The Okinawa Trough on the east side of the East China Sea(ECS)continental shelf is a dive-collision, back-arc basin between the Philippines and Eurasia plates(Sibuet et al., 1998). Hydrology and climatology in the ECS are dominated by the Kuroshio Current(KC), Taiwan Warm Current, Min-Zhe Coastal Current(MZCC)in winter, and Changjiang Diluted Water(CDW)in summer(Lee and Chao, 2003). Transport mechanisms of sediments are controlled by northward and southward currents. During the Last Glacial Maximum, the ECS current system was very different from modern times, because of extensivecontinental shelf exposure and progradation of coastline(Saito et al., 1998)in response to low sea level(Lambeck and Chappell, 2001; Liu et al., 2004; Clark et al., 2009). However, the Okinawa Trough was still submerged and recorded the change of current and climate(Li et al., 2001). It has been demonstrated that in modern times, offshore export of particles in the bottom nepheloid layer occurs primarily with downwelling and seaward bottom flow induced by the northeast winter monsoon, which is inhibited by a transverse circulation pattern in summer(Yang et al., 1992; Yanagi et al., 1996; Yuan et al., 2008). Although there is a bottom nepheloid layer in the shelf area in summer, it contributes less to the Okinawa Trough because of resistance of the Taiwan Warm Current and a “water barrier” engendered by Kuroshio subsurface-intermediate water(Yang et al., 1992; Hoshika et al., 2003; Iseki et al., 2003). The timing and cause of the formation of the modern current system remain elusive. In this study, we retrieved core Oki01 from the middle of the Okinawa Trough to investigate this problem based on sedimentological records, including clay mineral assemblage and major element compositions. 2 OCEANOGRAPHIC SETTING

As a linking passage between the continental margin sea of China and western Pacific Ocean, the Okinawa Trough may provide a sensitive record of the environmental transition of ocean and continent(Fig. 1). The climate in the study area is influenced by the East Asian monsoon system, which modulateslocal temperature, humidity, and atmospheric circulation(Lee and Chao, 2003). In summer, low pressure above the central Eurasian continent favors southeasterly winds around 5 m/s that carry warm and moist air from the Pacific Ocean toward Taiwan Isl and and mainl and China(Bian et al., 2010; Zheng et al., 2014a). In winter, a reversed pressure gradient favors northwesterly winds around 12 m/s, bringing cool and dry air from the Eurasian continent toward the Pacific(Bian et al., 2010; Zheng et al., 2014a). Seasonal variation of the monsoon system strongly influences hydrographic conditions and the KC in the Okinawa Trough(Lee and Chao, 2003; Diekmann et al., 2008; Yuan et al., 2008; Yuan and Hsueh, 2010). Poleward transport of tropical waters in the KC increase during summer, because of the southern location of the North Equatorial Current(NEC)bifurcation. A northward location of this bifurcation in winter causes less transport in the KC(Kagimoto and Yamagata, 1997; Qu and Lukas, 2003; Qu et al., 2004).

Fig. 1 a. schematic map of main rivers and provenance of research area; b. maps of regional circulation and bottom topography MZCC: Min-zhe Coastal Current; TWC: Taiwan Warm Current; KC: Kuroshio Current. Black dash line represents -40 m isobath. Topographic map was created by GMT.

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Core Oki01(28°19′31″N, 127°15′41″E), 380 cm in length, was retrieved from the west slope of the middle of the Okinawa Trough at water depth 1 010 m during the “autumn open offshore cruise” of R/V Kexue No. 1, operated by the Institute of Oceanology, Chinese Academy of Sciences, in September 2012(Fig. 1). This core consisted of gray silty clay with a turbidity layer at 182–194 cm and ash layer at 194–202 cm(Fig. 2). The turbidity layer was mainly composed of clay silt with high abundance of planktonic and benthic foraminifera(Fig. 2). The ash layer with less abundance of foraminifera is probably related to widespread K-Ah tephra(7.3 cal kyr BP)from southern Kyushu, Japan(Machida and Arai, 1983; Zheng et al., 2014b)(Fig. 2). About 20 mg of mixed planktonic foraminifera were identified from four horizons and dated at the Accelerator Mass Spectrometry(AMS)laboratory Beta Analytic(Table 1)(Zheng et al., 2014b). All AMS ¹⁴C ages were converted to calendar years using Calib 7.0 software, with surface ocean reservoir age 400 years(Fig. 3)(Zheng et al., 2014a). Sample age was acquired through linear interpolation among the dated layers, and adjusted with the K-Ah ash layer. The turbidity and ash layer were excluded before the age interpolation. The linear sedimentation rate was ~25 cm/kyr(Fig. 3).

Fig. 2 Vertical variation of(a)dry density, (b)log(Ti/Ca), clay mineral assemblages(in %)for(c)smectite, (d)illite, (e)kaolinite, and (f)chlorite with change of depth Light grey and dark gray bars indicate turbidity and ash layers, respectively(Zheng et al., 2014b).

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Table 1 AMS ¹⁴C ages measured in core Oki01

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Fig. 3 Calendar age versus depth in core Oki01

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Core Oki01 sediments were chosen at 4-cm intervals for clay mineral analysis(<2 μm). Pretreatment and measurement were processed based on methods described by Wan et al.(2007). Semiquantitative estimates of peak areas of basal reflection for the main clay mineral group(smectite, 17 Å, illite, 10 Å, and kaolinite/chlorite, 7 Å)were made for the glycolated samples using Topas 2.0 software. Clay mineral abundances were estimated with the empirical factors of Biscaye(1965). For consistent comparison, we recalculated those abundances for potential sources, including Changjiang, Huanghe Rivers and Taiwan Isl and , using Topas 2.0 based on published data(Zheng et al., 2014a). 3.2 XRF scanning

Major element compositions of core Oki01 sediment were measured with an Itrax X-ray fluorescence(XRF)core scanner at The First Institute of Oceanography, State Oceanic Administration(China). Measurements were done at 30 kV and 40 mA with 1-cm intervals. Elemental composition from XRF core scanning is given as count rates, based on elemental intensity. Elemental ratios are more useful than single elements because of their insensitivity to closure effects(Weltje and Tjallingii, 2008; Zheng et al., 2014a). Ratios of Ti/Ca are commonly used to indicate the influx of terrigenous material in marine sediments, thereby reconstructing paleoclimate(Arz et al., 1998). To reduce the influence of sample geometry and physical properties, we used log(Ti/Ca)as a proxy, which provides interpretable data of relative change in chemical composition by eliminating the constant-sum constraint(Weltje and Tjallingii, 2008). 4 RESULT AND DISCUSSION

The clay mineral assemblage of core Oki01 mainly consisted of illite(~77%) and chlorite(~11%), with minor abundance of smectite(~10%) and kaolinite(~4%)(Figs.2 and 4). The relative abundance of smectite showed a trend opposite to that of illite from 16.0 to 0 ka(Fig. 4). In general, contents of smectite and illite were stable from 16.0–11.6 ka, the same as kaolinite and chlorite. The percentage of smectite and kaolinite decreased considerably beginning at 11.6 ka and remained relatively stable beginning at 10.0 ka. In contrast, illite and chlorite displayed a opposite trends with increasing values within 11.6–10.0 ka. The abundance of smectite and kaolinite increased abruptly within 7.5–7.3 ka, but illite and chlorite contents declined drastically.

Fig. 4 Secular variation of(a)dry density, (b)log(Ti/Ca), clay mineral assemblages(in %)for(c)smectite, (d)illite, (e)kaolinite, and (f)chlorite

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The log(Ti/Ca)ratio was relatively stable from 16.0–11.6 ka, consistent with a high dry density in this period(Fig. 4). Beginning at 11.6 ka, log(Ti/Ca)decreased considerably, and then increased gradually beginning around 8 ka. However, dry density decreased throughout the Holocene. 4.1 Clay mineral and provenance implication

Detailed underst and ing of potential sources and transport processes are vital for sediment provenance identification of clay minerals(Chamley, 1989; Gingele et al., 1998; Diekmann et al., 2008; Steinke et al., 2008). According to earlier studies, terrigenous clastic sediments in the middle Okinawa Trough derived mainly from the Changjiang River, Huanghe River, ECS continental shelf, and rivers in eastern Taiwan Isl and (Meng, 1997; Iseki et al., 2003; Katayama and Watanabe, 2003; Liu et al., 2006; Liu et al., 2007; Diekmann et al., 2008; Kao et al., 2008; Dou et al., 2010; Zheng et al., 2014a). The Changjiang River, the fourth and fifth largest river in the world in terms of sediment and water discharge, respectively, delivers approximately 480 Mt of sediment to the ECS annually(Liu et al., 2006). Clay minerals in these sediments contain primarily illite followed by chlorite, with less smectite and kaolinite content(Diekmann et al., 2008; Xu et al., 2009; Dou et al., 2010; Zheng et al., 2014a). The Huanghe River, known for the greatest sediment load on earth(around 1××10⁹t/a), has also contributed tremendous sediment to the study area since the late Quaternary(Milliman and Meade, 1983; Milliman and Syvitski, 1992). Clay minerals of Huanghe sediments are mainly composed of illite, with lesser amounts of chlorite, kaolinite and smectite(Yang et al., 2003). Clay mineral assemblages of Huanghe sediment are comparable to those of the Changjiang, although the Huanghe has a higher smectite percentage and mineral crystallinity(Yang et al., 2003; Zheng et al., 2014a). Strong tectonic activity and strong storms and monsoon give Taiwan Isl and the highest soil erosion rate in the world(Li, 1976; Li et al., 2012). Sediments exported to the Okinawa Trough derive mainly from rivers in the eastern part of the isl and , reaching 150 Mt/a(Diekmann et al., 2008; Liu et al., 2008). Clay mineral assemblages in eastern Taiwan Isl and river sediments consist of illite and chlorite, with a near absence of smectite and kaolinite(Fig. 5)(Li et al., 2012).

Fig. 5 End-member analysis of provenance using(a)a ternary diagram of smectite-kaolinite-(illite+chlorite) and (b)smectite/illite vs. kaolinite/chlorite ratios Green triangle represents interval of 11.6–0 ka for core Oki01, and violet triangle 16.0–11.6 ka. Brown inverted triangle represents eastern Taiwan Isl and river samples(Li et al., 2012), black crosses East China Sea continental shelf samples, orange circles Changjiang Riversamples, and blue crosses Huanghe River samples. Referenced samples from Changjiang, Huanghe and East China Sea are from Zheng et al.(2014a).

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Interdisciplinary studies in sedimentology, geochemistry/biogeochemistry, mineralogy and physical oceanology concur that sediments in the Okinawa Trough are mainly terrigenous; however, dispute persists concerning the sediment origin and transport mechanisms(Iseki et al., 2003; Hsu et al., 2004; Lee et al., 2004; Kao et al., 2005; Yuan et al., 2008; Bian et al., 2010; Dou et al., 2010). Some scholars believe that a large volume of sediments from eastern Taiwan Isl and could be transported by the KC to the southern Okinawa Trough, even dispersing further north into the middle and northern part of the trough(Hsu et al., 2004; Lee et al., 2004; Dou et al., 2012). Others, however, have demonstrated that sediments along the middle and northern slope of the trough derive mainly from the Changjiang or old Huanghe River. Some in the latter group believes that wind-driven transverse circulations in winter could resuspend inner shelf sediments of the Changjiang River and produce seaward bottom flows that transport these sediments to the Okinawa Trough(Hu, 1995; Yanagi et al., 1996; Peng and Hu, 1997; Hoshika et al., 2003; Iseki et al., 2003). Other researchers in this group, however, believe that the source of terrigenous materials in the trough is mainly the old Huanghe River submarine delta, and that these materials are resuspended by strong wind-driven currents and are subsequently carried to the Okinawa Trough by crossshelf flow forced by collision of the Yellow Seacoastal current and Taiwan warm current(Milliman et al., 1985; Wang and Jiang, 2008; Yuan et al., 2008).

For elucidating the provenance and associated current systems in the middle of the Okinawa Trough, we analyzed clay mineral provenance of core Oki01 since 16 ka using smectite/illite vs. kaolinite/chlorite ratios and a ternary diagram of smectite–(illite+chlorite)–kaolinite(Fig. 5). The clay mineral assemblages in this core were compared with potential sources, including the Changjiang River, Huanghe River, and Taiwan Isl and (Zheng et al., 2014a). We found that the interval of 16.0–11.6 ka is situated near the Huanghe River and /or Changjiang River provenance but is different from Taiwan Isl and sediment, indicating that the clay minerals derived mainly from the Huanghe or Changjiang Rivers. From 11.6 to 0 ka, the sediment source was more similar to a mixture of Changjiang River, ECS continental shelf, and Taiwan Isl and sources(Fig. 5). This result is consistent with the alternation of ⁸⁷Sr/ ⁸⁶Sr and εNd values during 14–7.1 ka in nearby core DGKS9603(Dou et al., 2012). Clay mineral and geochemical records in the southern Okinawa Trough also indicate the change of provenance since the early Holocene as a response to KC intensification(Diekmann et al., 2008). Our result indicates that sediments from Taiwan Isl and could be transported by the KC to the middle Okinawa Trough, where it could mingle with sediments from the Changjiang River and ECS continental shelf, triggered by a monsoon-induced current in winter. 4.2 Complex relationship between current and

monsoon and formation of modern current system since the early Holocene The variation of log(Ti/Ca)ratios has strong correlation with smectite/illite ratios, indicating that change of clay minerals could be an indicator of sediment provenance and influx(Fig. 6). Based on clay mineral analysis, we found that the sediment source derived mainly from the Huanghe and Changjiang Rivers during 16.0–11.6 ka and shifted to the mixture of Changjiang and Taiwan Isl and beginning in the early Holocene(Figs.4 and 5). The provenance transition is synchronous with the drastic decrease of log(Ti/Ca) and smectite/illite ratios, indicating that terrigenous materials from East China decreased significantly beginning in the early Holocene, especially those from the Huanghe River with higher smectite/illite ratio. This transition is possibly associated with the resistance to sediments from East China by intensification of the KC and water barrier in summer(Zheng et al., 2014a).

Fig. 6 Temporal variations of(a)smectite/illite ratio;(b)log(Ti/Ca); and (c)sea level pattern in western Pacific since LGM(Liu et al., 2004), (d)P. Obliquiloculataabundance, core A7(Xiang et al., 2007);(e)sortable silt(10–63 μm)of core ODP1202(Diekmann et al., 2008);(f)magnetic susceptibility of sediment core from Huguang maar lake, Zhanjiang, China(Yancheva et al., 2007);(g)Mg-Ca sea surface temperature, core A7(Xiang et al., 2007), (h)median grain size of loess at Duowa(Maher et al., 2006)

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Moreover, the coastline location since the early Holocene approached that of the present. This signifies stable provenance input and therefore hints at the formation of the modern ECS current system(Zheng et al., 2014a)(Fig. 1). Studies have shown that seasonal variation of the monsoon system strongly affects hydrographic conditions and thereby the KC in the Okinawa Trough(Lee and Chao, 2003; Diekmann et al., 2008; Yuan et al., 2008; Yuan and Hsueh, 2010). Poleward transport of tropical waters in the KC strengthens during summer because of the southern location of NEC bifurcation. A northward location of that bifurcation in winter favors less transport in the KC(Kagimoto and Yamagata, 1997; Qu and Lukas, 2003; Qu et al., 2004). The East Asian winter monsoon drastically weakened beginning in the early Holocene, whereas the East Asian summer monsoon became more intense. This intensified monsoon might have favored a larger volume of poleward water transport in the KC beginning in the early Holocene(Fig. 6). The enhanced East Asian summer monsoon may have reduced sediment export from the shelf area.

Hydrologic observation and modeling indicate that the Taiwan Warm Current is composed of upper layer water from the Taiwan Strait and sub-surface layer water from the intrusion of the KC(Zhu et al., 2004; Chen and Sheu, 2006). However, sea level beginning in the Holocene was around -60 m(Xu et al., 2009), close to the average depth(-60 m)of the Taiwan Strait(Jan et al., 2002); this, coupled with the enhanced KC, facilitated Taiwan Warm Current entrance into the ECS(Figs.1 and 6). Thus, we believe that the modern ECS current system formed beginning in the early Holocene, in response to the favorable sea level, monsoon, and currents.

The general trend of log(Ti/Ca)beginning in the early Holocene is similar to proxies of the East Asian winter monsoon, including magnetic susceptibility of sediment in Huguangyan maar lake of Zhanjiang and median loess grain size in Duowa village, Qinghai Province(Maher and Hu, 2006;Yancheva et al., 2007; Zheng et al., 2014a)(Fig. 6). Titanium is found in both Taiwan Isl and and East China, but is greater in East China(upper continental crust), indicating the provenance from there(Yang et al., 2004). Therefore, log(Ti/Ca), an indicator of terrigenous flux from East China, represents sediment contributions in winter beginning in early Holocene(Fig. 6). This is consistent with our results from core Oki02, which sensitively recorded the East Asian winter monsoon signal during the Holocene(Zheng et al., 2014a).

Interestingly, there is an abrupt change of clay mineral, log(Ti/Ca), and dry density around 7.5–7.3 ka, consistent with other cores from Okinawa Trough(Zheng et al., 2014b)(Figs.4 and 6). The low P. obliquiloculata abundance and Mg/Ca sea surface temperature at nearby core A7 indicate a weaker KC(Xiang et al., 2007)(Fig. 6). In the aforementioned period, the magnetic susceptibility of sediment at Huguang maar Lake and median loess grain size at Duowa increased abruptly(Fig. 6). The sudden enhancement of the East Asian winter monsoon and synchronous attenuation of the KC coupled with active volcano events may favor the occurrence of gravity currents through more sediment input from the shelf area, owing to less KC-induced current resistance. 5 CONCLUSION

The Okinawa Trough is an excellent laboratory for the study of later Quaternary air-sea interaction and paleoenvironmental change. Clay mineral, dry density, elemental(Ti, Ca)composition analysis was done on a sediment core in the middle of the trough. Our results indicate that clay minerals are mainly from the Huanghe and Changjiang rivers from 16.0 to 11.6 ka, and the modern ECS current system began to form in the early Holocene. As a result, the ECS, Changjiang River and partially Taiwan Isl and mixture is the major contributor of clay minerals. A decrease of log(Ti/Ca) and an alternating provenance beginning in the early Holocene indicate less sediment from East China in summer because of resistance by the modern current system(i.e., a water barrier and upwelling), when an intensified East Asian summer monsoon favors a more poleward KC. However, sediment delivery persists in winter and log(Ti/Ca)represents the winter monsoon signal beginning in the early Holocene. Our evidence also suggests that sediments from Taiwan Isl and could be transported by the KC to the middle Okinawa Trough. There, it could mingle with sediments from the Changjiang River and ECS continental shelf, triggered by a monsoon-induced current in winter. The occurrence of turbidity and synchronous change of log(Ti/Ca) and dry density during 7.3–7.5 ka may be related to enhancement of the East Asian winter monsoon and subsequent weakness of the KC. 6 ACKNOWLEDGEMENT

We thank YAO Zhengquan for XRF measurements.