenous material from rivers on nearby islands and secondarily of marine biogenic carbonates and opal, with a small portion of volcanic materials. The deposits in the vicinity of the drilling site are primarily clay- to silt-sized sediments with a moderate content of biogenic carbonate (~20%) (Chen et al., 1992b; Lou and Chen., 1996). Sediment trap studies in the southern Okinawa Trough (Hung et al., 1999) indicate that the abundance of suspended particulate matter decreases with increasing distance from the East Asian continent but increases with water depth, implying effective resuspension and lateral transport processes across this area. Studies of short piston cores (Lou and Chen, 1996; Shieh et al., 1997; Ujiié and Ujiié, 1999) taken from the area have provided estimates of Holocene sedimentation rates of ~20 cm/k.y.
Our current understanding of the past history of the Kuroshio Current is based on studies of short piston and gravity cores obtained from offshore areas off central Japan (e.g., Chinzei et al., 1987; Takemoto and Oda, 1997; Sawada and Handa, 1998) and the eastern side of the island of Taiwan (Chen et al., 1992a; Lou and Chen, 1996; Shieh et al., 1997). These studies emphasized glacial to interglacial (Chinzei et al., 1987; Oda and Takemoto, 1992; Takemoto and Oda, 1997) and millennial (Kallel et al., 1988; Sawada and Handa, 1998) variations.
It has been suggested that the Okinawa Trough changed from an open-sea environment to a semienclosed marginal basin because of a 120-m sea level drop (Fairbanks, 1989) during the last glacial maximum (Ujiié et al., 1991). Consequently, the Kuroshio Current might have moved east of the Ryukyu Island arc (Ujiié et al, 1991; Ahagon et al., 1993) and outside the Okinawa Trough until ~7.5 ka, during the Holocene (Shieh and Chen, 1995). Glacial-interglacial sea level fluctuations must have resulted in significant changes in the configuration and distribution of continental shelves in the region, particularly in the East China Sea. Consequently, such changes must have caused dramatic hydrographic changes and sediment redistribution in the Okinawa Trough.
The Okinawa Trough, extending from southwest Kyushu, Japan, to the northeast side of the island of Taiwan, is an active, incipient, intracontinental backarc basin formed behind the Ryukyu arc-trench system in the western Pacific (Lee et al., 1980; Letouzey and Kimura, 1985; Sibuet et al., 1987). The trough was formed by extension within continental lithosphere already intruded by arc volcanism (Uyeda, 1977; Sibuet and Hsu, 1997). There is considerable controversy about the age of the early rifting phase of the Okinawa Trough, although most researchers agree that the two most recent phases of extension have taken place since 2 Ma (Sibuet et al., 1998, and references therein). The southernmost part of the Okinawa Trough (southwest of 123°E) (Fig. F1) is characterized as a rifting basin with incipient arc volcanism opening in the middle of a foundered orogen caused by previous arc-continent collision (Teng, 1996). Wang et al. (1999) proposed that postcollisional lithospheric extension in the northern Taiwan mountain belt during the Pliocene-Pleistocene triggered the Pleistocene rifting in the middle Okinawa Trough, making room for its subsequent southwestward propagation in association with the development of the southernmost Ryukyu subduction zone near the island of Taiwan.
The recent phase of extension of the Okinawa Trough occurred in late Pleistocene time (~0.1 Ma) (Furukawa et al., 1991), based on seismic correlation with drilling stratigraphy (Tsuburaya and Sata, 1985), but the exact timing of this recent phase of extension in the SPOT area is unknown. The extension is characterized by normal faults on both sides of the trough. The amount of extension during this recent phase has been estimated to be 5 km, both in the middle (Sibuet et al., 1995) and in the southwest end of the Okinawa Trough (Sibuet et al., 1998).
The penultimate rifting phase started at ~2 Ma (Sibuet et al., 1998). This assessment is based on the coincidence in timing between the development of the sedimentary basins of the Okinawa Trough (Kimura, 1985) and the uplift of the Ryukyu arc at the Pliocene/Pleistocene boundary (Ujiié, 1980). Sibuet et al. (1998) argued that the initiation of subsidence and block faulting along the central axis of the trough took place at ~2 Ma. The total amount of extension in the SPOT area is ~30 km.
Extensive marine geophysical surveys have been carried out in the southernmost Okinawa Trough, and detailed bathymetry data (Sibuet et al., 1998; Liu et al., 1998) and gravity and magnetic anomaly maps (Hsu et al., 1998) have been compiled. The major structural patterns in the Okinawa Trough are a series of normal faults dipping toward the center and a series of volcanic edifices piercing through the sedimentary layer. A prominent series of reflectors is observed from 250 to 350 ms below the seafloor (Fig. F2). This prominent reflection has been suggested to be the unconformity marking the onset of the most recent phase of extension of the southern Okinawa Trough (Hsu, 1999). Site 1202 was proposed to penetrate this sequence to a depth of ~410 mbsf. It was expected that the cores collected would not only address the paleoceanography of the Kuroshio Current, but would also provide constraints on the timing of the most recent phase of extension in the Okinawa Trough.
Four major paleoceanographic objectives were identified for Site 1202. These include the following:
