The upper 120 m of Hole 1202B, deposited during the past 30 k.y., is free of major turbidite disturbance. The sedimentation rates are relatively high, varying between 1.5 and 9 m/k.y. (Fig. F10C) (Wei et al., 2005). Such high sedimentation rates are unprecedented in the Okinawa Trough and were unexpected. Previous published results from southern Okinawa Trough sites recorded sedimentation rates of ~40–115 cm/k.y. For instance, site RN96-PC 1 (24°58.5´N, 122°56.1´E) records an average rate of 40 cm/k.y. and site RN 93-PC5 a rate of 116 cm/k.y. (Ujiié and Ujiié, 1999; Ujiié et al., 2003), site MD12404 (26°38.84´N, 125°48.75´E) shows 50 cm/k.y. (Chang, et al., 2005), and core 255 (25°12´N, 123°7´E) shows 60 cm/k.y. (Jian et al., 2000). Hole 1202B, on the other hand, has accumulated ~30 m of sediments over the last 8 k.y. and thus displays an average sedimentation rate an order of magnitude higher than that at any of the other sites. The high accumulation rate suggests that Site 1202 is located at a localized deposition center. The sediments are mainly composed of fine terrestrial clay and silt (Diekmann et al., submitted [N1]). The coarse-sized fraction (>63 µm), made up chiefly of foraminifers, only accounts for 1% to 6% of the sediments (Fig. F10D). The coarse fraction decreased to <1% in intervals with the highest sedimentation rate (~9 m/k.y.) from 11 to 15 ka (Fig. F10C, F10D).
A chronicle of the changes in the modes and sources of detrital input was inferred based upon sediment granulometry, clay mineralogy, and major element data (Diekmann et al., submitted [N1]). Several factors influenced the depositional regimes, including path of the Kuroshio Current, sea level, modes of fluvial runoff, neotectonics, and possibly changes in the monsoon system. In descending order of abundance, the clay mineral assemblage consists of illite (55%–75%), chlorite (15%–25%), kaolinite (5%–13%), and smectites (3%–12%). In Taiwan, illite and chlorite are abundant in widespread slates and schists as well as in soils, whereas kaolinite and smectites only occur locally (Chen, 1973; Dorsey et al., 1988). In contrast, the solid load of the Yangtze River provides conspicuous amounts of kaolinite in addition to illite and chlorite (Chen, 1973). The modern distribution of kaolinite in surfacial sediments in the East China Sea and the Okinawa Trough traces the spread of sediments from the Yangtze River (Aoki and Oinuma, 1974). Here, the chlorite/kaolinite ratio is used to gauge the relative contribution of sediments supplied from Taiwan vs. East China (Fig. F10A). It is clear that the Taiwan source became dominant only during the Holocene.
Grain size data display clearly that there are two major modes of sedimentation in the southern Okinawa Trough: (1) between 30 and 11.5 ka (MIS 3–2), when the sortable silt was low, and (2) afterward, when it was high (Fig. F9C). Apparently, sea level plays a crucial role in regulating the depositional regime. Diekmann et al. (submitted [N1]) interpreted that when sea level was low from 30 to 11.5 ka the Kuroshio Current did not enter the Okinawa Trough and water circulation in the trough was relatively sluggish. The influence of the Kuroshio Current began to increase between 11.5 and 9.5 ka, and the detrital sediment supply from Taiwan increased significantly, reaching a high stable value at 9 ka (Fig. F10A). It appears that only when sea level rose to –50 to –40 m relative to the present stand did the Kuroshio Current begin to invade the trough. On the other hand, Ujiié and Ujiié (1999) postulated that a major tectonic subsidence of Ilan Ridge occurred at 10 ka. More recently, Ujiié et al. (2003) inferred from planktonic foraminiferal census data that the Kuroshio Current strengthened after 13 ka and reached its full strength at ~10 ka.
The last 25 k.y. can be subdivided into three stages based upon all available proxies (Fig. F10).
Before 19.5 ka (Stage III in Fig. F10), when sea level was low, the sediments deposited at Site 1202 were an admixture from Taiwan island and East China continent sources as indicated by the intermediate chlorite/kaolinite ratio (Fig. F10A); because of low sea level, both the input sources were located relatively close to the site, and the percentage of the coarse fraction (>63 µm) was high (Fig. F10C). The sediments deposited during Stage III are characterized by high K/Ti (Fig. F10B). The potassium is derived from K-bearing feldspars and sheet silicates from both the East China continent and Taiwan and is particularly enriched in the clays. On the other hand, titanium is found in both sources but is higher in the upper continent crust, indicating provenance of the East China continent (Yang et al., 2004).
When sea level began to rise during the early stage of deglaciation between 19.5 and 15 ka (Stage IIb) (Fig. F10E), the sediment source from the Yangtze River became more distant and sedimentation rates began to drop. Meanwhile, the rising sea level washed and winnowed the East China Sea continental shelf and continuously released reworked fine particles into the water column. This washing and winnowing processes became more intense when the sea level rose quickly during Stage IIa (15–11 ka) (Fig. F10E), and the sedimentation rates reached their highest levels of 9 m/k.y. (Fig. F10C). The sediments were dominated by fine particles (Figs. F9C, F10D) transported from the East China Sea continental shelf, and clay content reached a maximum of ~40%. The reason that Taiwan became a less important source during 15–11 ka is because a good portion of the sediments supplied by the Lan-Yang River was trapped in the Lan-Yang Plain. Wei et al. (2003a) showed that sediments as thick as 230 m accumulated between 15 and 3 ka to form the Lan-Yang Delta. Sedimentation of the Lan-Yang Delta was in aggredation mode from 15 to 5 ka and switched to progradation after 5 ka (Chen et al., 2004). Thus, the southern Okinawa Trough received less sediment from the Lan-Yang River during Stage IIa, particularly the period of rapid sea level rise.
Between 11 and 9 ka, when sea level rose to a stand of approximately –50 m to –40 m relative to the present sea level (Lambeck et al., 2002), the Kuroshio Current entered the Okinawa Trough and the sediments from Taiwan began to increase, as indicated by an increase in the chlorite/kaolinite ratio (Fig. F10A). This interval is coined as "transitional" by Diekmann et al. (submitted [N1]). The Taiwanese source became dominant after 9 ka and has maintained a stable, high level of contribution until the present. The increase in sediment grain size (Figs. F9C, F10D) is believed to reflect the closeness of the sediment source (Taiwan) on one hand and a stronger current velocity on the other hand. The increase in abundance of sortable silt (10–63 µm size range) implies an increase in strength of the contour currents (McCave et al., 1995). Overall, the increase in grain size, particularly the sortable silt portion, is related to the vigor of the Kuroshio Current, which enhances both surface and undercurrents in the southern Okinawa Trough. Diekmann et al. (submitted [N1]) envisioned that once the strong Kuroshio Current entered the trough after 11 ka, it transported the coarse terrigenous silts from Taiwan over long distance into the southern Okinawa Trough, and the associated eddies formed on the edge of the continental shelf enhanced trapping and sucking of sediments into deeper water levels through the Mien-Hua Canyon systems, feeding the lateral-flowing nepholoid layers along the continental slope. Therefore, the increase in sortable silt content at ~11 ka marks a major change in the circulation regime brought about mainly by invasion of the Kuroshio Current into the trough. The strength of the Kuroshio Current remained constant throughout the Holocene (Fig. F9C). Interestingly, there was an increase in sediment input from Taiwan at ~1.5 ka that resulted in an increase in the chlorite/kaolinite ratio and a higher sedimentation rate (Fig. F10A, F10C), even though the percentage of sortable silt remained the same. The increase in the sedimentation rate is mainly due to a higher input of sediments, caused probably by heavier precipitation or more frequent seismic activity. Coincidentally, pollen and grain size data from sediments from northern Taiwan suggest that the precipitation increased at ~2–1.5 ka (Liew and Tseng, 1999; Liew and Hsieh, 2000).