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 thickness (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 Current 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 the eastern side of the island of 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 the island of 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 the northeastern 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). 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) (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 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 semienclosed 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 been located on 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 the island of 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/ky.
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.
The primary objective of drilling at Site 1202 was to obtain a high-resolution record of the paleoceanographic history of the Kuroshio Current. Such a record might make it possible to identify long-term patterns of climate change associated with the western Pacific boundary current during the past 1.5 m.y. For example, the Kuroshio Current passes over the Ryukyu arc and into the Okinawa Trough before turning northeast and continuing toward Japan, but changes in sea level associated with glacial-interglacial cycles could well redirect the Kuroshio Current outside the arc and isolate the Okinawa Trough on a cyclic basis. Changes in sedimentation caused by such deflections, coupled with periodic exposure of the continental shelf to the northwest during low sea level stands, should be easily detectable by drilling at Site 1202.
We also hoped that drilling at Site 1202 would enable us to detect the effects of orbital forcing in the Pacific during the mid-Pleistocene (~0.7 Ma), when the Earth's climate system switched from a regime of dominant 41-k.y. cycles to 100-k.y. cycles. Oxygen isotope measurements on foraminifers from Site 1202, for example, should reflect surface temperature cycles over a large area of the western equatorial Pacific because the Kuroshio Current is a composite current assembled from numerous smaller currents before it reaches the site.
We also hoped to document the temporal and spatial variability of millenial climate changes in the Kuroshio Current and the catchment basins that deliver sediments to the Okinawa Trough. These changes should be reflected not only in the oxygen isotopic composition of microfossils but in the grain size and composition of sediments from some of the largest river systems in east Asia, including the Yangtze, which rises in Tibet and samples much of southern China.
Finally, we hoped to examine long-term changes in El Niņo/La Niņa-style climate oscillations in the low-latitude Pacific by comparing the Kuroshio Current record to other Pacific ODP records (Andreasen and Ravelo, 1997; Clement et al., 1999).
After arriving on station and lowering the pipe, we planned to triple-APC core the sediment section to refusal, which was estimated to be at ~250 mbsf, to obtain overlapping, and thus complete, coverage for high-resolution paleoenvironmental studies. If time allowed, we planned to deepen the third hole to 410 mbsf using the XCB. We then hoped to log the open hole using the triple combo and the FMS-sonic tools to provide a quantitative basis for comparison with the multisensor track (MST) data, which could be used to reconstruct a continuous sediment section for the site.
The transit to Site 1202 (proposed Site KS-1) was made in good time with fair seas and favorable currents. The 784-nmi distance was covered in 64.9 hr at an average speed of 12.1 kt. At 0254 hr on the morning of 28 April, the vessel arrived at 24°48.24´N, 122°30.00´E, the coordinates for the drilling location. The crew began lowering thrusters and hydrophones, and at 0515 hr on 28 April the positioning beacon was deployed.
A standard APC/XCB bottom-hole assembly (BHA), including a lockable float valve (LFV) to allow wireline logging of the deepest hole of the site, was made up. The drill string was tripped to the bottom, and Hole 1202A was spudded at 0810 hr. APC coring continued through Core 195-1202A-9H to a depth of 83.1 mbsf (Table T1) when the APC failed to stroke out. Core 195-1202A-10H fully stroked; however, Cores 11H and 12H did not fully advance. Advance by recovery was used for the two incomplete cores in the hope that the hard layer would be limited in thickness and piston coring would once again become viable. APC refusal was finally accepted when Core 195-1202A-13H at 119.5 mbsf had not only failed to scope, but the core liner failed at the midpoint of the barrel. Core orientation using the Tensor tool was initiated with Core 195-1202A-4H and continued through Core 13H. Temperature measurements were taken on Cores 195-1202A-4H, 7H, 10H, and 13H using the Adara temperature tool. Two of the four temperature measurements were good (see "Physical Properties" in the "Site 1202" chapter). The developmental APC-methane tool was deployed on Core 195-1202A-4H and then on Cores 7H through 13H. All runs were successful in acquiring data. Hole 1202A officially ended with the clearing of the seafloor at 1900 hr on 28 April.
The vessel was offset 15 m to the east, and Hole 1202B was spudded at 1935 hr on 28 April. APC coring continued through Core 195-1202B-13H to a depth of 111.6 mbsf (Table T1) before APC refusal was defined by two successive incomplete strokes on Cores 195-1202B-12H and 13H. Because the ultimate depth objective at this site was 410 mbsf, we decided to cut three XCB cores before terminating the hole to obtain an idea about penetration rates, core recovery, and quality. Coring continued with the XCB through Core 195-1202B-16X to a depth of 140.5 mbsf. The drill string was pulled clear of the seafloor, officially ending Hole 1202B at 0445 hr on 29 April.
The ship was offset 15 m to the east, and Hole 1202C was spudded at 0540 hr on 29 April. APC coring continued through Core 195-1202C-11H to a depth of 97.5 mbsf (Table T1), where APC refusal was encountered as defined by three consecutive incomplete strokes. Core orientation using the Tensor tool was initiated with Core 195-1202C-4H and continued through Core 11H. The drill string was pulled clear of the seafloor, officially ending Hole 1202C at 1215 hr on 29 April.
For the third time, the ship was offset 15 m to the east, and Hole 1202D was spudded with the APC at 1245 hr on 29 April. Recovery of the first core was only 15 cm. APC coring continued in this hole through Core 195-1202D-9H to a depth of 76.2 mbsf (Table T1), when the first core did not achieve full stroke. Coring with the XCB proceeded through Core 195-1202D-32X to a depth of 297.4 mbsf, where a short wiper trip was made to 221.3 mbsf, above an area of poor recovery. The wiper trip was uneventful, and coring continued through Core 195-1202-44X to a depth of 410.0 mbsf, the approved target depth for Site 1202.
In preparation for logging, a wiper trip was initiated at 1930 hr on 30 April and reached the logging depth of 80.0 mbsf without incident. The hole was displaced with 150 bbl of sepiolite logging mud, and the bit was pulled back to a logging depth of 80.0 mbsf.
The triple combo tool string was made up with the Lamont Doherty Earth Observatory (LDEO) temperature/acceleration/pressure (TAP) tool. A nuclear source was not incorporated because the loss of density data did not outweigh the risk of losing the source in disputed waters under hostile current conditions. Throughout operations at the site, the Kuroshio Current was strong. Heavy pipe vibration was experienced continually, while the currents varied cyclically between 2.6 and nearly 4.0 kt. The first tool string was deployed to a depth of ~215 meters below rig floor. At that point, the logging engineer reported losing all power to the logging tools. After bringing the tool string to the surface, the tools showed several loose connections caused by the current-induced drill sting vibrations. All joints were retightened and taped with duct tape. The tool string was once again deployed inside the drill pipe; however, the winch operator only reached 72 mbsf before losing weight with the logging line, as if setting down on an obstruction. Upon recovery, the connections were once again found to be loose and the lower portion of the TAP tool was missing.
At 0430 hr, the circulating head was rigged up and the coring line was run in the hole to determine if the TAP tool was lodged in the drill string. Results were inconclusive. We decided to abandon further wireline logging efforts because of the intensity of the current-induced drill string vibration. The drill string was pulled clear of the seafloor by 0730 hr. By 1100 hr, preparations were under way to secure and clean the ship for transit into port. The 55-nmi transit to Keelung, Taiwan, was made at reduced speed for an 0815-hr arrival at the pilot station on 2 May 2001. The ship was dockside at 0904 hr, ending Leg 195.
The principal objective at Site 1202 was to obtain a continuous sediment section deposited beneath the Kuroshio Current that would allow high-resolution studies of climate change in East Asia associated with late Quaternary glaciation and deglaciation cycles. The strategy adopted was to drill at an extremely high sedimentation rate site in relatively shallow water above the CCD in the Okinawa Trough where calcareous microfossils would be preserved in an expanded section.
The objective was met with the recovery of a 410-m section of dark grayish green, bioturbated clayey silt with abundant sandy turbidites between 220 and 280 mbsf and occasional thin (<1 cm) turbidites scattered throughout the rest of the section. The sediments were rich in organic carbon and charged with H2S. Smear slide analysis shows that the nonbiogenic component of the sediments is composed predominantly of quartz, feldspar, and detrital carbonate, whereas the turbidites also contain micas, heavy minerals (green hornblende, tourmaline, epidote, and zircon), and opaques. Surprisingly, no tephra layers were observed and glass shards are rare. Although the site is located in the Okinawa Trough, heat flow values (0.040 W/m2) were slightly lower than the global average and no evidence of diagenesis was observed.
As anticipated, the preservation of calcareous microfossils was excellent, with planktonic and benthic foraminifers and calcareous nannofossils present in small amounts (<1% by volume) throughout the section, except in the turbidites, where they are abundant. Also present, especially in the turbidites, are diatoms, ostracodes, radiolarians, sponge spicules, the plates and spines of echinoderms, fragments of molluscs and pteropods, copepod remains, and fragments of bark, roots, and leaves, the latter indicating rapid burial and high sedimentation rates.
Despite the excellent preservation of microfossils, shipboard determination of the age of the section proved difficult. The presence of Emiliania huxleyi throughout the section suggests that the section is very young (<0.26 Ma, or latest Quaternary) and the absence of pink G. ruber indicates that the entire section is younger than 127 ka. This is consistent with shipboard paleomagnetic inclination data, which shows that the entire 410-m section lies within the Brunhes C1n normal polarity chron and is thus <0.78 Ma in age. Several excursions are seen in the data, including one at 110 m, which may correspond to the Laschamps event (40-45 ka), but given the absence of biostratigraphic markers and reversals, an accurate age determination for the section will have to be based on paleointensities.
If the age of the section is <127 ka, as suggested by the absence of pink G. ruber, then the sedimentation rate at the site was at least 3 m/k.y., one of the highest rates ever observed in the ocean basins for fine-grained, fossiliferous sediments. Given the relatively low biogenic content of the sediments, this requires a large terrigenous source. We infer that the section is composed of reworked sediments from the Chinese mainland that were delivered to the East China shelf by the Yangtze River. The presence of mica and other metamorphic minerals in the coarse fraction of the turbidites suggests that an additional component is derived from nearby metamorphic terrains on the island of Taiwan.
If this initial interpretation is borne out, then the section recovered at Site 1202 is almost ideal for studying climate change associated with glaciation and deglaciation in East Asia during the Holocene and latest Pleistocene. The section is relatively homogeneous; it is continuous, at least in the top 130 m where it was APC cored; it contains excellent paleomagnetic, lithologic, and biogenic proxies for climate; and it displays extraordinarily high resolution (<100 yr, assuming bioturbation to 20 cm). In principle, this resolution should be sufficient to study the influence of climate on the rise of Chinese civilization.