The Kuroshio (Black) Current is the biggest western boundary surface current in the western Pacific. Because of its high speed (2.7-3.6 km/hr), great depth (1.0 km) and width (150-200 km), and high temperature (28°-29°C in summer and 22°-25°C in winter), 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 that of the Gulf Stream in the North Atlantic, almost nothing has been learned about its evolution during the past 32 yr of drilling by DSDP and ODP because there are almost no locations beneath the Kuroshio where a deep-sea sedimentary section with high sedimentation rates can contain well-preserved calcareous microfossils. Because the CCD is shallow in the western Pacific (<3500 m) and the water depth is great (generally >4000 m), foraminifers and calcareous nannofossils are rarely preserved. Conditions appear to be ideal for obtaining a such a section, however, in the southern Okinawa Trough. As the Kuroshio Current passes between eastern Taiwan and the southernmost part of the Ryukyu Island arc, it is deflected upward when it approaches the Ilan Ridge and then flows northeastward in the Okinawa Trough (Ono et al., 1987; Chen et al.,1992) (Fig. F29). The seafloor in the Okinawa Trough lies above the CCD, and sedimentation rates are high because of terrigenous input from the East China Sea shelf and Taiwan (Boggs et al., 1979; Lin and Chen, 1983). Site 1202 was accordingly proposed on the southern slope of the Okinawa Trough to obtain a high-resolution record of the history of the Kuroshio Current during the Quaternary.
The Okinawa Trough, which extends from southwestern Kyushu, Japan, to northeastern 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). Although there is considerable controversy about the age of early rifting, most researchers agree that the most recent phases of extension have taken place since 2 Ma (Sibuet et al., 1998). The southernmost part of the Okinawa Trough 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).
The recent phase of extension of the Okinawa Trough occurred in the late Pleistocene (~0.1 Ma) (Furakawa 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 area of the site is unknown. The extension is characterized by normal faulting 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 and the southwest end of the Okinawa Trough (Sibuet et al., 1995, 1998). Based on the coincidence in timing between the development of the sedimentary basins in the Okinawa Trough (Kimura, 1985) and the uplift of the Ryukyu arc at the Pliocene/Pleistocene boundary (Ujiie, 1980), Sibuet et al. (1998) concluded that the penultimate phase of rifting, including subsidence and block faulting along the central axis of the trough, started at ~2 Ma. The total amount of extension in the area is ~30 km.
It has been suggested that the Okinawa Trough changed from an open-sea environment to a semi-enclosed marginal basin because of a 120-m drop in sea level (Fairbanks, 1989) during the last glacial maximum (Ujiie et al., 1991). Consequently, the Kuroshio Current may have moved to the trench side of the Ryukyu arc until ~7.5 ka during the Holocene (Ujiie et al., 1991; Ahagon et al., 1993; Shieh and Chen, 1995). Glacial-interglacial sea level fluctuations are likely to have caused significant changes in the configuration and distribution of continental shelves in the region, particularly in the South China Sea, and these changes must have caused dramatic hydrographic changes and sediment redistribution in the Okinawa Trough.
The southern Okinawa Trough is currently an area of high sedimentation because of the enormous terrigenous sediment supply from the East China shelf and Taiwan. Modern sediments in this area consist mainly of clay to silt-sized terrigenous sediments with a moderate (~20%) biogenic carbonate content (Chen et al., 1992; Lou and Chen, 1996). Sediment trap studies in the southern Okinawa Trough (Hung et al., 1999) indicate that the abundance of suspended particulate material decreases with increasing distance from the East Asian continent but increases with depth, implying effective resuspension and lateral transport across the area. Studies of short piston cores (Lou and Chen, 1996; Shieh et al., 1997; Ujiie and Ujiie, 1999) taken from the area suggest that sedimentation rates during the Holocene were ~20 cm/k.y.
Extensive geophysical surveys conducted in the area (Sibuet et al., 1998; Liu et al., 1998) show that the trough is marked by a series of normal faults dipping toward the center and a series of volcanic edifices and hydrothermal vents piercing through the sedimentary layer. Based on low-interval velocities (<2.0 km/s) determined from the analysis of seismic data, a prominent series of reflectors is observed from 250 to 350 mbsf (Fig. F30). 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, not only to study the paleoceanography of the Kuroshio Current but to provide constraints on the timing of the most recent phase of extension in the Okinawa Trough.
Next Section | Table of Contents