Clay mineral and grain size studies of sediment provenances and paleoenvironment evolution in the middle Okinawa Trough since 17 ka

Marine Geology 366 (2015) 49–61

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Clay mineral and grain size studies of sediment provenances and paleoenvironment evolution in the middle Okinawa Trough since 17 ka Jiaze Wang a,b,c,⁎, Anchun Li a, Kehui Xu c,d, Xufeng Zheng a, Jie Huang a a

Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China University of Chinese Academy of Sciences, Beijing 100049, China Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA d Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA b c

a r t i c l e

i n f o

Article history: Received 25 July 2014 Received in revised form 22 April 2015 Accepted 25 April 2015 Available online 27 April 2015 Keywords: Grain size Clay minerals Sediment provenance Kuroshio Current East Asian Winter Monsoon Okinawa Trough

a b s t r a c t Okinawa Trough receives a large amount of fluvial sediment transported by complex oceanographic circulations, and is an ideal location for the study of paleoceanography and paleoenvironment changes. However, our knowledge of the sediment provenance and paleoenvironment evolution in the Trough during the past 17 ka is still limited. Based on high-resolution grain size, clay minerals and AMS 14C data of the Core OKI04, we present new evidences of the provenances and paleoenvironment evolution in the middle Okinawa Trough during the last 17 ka. Our results indicate that clay-sized terrigenous sediment deposited in the middle Okinawa Trough is mainly from those rivers flowing into East China Sea and Yellow Sea (e.g., Yangtze and Yellow Rivers) and Taiwanese Rivers. Their contributions varied greatly in space and time during the past 17 ka. From 16.5 to 13.8 ka, sediment was mainly derived from the Yangtze and Yellow Rivers, with little contribution from Taiwan. From 13.8 to 5.4 ka, the contributions from the Yangtze and Yellow Rivers oscillated multiple times: low in 13.8–11.6 ka, high in 11.6–7.6 ka, and back to low in 7.6–5.4 ka, whereas the pattern of Taiwanese contribution was opposite. After 5.4 ka when the sea level was high and shoreline was far away from the Okinawa Trough, the Yangtze River and eastern Taiwanese Rivers became the main provenances. The contribution from the Yellow River decreased, which is probably because Yellow River mouth was far away from the middle Trough. Sediment grain size data show that two environmentally sensitive populations are 71–7 μm and 7–0.5 μm. The variation of the populations' mean grain size and content shows that the strength of Kuroshio Current was weak during 16.5–11.6 ka, and became strong in 11.6–5.4 ka, in which Tsushima Warm Current and Yellow Sea Warm Current formed around 8.5 and 6.5 ka, respectively. The strength of Kuroshio Current decreased in 5.4 ka and increased since 3.2 ka. Based on clay mineralogical analysis, we find that the kaolinite/chlorite could be used as a new effective indicator for the East Asian Winter Monsoon's evolution in the Trough since 17 ka. © 2015 Elsevier B.V. All rights reserved.

1. Introduction and background Located between the East China Sea and Ryukyu Islands (Fig. 1), Okinawa Trough is a back-arc spreading basin in the western Pacific marginal sea which receives sediment delivered by nearby large and small rivers during the Quaternary period. The successive sedimentation and complex dynamic environment in the Trough make it an ideal location for the study of high resolution paleoenvironment evolution. The transport of fine-grained sediment in the East China Sea is mainly affected by the sediment supplies from major rivers in Eastern mainland China, ocean circulations, sea level variations and monsoon

⁎ Corresponding author at: 2151 Energy, Coast and Environment Building, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA. E-mail address: [email protected] (J. Wang).

http://dx.doi.org/10.1016/j.margeo.2015.04.007 0025-3227/© 2015 Elsevier B.V. All rights reserved.

dynamics (Diekmann et al., 2008; Dou et al., 2010a,b, 2012; Katayama and Watanabe, 2003; Lee and Chao, 2003; Lee et al., 2004; Yuan et al., 2008). Other influencing factors include submarine canyon transport (Chung and Hung, 2000), seafloor earthquakes (Hsu et al., 2004), episodic volcanic eruptions (Machida, 1999; Miyairi et al., 2004a), and submarine hydrothermal activity (Glasby and Notsu, 2003). Previous studies show that clay minerals and grain size data collected from sediment can be used as effective indicators for the sediment provenance and ocean circulation (Dou et al., 2010a; Huang et al., 2011; McCave et al., 1995; Wan et al., 2010; Xiao et al., 2006; Xu et al., 2014). During the past several decades, numerous studies have been conducted in the Okinawa Trough, but there have been debates on the provenance of sediment in the Trough and their transport mechanisms (Table 1, Figs. 1 and 2). 1) Many geoscientists believe that sediment deposited in the southern and middle Trough is mostly from the Yangtze River and East China Sea Continental Shelf (ECSCS), especially before 7 ka when sea level fluctuated greatly and the Kuroshio Current (KC)

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Fig. 1. Locations of Core OKI04 in Okinawa Trough and other cores used in this study, including B-3GC and 255 (Jian et al., 2000); PC-6 (Xiao et al., 2006); E017 and A7 (Xiang et al., 2007); YSDP102, CSH1 and DGK03 (Li et al., 2009); DGK04 (Dou et al., 2010a,b, 2012); PC-1 (Xu et al., 2014); and Oki02 (Zheng et al., 2014). Regional ocean circulation pattern in the East China Sea is modified from Guo et al. (2006), Jan et al. (2002), and Liu et al. (2013). Water depths are in meters. KC, Kuroshio Current; YSWC, Yellow Sea Warm Current; TC, Tsushima Current; TWC, Taiwan Warm Current; YSCC, Yellow Sea Coastal Current; NJCC, North Jiangsu Coastal Current; CDW, Changjiang Diluted Water; ECSCC, East China Sea Coastal Current.

was weak or moved out of the Trough (Dou et al., 2010b; Ijiri et al., 2005; Iseki et al., 2003; Katayama and Watanabe, 2003; Kawahata, 2004; Li et al., 2001, 2007; Liu et al., 2007; Oguri et al., 2003; Saito et al., 1998; Xu and Oda, 1999; Yoo et al., 2002). About 70–80% Yangtze-derived sediment deposits near the Yangtze estuary and the inner shelf of East China Sea (Milliman et al., 1989). During winter time the energetic monsoon-driven currents can resuspend Yangtze River sediment which is deposited in the East China Sea inner shelf and transport sediment towards the middle Trough (Gao et al., 1999; Peng and Hu, 1997; Yanagi et al., 1996; Yuan and Hsueh, 2010; Yuan et al., 2008). Several previous studies of current velocity, sediment flux and minerals supported this mechanism (Hoshika et al., 2003; Iseki et al., 2003). 2) Some scientists believe that sediment from the Yellow River can be one of the contributors to the northern Trough during the Last Glacial Maximum (LGM, about 18–20 ka before present). During the LGM period, sea level was in lowstand and experienced the

subsequent transgression period. At the same time, the positions of the coastline and river mouth prograded towards the Trough, which probably influenced the sedimentation in the Trough (Saito et al., 1998; Ujiié et al., 2001; Ujiié and Ujiié, 1999; Xu and Oda, 1999; Xu et al., 2014; Youn and Kim, 2011). According to the observation data, under the influence of strong winter monsoon, sediment in the old Yellow River subaqueous delta can be resuspended and transported to the East China Sea Continental Shelf by the Yellow Sea Coastal Current (Milliman et al., 1985; Wang and Jiang, 2008). Other study shows that the convergence of the Yellow Sea Coastal Current and Taiwan Warm Current could lead to an eastward movement of water mass, which could deliver sediment to the northern Trough and even to the middle Trough area during the winter time (Yuan et al., 2008). 3) The third possible sediment source is the Taiwanese Rivers. The main axis of KC probably reentered into the Trough, and this could result in a natural “water barrier” in the Trough area (Dou et al., 2010b; Guo et al.,

Table 1 Possible sediment provenances of Okinawa Trough and sediment transport mechanisms proposed by past studies. Possible provenances Potential input area Possible transport mechanisms

Taiwanese Rivers

Yangtze River and East China Sea Continental Shelf

Yellow River

South and Middle Okinawa Trough North Okinawa Trough Kuroshio Current and Taiwan Warm current Lower sea level, river mouth progradation and Monsoon driven currents

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Fig. 2. The sediment contribution from the possible provenances to the Okinawa Trough since 22 ka (based on past studies). The gradient gray bars are the sediment contribution of main provenances to different areas in the Trough since 22 ka (dark gray means high contribution and light gray means low contribution). The dashed lines are the debatable influences on Trough area by the potential provenances. The blue line is the sea level curve in the Western Pacific Ocean (Liu et al., 2004). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2001). The “water barrier” could prevent sediment of the Yangtze and Yellow Rivers from entering into the Trough. Both KC and Taiwan Warm Current can potentially carry Taiwan sediment to the Trough. However, whether sediment from Taiwan Island could be delivered to the middle and northern Trough, and whether sediment from the Yangtze and Yellow Rivers could be transported into the middle Trough since 7 ka remain unclear in our scientific community (Fig. 2) (Diekmann et al., 2008; Dou et al., 2010a,b, 2012; Hsu et al., 2004; Xu et al., 2014). Our knowledge of which river(s) was the dominant sediment source and how did its or their contribution(s) change over time remain rather limited for the middle Okinawa Trough area. In the sediment transport processes, KC and East Asian Winter Monsoon play important roles. KC carries sediment from Taiwan to the Trough and also keeps sediment of eastern China from entering into the Trough (Diekmann et al., 2008; Dou et al., 2010a,b, 2012; Guo et al., 2001; Xu et al., 2014). Many studies have been done on the KC evolution since LGM (Dou et al., 2010b; Ijiri et al., 2005; Jian et al., 1998, 2000; Kawahata, 2004; Li et al., 1996, 2001; Ujiié et al., 1991, 2003; Ujiié and Ujiié, 1999; Xiang et al., 2007). Ujiié and Ujiié (1999) reported that there was a continental bridge between Taiwan Island and Ryukyu Island formed during LGM, which prevented KC entering into the Trough. Some foraminifer records also show that KC has moved to the southeastern part of the Trough during the LGM (Jian et al., 2000). However, other research findings indicate that KC weakened but still impacted the Trough during the LGM (Ijiri et al., 2005; Kawahata, 2004; Li et al., 2001, 2007; Xu and Oda, 1999; Zheng et al., 2014). Whether the KC moved out of the Trough or not is still debatable. East Asian Monsoon is another important force which affects the sediment transport process into the Trough. In recent years many geoscientists use terrestrial records like stalagmites and Maar Lakes sediment (Duan et al., 2014; Dykoski et al., 2005; Fleitmann et al., 2003; Griffiths et al., 2009; Wang et al., 2001, 2012; Yancheva et al., 2007) to

study East Asian monsoon evolution. The flux and transport of sediment to coastal environment are highly influenced by the East Asian monsoon evolution. As now only a handful of studies have been done with marine sediment to study the high resolution record of East Asian monsoon evolution (Kao et al., 2006; Steinke et al., 2011; Sun et al., 2005; Xiao et al., 2006; Xu et al., 2009), but no high resolution monsoon studies have been performed in the Trough area yet. In this study we strive to identify the provenance of sediment in the middle Okinawa Trough and try to reconstruct the paleoenvironment evolution during the past 17 ka using high-resolution mineralogical and texture data of Core OKI04. We study KC evolution by identifying environmentally sensitive grain-size fractions, and propose kaolinite/ chlorite ratio in marine sediment as a new indicator for the East Asian Winter Monsoon evolution. Through a comprehensive synthesis of grain size, mineralogy, radiocarbon chronology, and sea level change, we discuss the relative contributions from Yellow, Yangtze and Taiwanese Rivers in the Trough during the past 17 ka. 2. Materials and methods Core OKI04 (26°52.63′ N, 126°8.43′ E) is located at a water depth of 1407 m in the middle Okinawa Trough (Fig. 1). This 288-cm long core was collected using a piston corer by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) in 2012. It is mainly composed of dark gray clayey silt throughout the core, and there are four foraminifera-rich layers at 58–62, 98–99, 124–125 and 192–193 cm, respectively. Preliminary studies showed that water content decreases with depth and biological detritus was only found below 210 cm. No volcanic detritus layers or turbidite layers were found in the core. Mixed species of planktonic foraminifer for seven layers at 48–50, 62–64, 70–72, 88–90, 148–150, 204–206 and 286–288 cm were picked up to measure the AMS 14C ages at Beta Analytic Company, USA. The measured radiocarbon age data were calibrated to calendar age before

Table 2 AMS 14C age data and linear sedimentation rate of Core OKI04. Depth (cm)

AMS 14C age (a B.P.)

Calendar age (a B.P.)

48–50 62–64 70–72 88–90 148–150 204–206 286–288

4090 ± 30 5200 ± 30 6250 ± 30 7840 ± 30 10,190 ± 40 11,850 ± 30 14,060 ± 50

4112 5544 6683 8292 11,172 13,305 16,439

±1σ (a B.P.) 4049–4202 5489–5585 6627–6738 8240–8350 11,123–11,2230 13,245–13,363 16,309–16,550

Linear sedimentation rate (cm ka−1)

Sample resolution (a)

12.2 9.8 7.0 11.2 20.8 26.3 26.2

165 205 285 179 96 76 76

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Fig. 3. (a) The AMS14C age of Core OKI04; (b) silt/clay percentage, mean grain size and sorting variations of Core OKI04.

1950 A.D. (cal B.P.) using the software CALIB 7.0.4 (Hughen et al., 2004) with one standard deviation uncertainty (Table 2 and Fig. 3a). This core was sliced into 2-cm thick samples for grain size analysis, and a total of 144 samples were analyzed. Organic matters, carbonate and silica were removed using 15% H2O2, 20% Ac and 2 mol/l sodium carbonate, respectively. Grain size measurements of the samples were conducted using a laser diffraction particle size analyzer Cilas 940L in the laboratory of the IOCAS. The Cilas 940L laser grain size analyzer measures sediment particles between 0.5 and 2000 μm, with analytical precision better than 2%. Clay mineralogy analysis was performed on the b 2 μm sediment fraction of 72 samples at 4 cm intervals. All samples were pretreated to remove organic matters, carbonate and silica with 15% H2O2, 20% Ac and 2 mol/l sodium carbonate, respectively. Particles less than 2 μm were extracted according to the Stoke's settling velocity principle. Clay minerals analysis was carried out using a D8 ADVANCE X-ray diffractometer with Cuka radiation (40 kV, 40 mA) in the laboratory of IOCAS. Samples were measured four times: (1) air drying at room temperature (scanning within 3–30° 2θ, 0.02° scanning step); (2) heating at 490 °C for 2 h; (3) saturating in ethylene glycol solvent at 60 °C for 12 h (3–30° 2θ, 0.02° step); (4) saturating in ethylene glycol solvent at 60 °C for 12 h (24–26° 2θ, 0.01° step) (Huang et al., 2011; Wan et al., 2010). The fourth time was used to detect the double peak 3.54/3.58 Å of Chlorite/Kaolinite, respectively. Semi-quantitative calculation of peak areas for smectite (including mixed illite–smectite species), illite, and kaolinite/chlorite was processed on glycol-saturated samples using Topas 2P software with the empirical factors from Biscaye (1965). The empirical factors for smectite: illite: kaolinite/chlorite were 1:4:2. The 7 Å peak area for kaolinite and chlorite was subdivided in proportion to the relative areas of 3.54 Å (chlorite) and 3.58 Å (kaolinite) peaks. The sum of all four clay minerals content was 100%. The approximate accuracy of the XRD analysis is around ±5–10% (Biscaye, 1965). Illite chemical index (CI) was calculated from the ratio of the 5 Å and 10 Å illite peak areas in glycol-saturated samples (Esquevin, 1969). Ratios N 0.5 represent Al-rich illite formed in strong hydrolysis environment. However, ratios b 0.5 correspond to Fe–Mg illite, which is the product of physical weathering (Gingele, 1996). Illite crystallinity was estimated with the Full Width at Half Maximum (FWHM) of illite 10 Å

peak. In general, high values of FWHM represent poor crystallinity (Ehrmann, 1998; Gingele et al., 2001). 3. Results 3.1. Grain size The sediment in Core OKI04 is mainly silt and clay, and there is essentially no sand (Fig. 3(b)); the mean grain size of sediment increases with depth from top to 63 cm, and decreases from 63 to 165 cm, below which depth it is stable. The sediment sortings are between 1.3 and 1.7 μm at layer 0–120 cm, and are around 1.3 μm from 120 cm to the core bottom. The high values of sortings indicate that sediment is poorly sorted. The grain size frequency distribution curves at five selected depths (12, 50, 100, 150, 260 cm) in the core are shown in Fig. 4. The grain size distributions at 12 and 50 cm display a bimodal pattern near 1.5 and 14 μm. Sediment at 100 and 150 cm depths develop two types of

Fig. 4. Grain-size frequency distribution curves of Core OKI04 sediment at 12, 50, 100, 150 and 260 cm depths, respectively. Note that most of the curves are bimodal.

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Fig. 5. Variations of mean grain size, clay mineral content (illite, chlorite, kaolinite and smectite), illite crystallinity and illite chemical index and 5 point running averages (gray lines) of clay mineral assemblages in Core OKI04. YD = Younger Dryas; West Pacific sea level curve is from Liu et al. (2004).

Table 3 Clay mineral characteristics of the main rivers in different provenances (SD is the standard deviation). Samples Yellow River SD Yangtze River SD Rivers in east Taiwan SD Rivers in west Taiwan SD Continental shelf off southwest Taiwan SD OKI04 SD

# of samples analyzed

Smectite

Illite

Chlorite

Kaolinite

Illite crystallinity

Illite chemical index

References

22

27% 8.7 10% 3 0 0 1% 2 0 0 16% 3

54% 7.1 66% 2 64% 12.5 55% 4 59% 1.5 63% 3

12% 2 18% 1.7 36% 12.5 43.% 3 41% 1.5 5% 1

7% 1.5 6% 1.4 0 0 1% 1 0 0 16% 1.6

0.35 0.03 0.32 0.03 0.28 0.05 0.16 0.02 0.16 0.01 0.25 0.05

0.33 0.04 0.43 0.02 0.46 0.02 0.33 0.03 0.36 0.02 0.37 0.05

Lu et al. (2015)

6 3 18 11 72

Fig. 6. The histogram of the average clay mineral contents in Taiwanese Rivers, Yangtze and Yellow Rivers.

Lu et al. (2015) Li et al. (2012) Liu et al. (2008) Liu et al. (2008) This study

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Fig. 7. Clay mineral provenance analysis. (a) A ternary diagram of (smectite + kaolinite)–chlorite–illite; and (b) illite crystallinity vs. kaolinite/chlorite ratio of Core OKI04. Data of Western Taiwanese Rivers and Southwestern Taiwan shelf are from Liu et al. (2008); data of Eastern Taiwanese Rivers are from Li et al. (2012). Yangtze and Yellow Rivers data are from Lu et al. (2015). All the blue symbols in the oval are the data from Core OKI04. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

distribution modes: mode at 100 cm depth has a bimodal cure with peaks at 8.5 and 2.6 μm, while that at 150 cm depth shows a trimodal curve with peaks at 1.7, 12 and 45 μm, respectively. Both 45 and 1.7 μm peaks have low contents. The frequency distribution at 260 cm has flat bimodal curve, and two peaks are at 10 and 2 μm, respectively. Generally the grain size distributions indicate that two peaks are around 1.4–3 and 10–16 μm, respectively.

Fig. 8. Kaolinite content vs. illite crystallinity in Core OKI04. Data of Western Taiwan Rivers and South West Taiwan shelf are from Liu et al. (2008); data of Eastern Taiwan Rivers are from Li et al. (2012). Yangtze and Yellow River data are from Lu et al. (2015). The red crosses inside the rectangular are the data of Core OKI04. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.2. Clay minerals The clay minerals in the Core OKI04 include kaolinite, chlorite, smectite (including mixed illite–smectite mineral) and illite (Fig. 5 and Table 3). The average content of illite is about 63%, while smectite and chlorite are 16% and 16%, respectively; kaolinite has the lowest average content, only 5%. Illite crystallinity varies within a range of 0.15– 0.41°Δ2θ, with the average value of 0.25°Δ2θ, which shows that illite is in good crystallinity. Illite chemical index changes between 0.25 and 0.55, and the average is 0.37. The illite chemical index indicates illite in the core is mainly Fe–Mg rich, which is the result of physical weathering. Based on sea level change, grain size and clay mineralogy features, Core OKI04 can be divided into five units (Fig. 5): Unit 1 (16.5–13.8 ka), Unit 2 (13.8–11.6 ka), Unit 3 (11.6–7.6 ka), Unit 4 (7.6–5.4 ka), and Unit 5 (5.4 ka–present). In Unit 1 (16.5–13.8 ka), clay minerals' contents fluctuate greatly. The pattern of smectite is opposite to the illite, which increases firstly and then decreases with time. Chlorite content decreases first and then begins to increase, while kaolinite does not have much

Fig. 9. Grain size vs. standard deviation based on all sediment samples in Core OKI04.

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Fig. 10. Variations of grain size percentages of b7, 7–20 and 20–71 μm fractions, the mean grain sizes of the bulk samples, and mean grain sizes (μm) of the b7, 7–20 and 20–71 μm fractions, with a comparison with the Western Pacific sea level curve from Liu et al. (2004). YSWC, Yellow Sea Warm Current; TC, Tsushima Current; TWC, Taiwan Warm Current.

variation. Illite crystallinity shows strong fluctuations and relative high values, but illite chemical index has relative low values. In Unit 2 (13.8– 11.6 ka), illite and kaolinite contents increase at first, and then decrease. While smectite and chlorite show opposite change. Illite crystallinity and illite chemical index have stable low values. In Unit 3 (11.6–7.6 ka), the variations of illite and smectite are opposite to each other, which illite decreases firstly and then increases with time. Chlorite increases at the beginning, and follows by a decrease, and then it increases with time slightly. The content of kaolinite decreases firstly, and then follows by an increase trend. At the end of Unit 3, the kaolinite content begins to decrease. Illite crystallinity demonstrates strong fluctuation with high values, whereas illite chemical index shows small fluctuation. The illite chemical index decreases at the beginning, and then increases subsequently within a small value range. In Unit 4 (7.6–5.4 ka), illite content increases, while the others decrease. Illite crystallinity values are low and stable, whereas illite chemical index begins to decrease. In Unit 5 (5.4 ka–present), both illite and kaolinite decrease, while the variations of chlorite increase. Smectite does not have large changes. Illite crystallinity and illite chemistry index values increase gradually during this period and show relatively strong variations. 4. Discussions 4.1. Sediment provenances Sea level, KC, river discharge and East Asian monsoon all influence sediment transport and depositional processes in Okinawa Trough during the past 17 ka (Jian et al., 2000; Liu et al., 2004; Xiang et al., 2007; Yancheva et al., 2007; Zheng et al., 2014). In this study we compared clay minerals of Core OKI04 with those from the Yangtze and Yellow Rivers (Lu et al., 2015), Taiwanese Rivers (Li et al., 2012; Liu et al., 2008), and the continental shelf off southwest Taiwan (Liu et al., 2008) in order to study the evolution of sediment provenance in Okinawa Trough since 17 ka. Clay minerals in the Yangtze River mainly consist of illite (66–72%, averaging 66.6%) and chlorite (17–20%, averaging 18%), and less smectite (5–11%, averaging 10%) and kaolinite (2–7%, averaging 6%) (Table 3, Figs. 6 and 7). The Yellow River is composed of illite (37– 62%, averaging 55%) and smectite (17–48%, averaging 26%), and has less chlorite (6–15%, averaging 12%) and kaolinite (4–11%, averaging 7%) (Table 3, Figs. 6 and 7). Even though the smectite of the Yellow

River is higher than the Yangtze River, these two rivers are similar in other clay minerals' content. However, due to high precipitation, and strong physical erosion on the Tertiary sedimentary rock in Taiwan, illite and chlorite are predominant, and illite crystallinity is very good in Taiwan rivers, while kaolinite and smectite are extremely low (Li et al., 2012). Eastern Taiwan Rivers sediment consists of illite (52–77%, averaging 64%) and chlorite (36–48%, averaging 36%), while kaolinite and smectite are almost zero (Li et al., 2012). Western Taiwan Rivers also have high illite (46–66%, averaging 56%) and chlorite (33–46%, averaging 43%), and extremely low kaolinite (1–4%, averaging 0.7%) and smectite (1–8%, averaging 0.9%). Southwestern Taiwan continental shelf has no smectite and kaolinite, while illite (57–61%, averaging 59%) and chlorite (39–43%, averaging 41%) are dominant (Liu et al., 2008) (Table 3, Figs. 6 and 7). Compared with clay minerals' assemblages in the potential provenances (Fig. 7(a)), sediment in Core OKI04 is influenced by multiple potential provenances since last 17 ka. The relatively high smectite content in Core OKI04 indicates that the Yellow River is a potential dominant sediment contributor (Fig. 7(a)). However, there are authigenic smectites generated from volcanic activities, and may influence the smectite abundance. In other words, the high values of smectite content in Core OKI04 may be influenced by the volcanic materials, as well as the terrigenous contributions (Dou et al., 2010a; Jiang, 2001; Li, 1990; Machida, 1999; Miyairi et al., 2004a,b; Xu et al., 2014; Zeng et al., 2002). In considering about the difference of the other clay minerals' abundance and illite crystallinity among the sources (Table 3), we used the kaolinite/chlorite ratio vs. illite crystallinity to reveal the provenance evolution in detail (Fig. 7(b)). Illite crystallinity and kaolinite/chlorite ratio have shown that clay minerals in Core OKI04 seem to be the mixture of sediment from the Yangtze and Yellow Rivers, eastern and western Taiwanese Rivers, and also those from the continental shelf off southwest Taiwan (Fig. 7(b)). As the Fig. 7(b) has shown that, the OKI04 data points are located among Yangtze, Yellow, and Taiwanese Rivers’ range. The Yangtze and Yellow Rivers are dominant provenances. However, their contributions changed with time. In Unit 1 (16.5–13.8 ka), during which the sea level was about 120 m below the present, the Yangtze and Yellow Rivers were the dominant provenance of sediment in the Trough, while sediment from Taiwan contributed a little amount to the Trough. In Unit 2 (13.8–11.6 ka), when sea level began to rise, sediment contribution from Taiwan increased relatively, while the Yangtze and Yellow Rivers'

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Fig. 11. Variations of kaolinite/chlorite ratio in Core OKI04 compared with the kaolinite/chlorite ratio in Core OKI02 (Zheng et al., 2014), GISP2 δ18O (Stuiver et al., 1995), Dongge Cave δ18O (Dykoski et al., 2005), Ti content in Lake Huguang Maar (Yancheva et al., 2007), as well as Western Pacific sea level curve (Liu et al., 2004).

Fig. 12. Changes in ocean circulation, East Asian monsoon and sediment contribution (from the Yangtze and Yellow Rivers, and Taiwanese Rivers) to Core OKI04 in the middle Okinawa Trough during the past 17 ka. Multiple black arrows indicate the currents; single gray arrow indicates the monsoon; orange arrows indicate the sediment supply. The widths of arrows correspond to the relative strengths of winter monsoon and sediment inputs. Paleo-river valleys on the East China Sea/Yellow Sea shelves (Liu et al., 2004; Ujiié and Ujiié, 1999) show coastline retreat during the 5 units (brown bold lines show the paleo-river valleys; black thin lines show the paleo coastal line). KC, Kuroshio Current; TWC, Tsushima Warm Current; YSWC, Yellow Sea Warm Current; TWC, Taiwan Warm Current; EAWM, East Asian Winter Monsoon. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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contribution decreased. In Unit 3 (11.6–7.6 ka), the sea level kept rising and the transport of sediment from the Yangtze and Yellow Rivers increased again. In Unit 4 (7.6–5.4 ka), sea level reached to the modern sea level position around 7.5 ka. Western Taiwanese Rivers' input into the Trough came to reinforce again, whereas the Yangtze and Yellow Rivers' contribution began to reduce. In Unit 5 (5.4 ka–present), the Yangtze River and Eastern Taiwanese Rivers became the main provenances, and the input of the Yellow River and Western Taiwanese Rivers decreased. Clay minerals in Core DGKS04 (Fig. 1) located at the north of middle Okinawa Trough also show that sediment provenance in the Trough have significantly adjusted since 7 ka, which they thought was attributed to the reentry of KC into the Trough (Dou et al., 2010a). Considering the extremely low content of kaolinite in Taiwan and relative high percent in the Yangtze and Yellow Rivers (Table 3), we also studied kaolinite's provenance in the core (Fig. 8). The kaolinite content is primarily within the range of values for the Yangtze and Yellow Rivers, and it can be deduced that kaolinite in the core is probably originally from the Yangtze and Yellow Rivers, while Taiwanese Rivers' contribution is small.

4.2. Paleoenvironment evolution since the past 17 ka 4.2.1. Kuroshio Current evolution Grain size data contain key information on environment evolution and can indicate ocean circulations and current strength (McCave et al., 1995; Moerz and Wolf-Welling, 2001). It is key to extract grain size populations sensitive to the environment from complex grain-size data set to study marine paleoenvironment evolution. In order to further reveal the sediment transport processes in the Trough, we applied the method of grain-size vs. standard deviation to partition grain-size populations and indentify the sensitive grain size populations to the paleoenvironment evolution (Boulay et al., 2002; Huang et al., 2011; Sun et al., 2003; Xiao et al., 2005, 2006). Standard deviations are calculated for 144 samples in Core OKI04 and the results are shown in Fig. 9. Two sensitive grain size populations exist in the samples, namely coarse population at 71–7 μm (peak at 15 μm) and fine population at 7–0.5 μm (peak at 3 μm). Sediment population b 20 μm is mainly transported as suspended loads in the intermediate and surface water column, while the coarse population N 20 μm is mostly transported as bed loads in East China Sea (Sun et al., 2003; Xiao et al., 2005). Thus we use three grain size populations, 0.5–7 μm, 7–20 μm and 20–71 μm in the following analysis (Fig. 10). The two populations 7–20 μm and 0.5–7 μm are the dominant partitions in Core OKI04, due to their high percentages. However, there is sediment coarser than 20 μm in Core OKI04 with low percentage (Fig. 10). As shown in Fig. 10, the mean grain size of the population 7–20 μm and its percentage variation are consistent with the mean grain size for the total bulk samples, and it plays a dominant role in the overall mean grain size's variation in the core. However, the variation of population 0.5–7 μm is contrary to the bulk samples' mean grain size. For the coarse population 20–71 μm, it varies greatly in several layers, which probably leads to the large fluctuations in the bulk mean grain size in several layers. The variation of bulk samples' mean grain size is consistent with the provenances' change since 16.5 ka (Figs. 7(b) and 10). The change of dynamic environment probably resulted in the variation of grain size in the core (Boulay et al., 2002; McCave et al., 1995; Sun et al., 2003; Xiao et al., 2005). In Fig. 10, it displays that the environment was in a steady state from 16.5 to 11.6 ka (Units 1 and 2) when sea level was low (Fig. 10), and began to change since 11.4 ka. The strength of KC was possibly weak during 16.5–11.6 ka, but it still influenced the Trough, because Taiwanese Rivers contributed to the Trough during that period (Fig. 7(b)).

In Unit 3 (11.6–7.4 ka), the percentage of grain size population b 7 μm decreased, while the content of population 7–20 μm increased. Most notably the percentage of population 20–71 μm increased at 9.5 ka (Fig. 10). These grain size variations were probably related to the strengthening of KC. All of these records corresponded to the sea level rise in the South China Sea, East China Sea and Yellow Sea during 9.5– 9.2 ka (Xiang et al., 2007). Moreover, foraminifer data at the nearby Cores E017 and A7 (Fig. 1) in the north of the middle Trough showed that the amount of deep water foraminifer species increased since 9.4 ka (Xiang et al., 2007), which probably also indicated that the KC began to strengthen at around 9.5 ka. The mean grain size of population 20–71 μm reached its first peak at 8.5 ka (Fig. 10), which was possibly related to the enhancement of KC and the formation of the Tsushima Warm Current. Cores CSH1, PC-1 and DGSK03, which are located in the north of the Trough and north of the middle Trough, respectively (Fig. 1), also showed that the modern Tsushima Warm Current began to develop since 16 ka, and finally formed at about 8.5 ka. The strengthened KC became the main water source for the Tsushima Warm Current during the deglaciation period (Li et al., 2007; Xu et al., 2014). In Unit 4 (7.6–5.4 ka), the percentage of coarse population 20–71 μm had almost reached its highest value, and the mean grain size of this population also had large values at this period (Fig. 10). The mean grain size of the coarse population (20–71 μm) reached its highest value at about 7.5–6.5 ka. Meanwhile, the population 7–20 μm kept a high content during this time period (Fig. 10). According to these records, it is concluded that the strength of KC reached its peak in this period. Besides, the increasing strength of KC probably resulted in the formation of the Yellow Sea Warm Current, which was probably formed at about 6.5 ka. It is consistent with paleoceanographic records in Cores YSDP102, CSH1 and DGKS9603 (Li et al., 2009; Xu et al., 2014). In Unit 5 (5.4 ka–present), the percentage of population 20–71 μm decreased, while the content of population b 7 μm increased (Fig. 10). The content and mean grain size of the coarse population 20–71 μm had low values from 5.4 to 3.2 ka, which probably indicated the weakening of the KC during this stage (Jian et al., 2000; Li et al., 2001, 2009; Ujiié and Ujiié, 1999; Xu et al., 2014). After 3.2 ka, the mean grain size and content of the population 20–71 μm started to increase again, which was related to the strengthening of KC (Li et al., 2009; Xu et al., 2014). 4.2.2. Winter Asian Monsoon evolution Many studies show that the transport of sediment from the Yangtze and Yellow Rivers to the East China Sea mainly happens in winter season (Bian et al., 2010; Gao et al., 1999; Yang et al., 1992). The Core OKI04 is located in the middle Okinawa Trough, and sediment transported to this area through submarine canyons is limited (Chung and Hung, 2000). The strata of the core are successive, and there are no apparent turbidity current deposits. The kaolinite/chlorite ratio is within the Yangtze and Yellow Rivers’ range, and the kaolinite has a relatively clear provenance (Figs. 7(b) and 8), which is mainly from the Yangtze and Yellow Rivers and East China Sea Continental Shelf. Therefore, it is deduced that the transport of kaolinite is probably related to the currents moving to the Trough which are generated by the winter monsoon. In order to minimize the XRD-related error and amplify the clay signals, we chose to use the kaolinite/chlorite ratio to indicate the evolution of winter monsoon which drives currents into the Trough (Fig. 11). Moreover, we compared the kaolinite/chlorite ratio with a nearby core Oki02 (Zheng et al., 2014), multiple δ18O proxy records on land, including Dongge Cave and GISP2 (Greenland Ice Sheet Project 2) (Dykoski et al., 2005; Stuiver et al., 1995), and Ti records in Lake Huguang Maar (Yancheva et al., 2007) (Fig. 11). In Unit 1 (16.5–13.8 ka), the GISP2 and Dongge Cave δ18O records showed that the climate was cold at the beginning and then went into a warm period at about 14.4 ka. At the same time, Ti content in Lake Huguang Maar revealed that the winter monsoon was strong at early

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time of this period, and became weak subsequently (Fig. 11). The kaolinite/chlorite had high values during the cold time and showed relatively low values in warm period, which probably indicated that winter monsoon strengthened during the cold time and weakened in warm period. It is deduced that the winter-monsoon driven currents carry the Yangtze and Yellow Rivers sediment as well as the continental shelf sediment in the East China Sea to the Trough. During the low sea level period before 7 ka (Fig. 12), the Yangtze and Yellow Rivers cut through the exposed continental shelf of East China Sea, so it was easier for the two rivers to deliver sediment directly into the Trough. The proximity of river mouths, strong winter monsoon and weak KC resulted in that Yangtze and Yellow Rivers contributed more sediment than Taiwanese Rivers to the Trough during this period (Figs. 7(b), 10, 11 and 12). In Unit 2 (13.8–11.6 ka), both GISP2 and Dongge Cave δ18O recorded Younger Dryas cold event (11.6–12.8 ka) (Dykoski et al., 2005; Stuiver et al., 1995). The Ti content in Lake Huguang Maar also showed a strong winter monsoon period (Yancheva et al., 2007). Consistent with these records, the kaolinite/chlorite had high values in both core OKI04 and Oki02, and it probably indicated strong winter monsoon correlated with other proxies (Fig. 11). The clay mineral data displayed that the contribution of the Yangtze and Yellow Rivers was less than that of Unit 1 (16.5–13.8 ka) (Fig. 7(b)). Even though the winter monsoon was strong during this period, the high sea level and strengthening of KC could probably prevent the Yangtze and Yellow Rivers' sediment from entering into the Trough (Figs. 7(b), 10, 11 and 12). In Unit 3 (11.6–7.6 ka), the kaolinite/chlorite ratio decreased with time, which is consistent with the other three proxies (Fig. 11). However, the kaolinite/chlorite ratio began to increase at about 10.4 ka. The variations of kaolinite/chlorite ratio indicated that the winter monsoon strength decreased since the Younger Dryas cold event, and began to strengthen subsequently at about 10.4 ka. Even though the KC began to strengthen and the sea level was higher than that of Unit 2, the contribution of the Yangtze and Yellow Rivers to the Trough still increased, which was possibly related to the strengthening of winter monsoon (Figs. 7(b), 10, 11, and 12). In Unit 4 (7.6–5.4 ka), the kaolinite/chlorite ratio had relative high values, even though the ratio had a decreasing trend and reached the first lowest value at about 7.3 ka. The decrease of the kaolinite/chlorite ratio probably indicated the weakening of the winter monsoon since 10.4 ka and a warm period around 7.3 ka. As recorded in Cores B-3GC and 255, the sea surface temperature and the depth of thermocline increased at about 7.3 ka (Jian et al., 2000) (Fig. 1). The weakening of the winter monsoon probably influenced the sediment delivery to the Trough and resulted in the provenance change, as recorded by the Core DGSK04 (Dou et al., 2010a) (Figs. 1, 7(b) and 11). However, the winter monsoon strength increased after 7.3 ka. The KC strength is deduced to reach its highest peak at this period. Correspondingly, the clay mineral data show that the contribution of the Yangtze and Yellow Rivers to the Trough has decreased, which is probably related to the strong KC and weak winter monsoon (Figs. 7(b), 10, 11 and 12). In Unit 5 (5.4 ka–present), the kaolinite/chlorite had relatively low value at about 5.4 ka, which indicated weak winter monsoon, and it is also recorded by the grain size of Core PC-6 in the inner continental shelf of the East China Sea (Xiao et al., 2005, 2006) (Figs. 1 and 11). Another relatively low value is at about 4.1 ka. This indicated that the winter monsoon weakened a little. At the same time, the GISP2 and Dongge Cave δ18O also showed warm climate (Dykoski et al., 2005; Stuiver et al., 1995). Dunde ice core recorded the warm climate at 4.2–4 ka (Yao et al., 1997). The other weak winter monsoon period recorded by the kaolinite/chlorite ratio happened at about 3 ka, which is corresponded to the Dunde ice core's record that the climate was warmer before 3 ka, and became colder after 3 ka (Yao et al., 1997). Furthermore, according to the kaolinite/chlorite data, the winter monsoon strengthened at 4.3–4.6 ka and 3.4–4.0 ka, which were probably related to the global cooling events recorded by the GISP2 δ18O (Stuiver et al., 1995).

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Together with the decrease of KC strength since 5.4 ka, the strengthening of the winter monsoon in this period probably resulted in the increase of sediment contribution from the Yangtze River to the Trough (Figs. 7(b), 10, 11 and 12). 5. Conclusions Based on the grain size and clay mineral analysis of the Core OKI04 collected from the middle Okinawa Trough, we have investigated the sediment provenance evolution in the middle Okinawa Trough, explored the Kuroshio Current evolution with sensitive grain size population, and discussed the East Asian Winter Monsoon evolution with kaolinite/chlorite ratio since 16.5 ka (Fig. 12). The clay mineral data show that sediment in the Trough is mainly from the Yangtze, Yellow, and Taiwanese Rivers. However, the contribution of each provenance varies with time and space, and the provenance evolution is divided into 5 units: Unit 1 (16.5–13.8 ka), Unit 2 (13.8– 11.6 ka), Unit 3 (11.6–7.6 ka), Unit 4 (7.6–5.4 ka), and Unit 5 (5.4 ka– present). In Unit 1 (16.5–13.8 ka), the Yangtze and Yellow Rivers contributed more sediment than Taiwanese Rivers to the Trough. In Unit 2 (13.8– 11.6 ka), the Yangtze and Yellow Rivers' contribution decreased, whereas Taiwanese Rivers' contribution increased correspondingly. During Unit 3 (11.6–7.6 ka), the transport of sediment from the Yangtze and Yellow Rivers increased again, while Taiwanese Rivers' contribution declined. In Unit 4 (7.6–5.4 ka), contribution from Taiwan to the Trough came to reinforce again, whereas the Yangtze and Yellow Rivers' contribution reduced. During Unit 5 (5.4 ka–present), the Yangtze River and Eastern Taiwanese Rivers became the dominant provenances, and the effect of Yellow River and rivers in west Taiwan decreased. The grain size data indicate that the sediment was mainly transported to the Trough in the form of suspended load, and a small portion of the sediment was transported as bed load. The sensitive grain size populations demonstrate that Kuroshio Current was weak in Unit 1 and Unit 2 (16.5–11.6 ka), but became strong in Unit 3 (11.6–7.6 ka). The Tsushima Warm Current was probably formed at 8.5 ka. In Unit 4 (7.6–5.4 ka), the KC strengthened, and reached its strongest stage at about 7.5–6.5 ka. The Yellow Sea Warm Current was formed around 6.5 ka. In Unit 5 (5.4 ka– present), the strength of KC decreased gradually since 5.4 ka, and its strength began to increase after 3.2 ka. Kaolinite has a relatively single source and kaolinite/chlorite ratio can be used to indicate the East Asian Winter Monsoon's evolution in the Trough. The kaolinite/chlorite ratio clearly displays the winter monsoon evolution since 16.5 ka. The winter monsoon and Kuroshio Current have weakened and strengthened multiple times, impacting the sediment transport and fate in the Okinawa Trough. Acknowledgments We are grateful to two anonymous reviewers and the editor for their critical reviews and suggestions which helped in the improvement of our manuscript. We would like to thank the crew of the R/V KEXUEYIHAO for the assistance with sample collection. This study was supported by the National Natural Science Foundation of China (Grant no. 41430965). References Bian, C., Jiang, W., Song, D., 2010. Terrigenous transportation to the Okinawa Trough and the influence of typhoons on suspended sediment concentration. Cont. Shelf Res. 30, 1189–1199. Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep sea clay in the Atlantic Ocean and adjacent seas and oceans. Geol. Soc. Am. Bull. 76, 803–832. Boulay, S., Colin, C., Trentesaux, A., Pluquet, F., Bertaux, J., Blamart, D., Buehring, C., Wang, P., 2002. Mineralogy and sedimentology of Pleistocene sediments on the South China Sea (ODP Site 1144). Proc. Ocean Drill. Program Sci. Results 184, 1–21. Chung, Y.-C., Hung, G.-W., 2000. Particulate fluxes and transports on the slope between the southern East China Sea and the South Okinawa Trough. Cont. Shelf Res. 20, 571–597.

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