Neodymium isotopic variations of the late Cenozoic sediments in the Jianghan Basin: Implications for sediment source and evolution of the Yangtze River

Journal of Asian Earth Sciences 45 (2012) 57–64

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Neodymium isotopic variations of the late Cenozoic sediments in the Jianghan Basin: Implications for sediment source and evolution of the Yangtze River Lei Shao a,b, Chang’an Li a,c,⇑, Shengyuan Yuan a,b, Chunguo Kang b,d, Jietao Wang a, Ting Li e a

Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan 430074, China c Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, Wuhan 430074, China d Institute of Geophysics & Geomatics, China University of Geosciences, Wuhan 430074, China e Hubei Earthquake Administration, Wuhan 430074, China b

a r t i c l e

i n f o

Article history: Received 13 April 2011 Received in revised form 8 September 2011 Accepted 19 September 2011 Available online 25 September 2011 Keywords: Yangtze River Jianghan Basin Nd isotope Provenance

a b s t r a c t The Yangtze River originates from eastern Asia and is one of the most important components of the East Asia river system. In this study we applied bulk Nd isotopic analysis to identify the sediment provenance in the Jianghan Basin, middle Yangtze River and tried to provide useful information on the evolvement of the Yangtze River. The fine-grained (<0.058 mm) sediment samples were selected from a continuous borehole in the Jianghan Basin and analyzed for Nd isotopic compositions. The 143Nd/144Nd ratios vary between 0.512042 and 0.512239, with an average of 0.512144. The eNd(0) values vary between 11.6 and 7.8, with an average of 9.6. Nd isotopic compositions cannot provide compelling evidence to prove whether the Pliocene sediments in the Jianghan Basin were influenced by the source rocks in the Jinshajiang area characterized by extremely high (positive) eNd(0) values. While these source rocks made a great contribution to the Jianghan Basin during the Quaternary. Less negative eNd(0) values reflect preferential erosion of source rocks in the Jinshajiang drainage. The Nd variations reflected changes in erosion patterns during the Quaternary. Based on the provenance analysis of the Jianghan Basin sediments, we propose that the Yangtze River appears to develop into a large river similar as the modern Yangtze no later than the beginning of the Quaternary. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The major river systems in eastern Asia transport a large quantity of terrestrial materials eroded from the Tibetan into the marginal seas and have been highlighted for many years (Brookfield, 1998; Clift et al., 2004, 2006, 2008a,b; Liang et al., 2008). Besides the marginal seas, some sedimentary basins e.g. the Jianghan Basin (Fig. 1) also receive large amount of sediments transported by these large rivers. Thus they are considered as important linkage between the continents and the oceans (Clift et al., 2004; Wang, 2004; Zheng and Jia, 2009). Understanding the evolvement of these large rivers is important to understanding the process of orogeny. The uplift of the mountains and plateaus where these large rivers originate are proposed to profoundly affect the global climate system (An et al., 2001). Consequently the reconstructions of these rivers are important to understand the global change and its regional response (Wang, 2004).

⇑ Corresponding author at: Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China. E-mail addresses: [email protected], [email protected] (C. Li). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.09.018

The Yangtze River is one of the major rivers in the world (Chen et al., 2001). Originating from the Tibetan Plateau, the river flows eastward across the three major topographic levels. Its drainage covers more than one-fifth of the continental area of China before finally entering the East China Sea. Although study of the evolvement of the Yangtze River has a long history of more than 100 years, it is still controversial. Previous studies proposed that the Yangtze River can be dated back to the Cretaceous or the early Tertiary (Clark et al., 2004; Clift et al., 2006, 2008b; Jia et al., 2010; van Hoang et al., 2009), the early Quaternary (Fan et al., 2005; Kang et al., 2009; Yang et al., 2006; Zhang et al., 2008) or the late Pleistocene (Brookfield, 1998). Clark et al. (2004) proposed that the Yangtze River was once the tributary of the so called ‘‘paleo-Red River’’ draining into the South China Sea and reorganized by sequential river capture and reversal events. The Nd evolution in the Hanoi Basin showed that the middle Yangtze (downstream the First Bend) was once important source to the paleo-Red River and was lost from the paleo-Red River between 37 and 24 Ma (Clift et al., 2006). Nd, Pb data and U–Pb dating and Hf isotope analysis of zircons support the paleo-drainage pattern that the Songpan-Garze terrain once belonged to the paleo-Red drainage basin and was gone before 12 Ma or maybe much earlier (Clift et al.,

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Fig. 1. Location of the Jianghan Basin and the Zhoulao Borehole (modified after Yang et al. (2007a), Zhang et al. (2008)). (A) A sketch of Chinese tectonic units. (B) The Yangtze drainage basin with the location of the Sichuan Bain and Jianghan Basin. (C) Simplified geological map of the Jianghan Basin and the location of the Zhoulao Borehole. SC and JH denote the Sichuan Bain and Jianghan Basin respectively.

2006, 2008b; van Hoang et al., 2009). However no compelling evidence for the connection between the upper Yangtze (upstream the First Bend) and the paleo-Red River were found, but their data do not preclude this possibility (van Hoang et al., 2009). Recently many important researches based on the theory of ‘‘source to sink’’ have been carried out in the Yangtze delta to reconstruct the evolution history of the Yangtze River. These authors used the elemental compositions (Huang et al., 2009; Yang et al., 2006, 2007b), isotopic compositions (Yang et al., 2007b), and age patterns of monazite (Fan et al., 2005; Yang et al., 2006) and zircon (Jia et al., 2010) to trace the source of the late Cenozoic sediments in the Yangtze delta. Their researches provided direct evidence of evolution of the Yangtze River and showed that the wide Yangtze drainage basin similar as today’s dimension was formed mainly at the beginning of the Quaternary. Compared with the Yangtze Delta, the provenance study of the late Cenozoic sediments in the Jianghan Basin, another depocenter of the Yangtze sediment were not so integrated. The Nd isotope has been proved to be rarely influenced by weathering, sediment transport and deposition process (Goldstein et al., 1984). Thus the Nd isotopic compositions in sedimentary basin reflect the average composition of the source rocks that were providing materials at the sedimentation time. If a source with different isotopic character is gained or lost from the drainage then it will induce a measurable change in the Nd isotopic composition of the sediments carried by river (Clift et al., 2006). Based on this, the Nd isotope can be a reliable tool for tracing the source of sediments. Many ‘‘Source to Sink’’ researches have been carried out based on the Nd isotopic compositions (Clift et al., 2006; Singh et al., 2008; Yang et al., 2007a,b). Changes in eNd values have been well suited to defining sediment provenance in the Red River system e.g. the Nd isotopic change of sediments of the Hanoi Basin can

be well associated with some important drainage capture of within the paleo-Red River system (Clift et al., 2006). Yang et al. (2007a) reported the Nd isotopic composition of the suspended particulate and fine-grained floodplain sediments of the Yangtze River system, which showed that the Nd variations primarily reflect the control of source rocks such as the Himalayanian and Yesannian granite rocks and the Emeishan basalt in the upper valley and the sedimentary and low-grade metamorphic rocks in the middle and lower valleys. In this paper we report the Nd isotopic compositions of the late Cenozoic sediments of the Jianghan Basin for the first time. The main objective is to identify the provenance change of the late Cenozoic sediments of the Jianghan Basin. Based on that, we try to figure out the changing patterns of erosion of the Yangtze River system and provide more constraints on the evolution of the Yangtze River.

2. Yangtze River system 2.1. River setting The huge Yangtze drainage basin is more than 6300 km in length and has a catchment area of 1.8  106 km2. It is located between 24°270 –35°440 N and 90°330 –122°190 E. The Yangtze drainage basin spans the regional structure of China with three-grade relief and can be divided into three parts, the upper, middle and lower reaches. The upper Yangtze is from the source area to Yichang and has a total drainage area of about 100  104 km2. Within the upper Yangtze, the section from the source area to Yibin is usually called the Jinshajiang. The Jinshajiang is jointed by the Yalongjiang at Panzhihua. The river section from Yibin to Fengjie is the so called

L. Shao et al. / Journal of Asian Earth Sciences 45 (2012) 57–64

Chuanjiang, which is jointed by three major tributaries: the Minjiang, Jialingjiang and Wujiang. The famous Three Gorges reach is from Fengjie to Yichang which is composed of Qutang Xia, Wu Xia abd Xiling Xia (Xia means Gorge). The middle Yangtze is from Yichang to Hukou, jointed by the Dongting Lake drainage basin, Hanjiang and Poyang Lake drainage basin. Below the Hukou, several large interior lakes, such as the Chaohu Lake and Taihu Lake in association with many tributaries, drain into the lower Yangtze River. The Yangtze drainage basin consists of complex strata from Archean to Quaternary. The Emeishan Large Igneous Province is

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the typical basic source in the upper Yangtze especially in the Jinshajiang valley. Quaternary loose sediments and Paleozoic sedimentary rocks widely outcrop in the middle-lower reaches of the Yangtze River (Fig. 2. Detailed information seen Section 4.1). 2.2. Geological background of the Jianghan Basin The Jianghan Basin is a Mesozoic–Cenozoic basin developed along the southern margin of the Dabie Orogen. It is located between 29°260 –30°230 N and 111°300 –114°320 E, at the middle Yangtze River. The Yangtze River runs through the Jianghan Basin

Fig. 2. Regional geological map of the Yangtze River drainage basin (modified after Kang et al. (2009)). It showed the complicated compositions of lithology in the Yangtze drainage and the location of the largely distributed Emeishan basalts.

Fig. 3. Magnetic polarity and lithology of the late Cenozoic sediments in the Jianghan basin and the comparison of eNd(0) values between the Jianghan Basin and the Yangtze delta. The magnetic polarity data are from Zhang et al. (2008). Nd data of the Yangtze delta are from Yang et al. (2007b). The dashed lines a and b denote the average eNd(0) values of the modern upper Yangtze River (10.6, taken from the eNd(0) values reported by Yang et al. (2007b)) and the Jianghan Basin (9.6, this study). The dashed line c denotes the average eNd(0) values of the Yangtze Delta (11.2, Yang et al. (2007b)). Q4: Holocene strata; Q3: upper Pleistocene strata; Q2: middle Pleistocene strata; Q1: lower Pleistocene strata; N2: Pliocene strata.

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from west to east. Tectonic frameworks of the Jianghan Basin are controlled by two groups of tensional normal faults lying NNW and EW respectively. The basin has subsided ever since the late Cretaceous (Zhang, 1994). Such a geological controlled basin is favorable to siltation (Yin et al., 2007). It is an important catchment basin where a great amount of materials are deposited from the upper Yangtze. With continuous deposit for a long time especially ever since the late Cenozoic, the Jianghan Basin provides ideal materials for the reconstruction of the evolvement of the Yangtze River. 2.3. Late Cenozoic sediments in Core Zhoulao The continuously drilled core (Core Zhoulao) was taken from the Qianjiang Depression, Jianghan Basin. Zhang et al. (2008) reported the magnetic stratigraphic framework of Core Zhoulao which has been the best magnetic stratigraphic column of the Jianghan Basin so far. The Brunhes/Matuyama boundary (0.78 Ma) and Matuyama/Gauss (2.58 Ma) boundary were located at the depths of 82 m and 260 m respectively (Fig. 3). Besides several polarity subepoches were identified. The Jaramillo (0.99–1.07 Ma) and Olduvai (1.77–1.95 Ma) normal polarity subepoch were located at the depths of 90–100 m and 154.99–164.37 m respectively, while the Reunion normal polarity subepoch (2.14– 2.15 Ma) was located at the depth of 178.69–185.07 m. Based on the magnetic stratigraphy, the time framework of the Core Zhoulao can be established. The final drilling depth of the Core Zhoulao was 300.49 m, with the average recovery of 85%. The Quaternary strata of the Jianghan Basin primarily consist of fluvial facies interbedded with lacustrine facies and comprise several major fining-upward and coarseningupward sediment sequences (Zhang et al., 2008). 3. Sampling and methodology Thirty-five samples were collected from the borehole sediments. Fine-grained clastic sediments can best reflect the average composition of the large source area, so only the fine fraction (<0.058 mm) was selected for the determination of Nd isotopic composition. The <0.058 mm fine fraction was washed to remove salts and sieved from the bulk sediments using a 250-mesh sieve in deionized water and dried at room temperature in a clean oven. Then the dried samples were grounded into powder <200 mesh and prepared for the Nd isotope measurement.

The Nd isotope experiments were carried out at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences using a Finnigan Triton Thermal Ionization Mass Spectrometer (TIMS) according to the method presented by previous study (Ling et al., 2009). An aliquot of 0.1 g powders of each sample were measured and digested by Teflon bombs using mixed agents of HNO3 and HF acids at 190 °C for 72 h. Element Nd was separated and purified in a super clean laboratory using ion exchange columns of Dowex AG50WX12 cation resin and Eichrom Ln-Spec resin successively. Isotope ratios of 143 Nd/144Nd were normalized to 146Nd/144Nd = 0.721900. Measurement of standard La Jolla gives average value of 0.511847 ± 3 (2r external standard deviation, n = 25). The eNd(0) values were calculated using 143Nd/144Nd value of 0.512638 for the Chondritic Uniform Reservoir (Hamilton et al., 1983).

4. Results and discussions 4.1. 143Nd/144Nd and eNd(0) variations of the late Cenozoic sediments in the Jianghan Basin and the Nd isotopic compositions of the potential source rocks The Nd variations are listed in Table 1. The 143Nd/144Nd ratios of the <58 lm fractions range from 0.512042 to 0.512239, with an average of 0.512144. The corresponding eNd(0) values vary between 11.6 and 7.8, with an average of 9.6 (Table 1 and Fig. 3). The in-depth analysis of the Nd variations revealed that sediments from the Core Zhoulao below the depth of 280.0 m show the lowest eNd(0) values around 11.6 which is obviously lower than the average of the sediments from the Core Zhoulao and the average of the modern upper Yangtze mainstream (10.6 was taken to represent the average eNd(0) values of the upper Yangtze mainstream as reported by Yang et al. (2007a)). eNd(0) values of sediments between the depth of 280 m and 220 m are relatively stable and higher than the modern upper Yangtze. eNd(0) values of the sediments from the depth of 220 m upward till 82 m (the end of the early Pleistocene) are characterized by drastic fluctuations and several extremely positive values around 8.0. Ever since the mid-Pleistocene, sediments showed eNd(0) values around 10.6 which is close to the modern upper Yangtze mainstream, except one sample with eNd(0) value of 8.1. The Nd isotopic compositions can only be used as provenance indicator when the different Nd isotopic compositions between

Table 1 Nd isotopic compositions of the late Cenozoic sediments in the Jianghan Basin. Notes: eNd(0) = ((143Nd/144Nd)Measured/(143Nd/144Nd)CHUR  1)  104, is the normalized present-day isotopic composition. Depth (m)

Samples

143

Nd/144Nd

5.4 10.0 15.0 26.0 34.0 46.0 56.0 62.0 72.0 82.0 87.0 95.0 105.0 118.0 121.0 130.0 138.0 143.0

6-1 9-1-1-B 13-1 21-1 25-2 32-1 39-1-1 42-2-1 48-1 53-1-1 55-1-1 59-2-1 65-3-1 72-1-1 73-2-1 77-1 82-1-1 84-1-1

0.512102 0.512139 0.512179 0.512130 0.512125 0.512109 0.512223 0.512111 0.512099 0.512107 0.512178 0.512085 0.512165 0.512229 0.512123 0.512173 0.512171 0.512201

2r

eNd(0)

Depth (m)

Samples

143

Nd/144Nd

5 3 1 6 6 6 28 3 5 3 3 4 3 7 3 4 2 4

10.5 9.7 9.0 9.9 10.0 10.3 8.1 10.3 10.5 10.4 9.0 10.8 9.2 8.0 10.0 9.1 9.1 8.5

151.0 161.0 167.0 174.0 193.0 199.0 205.0 215.0 225.0 234.0 243.0 255.0 266.0 271.0 279.0 287.0 300.0

87-2-1 91-2-1 94-1-1 96-3-1 104-2-1 106-3-1 109-1-1 112-3-1 116-2-1 119-3-1 123-2-1 127-3-1 131-3-1 133-2-1 136-1-1 139-1-1 142-4-1

0.512223 0.512090 0.512175 0.512239 0.512112 0.512051 0.512157 0.512273 0.512160 0.512144 0.512155 0.512182 0.512180 0.512128 0.512152 0.512042 0.512044

2r

eNd(0)

3 5 3 4 9 25 4 23 5 24 4 4 6 5 8 8 11

8.1 10.7 9.0 7.8 10.3 11.4 9.4 7.1 9.3 9.6 9.4 8.9 8.9 9.9 9.5 11.6 11.6

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Fig. 4. Nd isotopic compositions of the Pliocene and Quaternary sediments in the Jianghan Basin, main tributaries of the Yangtze River and possible source terrains. The Nd data are from Yang et al. (2007a), Wu et al. (2010), Meng et al. (2008), Xiao et al. (2003), Zhang and Wang (2003), Wei et al. (2003), Zhu et al. (2003), Chen et al. (2006), Zhang et al. (2007) and Gao et al. (1999).

the potential sources exist. The source rocks in the Yangtze drainage basin are complicated including Archean metamorphic rocks, Paleozoic carbonate and sedimentary rocks, Mesozoic–Cenozoic igneous and clastic rocks, and Quaternary detrital sediments (Yang et al., 2007a). However according to the Nd isotopic compositions of these source rocks (Fig. 4), potential source of the sediments of the Jianghan Basin can be divided into three parts. 4.1.1. First, proximal area around the Jianghan Basin and the Three Gorges area Quaternary loose sediments widely outcrop in the Jianghan Basin and its surrounding area. Isotopic researches of these loose sediments have been rarely reported, except the Kongling Terrain which are associated with strongly negative eNd(0) values varying between 36.0 and 13.2 (Gao et al., 1999). Sediments from main tributaries of the middle Yangtze such as Qingjiang, Hanjiang, Yuanjiang and Xiangjiang showed negative eNd(0) values ranging from 15.3 to 11.0 (Meng et al., 2008; Yang et al., 2007a). 4.1.2. Second, source rocks in the Chuanjiang drainage basin The Chuanjiang rivers such as the Minjiang and Jialingjiang, drain the eastern flank of the plateau and are inferred to supply significant sediments to the Yangtze River (Changjiang Water Resource Commission, 2008). Detailed information has been extracted by the Ar–Ar muscovite data (van Hoang et al., 2010) and the 10Be budget (Chappell et al., 2006) that the Longmenshan area is the most important sediment supplier to the Chuanjiang River sediments. The Longmenshan range constitutes the eastern border of the Tibetan Plateau. It is characterized by a steep topographic transition from the Sichuan Basin to the plateau. Triassic flysh and Neoproterozoic-Permian passive margin sediments widely outcrop. Previous studies showed that these materials are associated with eNd(0) values ranging from 16.5 to 9.1, with an average of 12.7 (Chen et al., 2006; Zhang et al., 2007). Furthermore, Permian basalts with high eNd(0) values (Zi et al., 2008) can also be the possible basic sources of sediments. 4.1.3. Third, the Jinshajiang drainage basin Most source rocks in this area are characterized by high (less negative) eNd(0) values. The Emeishan large igneous province widely distributes at the west margin of the Yangtze Craton. It occupies an area over 250,000 km2 and is one of the most important igneous provinces in the world (Xiao et al., 2004). The Emei-

shan basalts are characterized by the strongly positive eNd(0) values varying from 9.0 to 3.3 with an average of 2.5 (Xiao et al., 2003,2004; Zhang and Wang, 2003). As pointed by Yang et al. (2007a), it is suffered strongly chemical weathering under the influence of humid and warm climate and thus should have much influence on the Nd isotopic composition of the river sediments. Besides, the Permian ophiolites and volcanic rocks are abundant in the Jinshajiang area. These rocks are associated with high eNd(0) values from 1.8 to +6.9 (Wei et al., 2003), erosion from these rocks can also make important contribution to the Nd isotopic composition of the downstream (Wu et al., 2010). Wu et al. (2010) reported three samples of Jinshajiang showed that they had high eNd(0) values ranging from 9.4 to 8.5. Generally the eNd(0) values of the source rocks become less negative (higher) from the proximal area of the Jianghan Basin to the Jinshajiang area (Fig. 4). This is confirmed by the fact that the eNd(0) values of the middle Yangtze River sediments are obviously lower than those of the upper reaches (Yang et al., 2007a). Nd isotope has a long history of application for provenance discrimination. The difference of Nd isotopic compositions of the end members and the successful application of Nd isotopes to constraining the provenance of sediments of the Yangtze River (Yang et al., 2007a,b) suggest that this method is appropriate for constraining sources of sediments in the Jianghan Basin. Here we synthesize our data with the Nd values of the late Cenozoic sediments of the Yangtze delta reported by Yang et al. (2007b) to generate a complete image of Nd isotopic variation in the Jianghan Basin and decipher some useful information on the changing patterns of erosion and the understanding of the evolution of the Yangtze River. 4.2. Provenance discrimination and changing patterns of erosion Studies of magnetism parameters and heavy mineral characteristics revealed that sediments in the Jianghan Basin were derived from the proximal area before the Yangtze River was formed (Kang et al., 2009; Zhang et al., 2008). After the Yangtze drain pattern became as large as its modern dimension, large amount of debris were eroded from the upper basin and deposited at the Jianghan Basin. Sediment flux analysis reveals that most of the sediment of the modern Yangtze River is derived from the upper basin (Chen et al., 2001). As mentioned above, these sediments from distal sources could dominate Nd isotopic compositions of the Jianghan Basin. Similar phenomena also exist at the Yangtze delta.

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Significant and large variations of Nd ratio not only occur at the depth of 280 m (close to the boundary of Pliocene and Quaternary strata), but also within the lower Pleistocene sediments and between the middle and upper Pleistocene sequences. In comparison, the upper Pleistocene sediments and Holocene sediments have relatively small variations in Nd ratio. Because the magnetic polarity cannot provide accurate age, we consider that the sedimentary age of sediments at the depth of 280 m was no later than the beginning of the Quaternary. Combined with the observation of the sediment lithology, it shows that the significant Nd isotopic variations could not be well correlated with particular sediment lithology or its variation. Even among sediments with same characters, significant Nd isotopic variations still exist. Thus the possibility that these Nd isotopic variations were caused by changing sediment characters could be excluded. Nd isotopic compositions of sediments have been proved to be rarely influenced by depositional process (Goldstein et al., 1984). So changing sediment provenance should be the only factor that could induce the Nd isotopic variations of the sediments in the Jianghan Basin. Sediments at the bottom (280 m below) of the core show much more negative values than the overlying sediments, which should be indicative of more input from older, radiogenic continental crust. As discussed above, proximal area of the Jianghan Basin and the tributaries in the middle Yangtze River are both associated with strongly negative eNd(0) values. It seems that these source rocks should made important contributions to the Jianghan Basin. However source rocks in the Chuanjiang drainage area are also capable of supplying sediments with similar Nd isotopic compositions (Fig. 4). This makes Nd a less powerful provenance tool for these sediments. It cannot provide compelling evidence to prove whether these sediments in the Jianghan Basin were influenced by the source rocks in the Jinshajiang area characterized by extremely high (positive) eNd(0) values. In contrast, eNd(0) values of sediments above the depth of 280 m are much more positive, which should be associated with more input from primitive crust. As mentioned above, the first part (280– 220 m) shows stable isotopic values around 9.5 which indicate a stable provenance. The eNd(0) values are obviously higher than the source rocks known from the Chuanjiang and middle Yangtze drainage basin and indicate significant input from sources with eNd(0) values higher than 9.5. That means the source rocks distributed in the Jinshajiang drainage are expected to be an important contributor to the sediment flux. From the early Pleistocene to the beginning of the mid-Pleistocene, sediments in the Jianghan Basin received more inputs of primitive crust as revealed by the higher eNd(0) values. It could be associated with the mantle-derived upper Permian–Triassic Emeishan basalt. The Emeishan LIP covers a large area over 250,000 km2 in the upper Yangtze valley. It could be an important supplier of sediments to the Yangtze River and thus can have much influence on the Nd isotopic compositions (Yang et al., 2007a). Besides, the Cenozoic volcanic rocks are also exposed in the Jinshajiang area. These rocks are also characterized by high eNd(0) values and thus could possibly make contributions to the Yangtze River sediments (Zhu et al., 2003). Furthermore, during this period the Nd variations were fluctuant and indicate that provenance of the sediments were not stable. Ever since the mid-Pleistocene, provenance of the sediments have stayed stable and been similar to the modern Yangtze River especially the upper Yangtze. Heavy mineral research of the Jianghan Basin sediments revealed that the sediments after 1.1 Ma show stable heavy mineral compositions were similar as the modern Yangtze River (Kang et al., 2009). Besides, magnetism parameters characters of these sediments were also similar to the modern Yangtze River (Zhang et al., 2008). These researches argue that the provenance of the sediments in the Jianghan Basin have been the same as the modern upper Yangtze River since 1.1 Ma, which are similar to our research. However,

those data could not show the provenance change before 1.1 Ma. It suggests that the Nd isotopic compositions could provide much more informations for the provenance of the sediments in the Jianghan Basin. Generally, source area of sediments of the Jianghan Basin might become as large as the modern ones no later than the beginning of the Quaternary. However, patterns of erosion were not totally the same during different stages of the Quaternary. Nd isotopic variations of the sediments in the Yangtze Delta also revealed that erosion patterns of the Yangtze River changed during different stages of the Quaternary (See Section 4.4). According to the division of the source area, sediments in the Jianghan Basin mainly derive from three end-members. The contributions from the surrounding area of the Jianghan Basin should significantly decrease after the Yangtze River was finally formed. It can be supported by the fact that most of the sediment of the modern Yangtze River is derived from the upper basin (Chen et al., 2001). Based on that, we can make reasonable assumption that the change of erosion rates in the Chuanjiang drainage and Jinshajiang drainage would significantly induce the Nd variations of the sediments in the Jianghan Basin. The patterns of erosion may be correlated with climate-tectonic coupling. Previous studies revealed that erosion rates are governed by crustal deformation, climate or both of them (Beaumont et al., 2001; Burbank et al., 2003; Clift, 2010; Zhang et al., 2001). The southeastern part of the Tibetan Plateau experienced rapid deformation during the late Cenozoic (Wang and Burchfiel, 2000). Besides, topography falls rapidly on the edges of the Tibetan Plateau, fast erosion could be expected in this area during the Cenozoic. Previous studies showed that high exhumation rates at the Longmen Shan area began at the end of the Cenozoic (Arne et al., 1997; Clark et al., 2005; Godard et al., 2009; Kirby et al., 2002), while the comparison of zircon and apatite ages indicated that the eastern part of the Longmen Shan range might have experienced a significant decrease in exhumation since 2– 3 Ma (Godard et al., 2009). Thus decreasing input from the Longmenshan area could be expected and would indirectly induce the increasing proportion of inputs from the Jinshajiang area. In that case, Nd values of the Yangtze sediments would become more positive. The relatively unstable Nd variations revealed the unstable patterns of erosion in the upper Yangtze drainage basin during the early Pleistocene. Ever since the mid-Pleistocene, the stable Nd variations of the Jianghan Basin indicated stable pattern of erosion in the source area. In particular, the middle Pleistocene–Holocene sediments have more negative Nd ratios than the lower Pleistocene sediments. We argue that it was due to the increasing input from the Longmenshan area. Ar–Ar muscovite data (van Hoang et al., 2010) and the 10 Be budget (Chappell et al., 2006) reveal that the Longmenshan area is the most important sediment supplier to the modern Yangtze River sediments. As shown by the Nd isotopic variations of the source rocks, source rocks in the Longmenshan area show more negative Nd ratios than the Jinshajiangjiang basin, thus, the increasing input from the this area could lead the Nd ratios of the sediments to become more negative. 4.3. Comparison with the Yangtze Delta Yang et al. (2007b) reported the Nd isotopic variations of the late Cenozoic sediments in the Yangtze Delta (Fig. 3), eNd(0) values of which vary between 12.9 and 7.8, with an average of 11.2. eNd(0) values of the late Cenozoic sediments in the Jianghan Basin are relatively higher than the Yangtze Delta, indicating that the source rocks with low eNd(0) values in the middle and lower Yangtze did exert a significant influence on the Yangtze delta. Based on the 10Be data, Zheng et al. (2005) assumed that sediments from the east Yangtze could account for 17–40% of the sediments in the Yan-

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gtze Delta. The Nd variations in the Yangtze Delta seem to be more complicated than those in the Jianghan Basin, especially since the mid-Pleistocene. As revealed by Nd variations in the Jianghan Basin, patterns of erosion in the upper Yangtze were stable ever since the mid-Pleistocene. Meanwhile Nd variations in the Yangtze delta were characterized by several periodic changes (Yang et al., 2007b). It indicates that provenance of the sediments in the Yangtze Delta changed periodically. During this period, the different Nd variations between the Jianghan Basin and the Yangtze Delta revealed that the provenance change of the Yangtze Delta actually indicated the changing patterns of erosion in the middle and lower Yangtze. Patterns of erosion in the Yangtze River drainage basin were different during different stages of the Quaternary. Even at the same time, they were still different in different reaches. However, sediment sources of both the Jianghan Basin and the Yangtze Delta have been stable ever since the late Pleistocene. It indicates that patterns of erosion in the whole Yangtze reaches have been stable since then. 4.4. Implications for the evolution of the Yangtze River The Nd data cannot provide compelling evidence for the provenance of the Pliocene sediments. We cannot confirm whether these sediments in the Jianghan Basin were influenced by the source rocks in the Jinshajiang area characterized by extremely high (positive) eNd(0) values. As the result, we do not know that whether the sharp changes in the Nd isotope compositions at the depth of 280 m was caused by river geometry or the changing erosion patterns. However the Nd isotope compositions do suggest that the Quaternary sediments were considerably influenced by distal source rocks with high eNd(0) values, including the Emeishan basalts (Fig. 4). Our borehole does not go deep enough, so it cannot provide more details on the river evolution history before, i.e. the relationship between the paleo-Yangtze and the paleo-Red. So we conclude that the Yangtze drainage basin extended to the upper valley including the Emeishan LIP and developed into a large river as the same dimension as the modern Yangtze River no later than the beginning of the Quaternary. What needed to be declared is that we do not try to prove a young birth to the Yangtze River, we just conclude that the birth of the Yangtze River should predate the beginning of the Quaternary. Provenance studies of the late Cenozoic sediments in the Yangtze Delta suggest that the Yangtze became a large river extending to the eastern Tibetan Plateau mainly at the beginning of the Quaternary (Fan et al., 2005; Huang et al., 2009; Jia et al., 2010; Yang et al., 2006). Those researches provided direct evidence of evolution history of the Yangtze River and did not contradict with our research results. 5. Conclusions Nd isotopic compositions of the sediments from Core Zhoulao in the Jianghan Basin, middle Yangtze River, have been used as proxies to trace sediment source and provide some useful constraints on the evolution history of the Yangtze River. Generally the Pliocene sediments have lower eNd(0) values compared to the Quaternary sediments, suggesting a different provenance. What can be confirmed is that the Quaternary sediments were considerably influenced by distal source rocks with high eNd(0) values, including the Emeishan basalts. In contrast, The Nd data cannot provide compelling evidence for the provenance of the Pliocene sediments. During the Quaternary, sources of the sediments did not stay stable. We conclude that the different sediment origins during the Quaternary indicated different patterns of erosion in the upper Yangtze River. More positive eNd(0) values reflect preferential erosion of source rocks in the Jinshajiang drainage. The source rocks

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