Tracing Sr isotopic composition in space and time across the Yangtze River basin

Chemical Geology 388 (2014) 59–70

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Tracing Sr isotopic composition in space and time across the Yangtze River basin Chao Luo a, Hongbo Zheng b,⁎, Ryuji Tada c, Weihua Wu a, Tomohisa Irino d, Shouye Yang e, Keita Saito c a

School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China School of Geography, Nanjing Normal University, Nanjing 210000, China Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113–0033, Japan d Graduate School of Environmental Science, Hokkaido University, N10W5 Sapporo, Hokkaido 060–0810, Japan e State Key Laboratory of Marine Geology, Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, Shanghai 200092, China b c

a r t i c l e

i n f o

Article history: Received 31 May 2014 Received in revised form 3 September 2014 Accepted 5 September 2014 Available online 16 September 2014 Editor: David R. Hilton Keywords: Yangtze River Temporal variation 87 Sr/86Sr Provenance

a b s t r a c t Variation of dissolved 87Sr/86Sr in the Yangtze River is poorly documented compared to other Tibetan-sourced rivers. Here, we trace the Sr isotopic composition in space and time across the Yangtze River basin using a systematic sampling strategy. The 87Sr/86Sr values of samples collected at different depths within the water column at three gauge stations located in the upper, middle and lower reaches of the river are very similar, indicating a wellmixed water body at a given location. Data from basin wide-samples shows low 87Sr/86Sr values in the upper reaches while higher 87Sr/86Sr values in the middle-lower reaches which mainly reflect the controls of source rocks. A time series record at the lowermost reaches of the river indicates a temporal variation of 87Sr/86Sr values from 0.7101 to 0.7109, with a discharge weighted value of 0.7106. We suggest that temporal variations of the isotopic composition result from changes in the relative contributions from different terrains associated with spatial variability of rainfall within the basin. A mixing model based on Sr values deduced from basin-wide sampling from different seasons yields similar temporal variations of Sr isotopic composition compared to experimental data. Calculation of Sr fluxes indicates that the upper reaches contribute 3 times as much as the middle and lower reaches to total Sr flux. The annual Sr flux of the Yangtze River to the East China Sea is estimated to be 1.9 × 109 mol · a−1, which is one magnitude higher than that of the Brahmaputra and Ganges. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Global cooling during the Cenozoic Era is hypothesized to have been caused by uplift and weathering of youngorogenes, especially the Himalaya and Tibetan Plateau (Raymo et al., 1988; Raymo and Ruddiman, 1992). Riverine Sr has been widely used as an effective proxy to tracer continental chemical weathering in major drainage basins, and to elucidate the provenance of the dissolved load. There have been a large number of studies on the dissolved Sr isotopic composition of Tibetan rivers during the past decades (Bickle et al., 2003, 2005; Dalai et al., 2003; Galy et al., 1999; Singh and Kumar, 2003; Singh et al., 2006; Wu et al., 2005). However, samples used in most of the previous studies were mainly collected mainly fromthe surface of river channels during a single season. Rivers in the monsoonal region exhibit significant temporal, vertical and lateral variability, with annual discharge varying significantly and seasonal variation up to an order of magnitude higher between monsson season and dry season. Consequently sampling at a single time slice of any season would lead to errors in estimates of average

⁎ Corresponding author. Tel.: +86 25 85891673. E-mail address: [email protected] (H. Zheng).

http://dx.doi.org/10.1016/j.chemgeo.2014.09.007 0009-2541/© 2014 Elsevier B.V. All rights reserved.

values and total fluxes of 87Sr/86Sr. Time-series observations of several Tibetan rivers record a seasonal variation in dissolved 87Sr/86Sr, with lower 87Sr/86Sr ratios occurring during the monsoon as a result of enhanced carbonate weathering (Tipper et al., 2006; Rai and Singh, 2007; Tripathy and Singh, 2010). However, unlike rivers from the Himalaya, Wei et al. (2013) found higher 87Sr/86Sr values during high discharge season in the Xijiang River in south China, indicating enhanced silicate weathering during rainy seasons. In the absence of accurate spatial and temporal characterization of the river basin, it is not possible to judge what causes the discrepancy. Much recently, Voss et al. (2014) observed the spatial and temporal variations in the Fraser River, British Columbia, attributed the temporal variation of Sr isotopes to varying contributions of different sources. In addition, Sr isotopic composition of suspended matter collected at varying water depths of a river profile also shows that depth variations can be as large as spatial and seasonal variations (Bouchez et al., 2011; Luo et al., 2012). Therefore, in order to characterize the Sr isotopic composition and estimate Sr fluxes of a river comprehensively, a systematic sampling strategy is a prerequisite. Compared with the Ganges, Brahmaputra and Indus rivers, the Yangtze River is much less investigated in terms of its Sr isotopic composition. Limitied studies of dissolved Sr isotopes in the Yangtze Tiver either focused only on a certain time slice of the year, or on specific sub-basin with certain rock

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C. Luo et al. / Chemical Geology 388 (2014) 59–70

area of about 100 × 104 km2 which is characterized by metamorphic sandstone and shale, carbonate rocks and igneous rocks,including the Emeishan Basalt (Fig. 2). The upper reaches of the main stream are joined by five major tributaries, the Yalong Jiang, Dadu He, Min Jiang, Jialing Jiang and Wu Jiang (WJ). Wu Jiang is the main tributary of the upper reaches of the Yangtze River with more than 90% of its catchment consisting of carbonate rocks. The middle reaches are dominated by the Dongting Lake and Poyang Lake drainage basins in the south and by the Han Jiang basin in the north. In the lower section of the river, from Hukou downstream, there are no large tributaries. The middle-lower basins are dominated by Paleozoic marine and Quaternary fluviolacustrine sedimentary rocks, together with intermediate-felsic igneous rocks, and older metamorphic rocks. Gan Jiang (GJ) is one of the branches in the middle reaches of the Yangtze River located mainly within Jiangxi province. The majority of the watershed drains Mesozoic granitoids with minor outcrops of carbonate rocks (Shen et al., 1999; Yang et al., 2004). Generally speaking, the upper reaches are dominated by carbonate and evaporitic rocks while the middle-lower reaches consist mainly of silicate rocks. Meteorologically, the Yangtze River catchment is strongly influenced by a subtropical monsoon climate, which causes large seasonal and spatial variability in temperature, precipitation, and runoff. The summer monsoon normally starts to influence the river basin in April and retreats in October; it accounts for more than 70% of the annual total rainfall (CWRC, 2002). The rainy season, when water discharge is high, is typically from June to September, whereas the dry season, when water discharge is low, is from December to March. Generally, the rainy season in the Yangtze River basin can be divided into several stages. The rainy season starts around April over the Dongting Lake and Poyang Lake drainage basin. Subsequently the rain band moves to the main stream of the middle and lower reaches during mid-June to

type such as silicate or carbonate (Wang et al., 2007; Chetelat et al., 2008; Jiang and Ji, 2011). Sr composition of suspended sediments collected at a water gauge located in the lower reaches of the river suggested large depth-variation (Luo et al., 2012). It is unknown whether similar variation exist in dissolved Sr,In this study, we present the results of the first comprehensive investigation of the Sr isotopic composition of the dissolved load in the Yangtze River based on a systematic sampling strategy. Our aims are 1) to determine whether vertical stratification of Sr isotope of dissolved load exists in the water profile; 2) to estimatethe Sr budget of the mainstream and major tributaries, and to calculated the annual Sr flux to the sea, and 3) to better understand the mechanism which controls temporal and spatial variations of dissolved 87Sr/86Sr. 2. Regional setting The Yangtze River extends over 6300 km from its headwaters in the northeast Tibetan Plateau to the East China Sea, covering about 1.8 × 106 km2 drainage area. Conventionally the river is divided by Yichang and Hukou (the outlet of Poyang Lake) into three sections, the upper reaches, middle reaches and lower reaches (Fig. 1). Zhutuo, Yichang and Datong hydrological stations are three well known stations located in the upper, middle and lower reaches of the Yangtze River, respectively. The upper reaches can be further divided into Jinsha Jiang and Chuan Jiang. The Jinsha Jiang descends from the plateau and flows through deep valleys until it reaches Yibin. The section between Yibin and Yichang is called Chuan Jiang. The Three Gorges Dam (TGD) is located within the Chuan Jiang section. The storage capacity of the Three Gorges Reservoir (TGR) was 39 km3, about 4.5% of the Yangtze's annual discharge (Xu and Milliman, 2009). Strata from Archean to Quaternary age are distributed in the Yangtze drainage basin (Shao et al., 2012). The upper Yangtze has a total drainage 96°

104°

100°

112°

108°

116°

(A) Upper Reaches

Middle Reaches

Lower Reaches 34°

Nanj ing

N 1

2

TGD

3

5 4

12 Yi c h

2

3

Wuh an

Dato ng

Huko u

ang

34°

14 Ch

Sampling-summer Sampling-winter

Yibin

Shig u

Time series

on

gq 1 ing Zhut uo 6

Depth sampling

11

7 8 9

13 10

34°

City

Sampling Tributaries and Hrdrological stations

Depth( m)

0

S

(B)

10

M 20

B

1-Yalong Jiang

7-Li Shui

13- Gan Jiang

2-Dadu He

8-Yuan Jiang

14-Poyang Lake

3-Min Jiang

9-Zi Shui

4-Tuo Jiang

10-Xiang Jiang

5-Jialing Jiang

11-Dongting Lake

6-Wu Jiang

12- Han Jiang

30 0

500

1000

1500

2000

1

Zhutuo (ZT)

2

Yichang (YC)

3

Datong (DT)

Distance from left Bank (m)

Fig. 1. (A) Locations of water samples in the Yangtze River drainage basin. Time-series sampling site is marked by a star. (B) Depth sampling profile at hydrological station.

C. Luo et al. / Chemical Geology 388 (2014) 59–70

96

100

108

104

61

112

116 N

Upper Reach

Middle Reach

Lower Reach

34

Tibetan Plateau

Yichang Sedimentary rocks Quaternary sediments

Hukou

30

Chongqing Yibin

Carbonate rocks

Slate, phyllite

Granitoids

Gneiss, schist, marble

Basalt

Basic and ultra-basic rocks

26

Fig. 2. Geological map of the Yangtze River basin. Modified from Yang et al. (2009).

July. From mid-July to August, the Sichuan basin and Han Jiang drainage basin experiences heavy rainfall. In September, the rain belt moves from western Sichuan to the northeast and the upper reaches of Han Jiang (The Changjiang Water Resources Commission. See http://www.cjw. gov.cn). 3. Methods 3.1. Sampling strategy In the present study, a comprehensive and systematic sampling program was conducted over the entire Yangtze River basin. Three sets of samples were collected between June 2010 and February 2012 (Fig. 1). A time series of samples was collected bi-weekly at the Datong Hydrological Station from June 2010 to June 2011. Hydrological information from this station can be regarded as representative of the entire river catchment since there are no large tributaries downstream and from Datong station upwards, the Yangtze River is no longer affected by tidal fluctuations. Approximately 1 liter of water was taken from a depth of 50% of the river depth at the time of sampling by submerging a point-sampler down to the desired depth. The water depth at Datong station varies on an annual basis, ranging from 23–28 m during the peak flow period and from 19–22 m during the dry season. A total of 21 samples were collected over one year providing a previously unavailable insight into the temporal variation in Sr-isotopic properties of the dissolved load of the Yangtze River. The second set of samples was collected during two basin-wide sampling campaigns which took place in the summer of 2011 and the winter of 2012. The sampling campaign in 2011 consisted of three separate field trips in July, August and September while the sampling campaign in 2012 was conducted continuously during February. The basin-wide sampling was carried out along the mainstream and at the confluences of the major tributaries (Fig. 1A). Water samples were collected from the river bank or from the midstream away from towns in order to avoid local anthropogenic contamination. In order to maximize the temporal aspect of the study, basin-wide samples were collected from almost the same locations during summer and winter. A total of 35 and 32 water samples were

collected during summer and winter, respectively. In order to assess the possible variation of Sr-isotopic composition with depth, depth series of samples were obtained from Zhutuo, Yichang and Datong hydrological station during the basin-wide surveys. All three stations are regarded as representative gauging stations for the upper, middle and lower reaches, respectively. At Zhutuo station, samples were collected during both summer and winter; at Yichang station they were only taken during summer and at Datong station they were taken during winter. At each station, samples were taken from midstream at the surface (S), middle (M) and bottom (B) of the depth-profile by submerging a point-sampler down to the desired water depth (Fig. 1B). Temperature, electrical conductivity (EC) and pH were measured in situ with a portable probe and meter. The water samples were stored in pre-cleaned high-density polyethylene (HDPE) bottles and filtered through 0.45 μm Millipore filters. Filtered water was acidified to pH b 2 with quartz-distilled HNO3. All samples were refrigerated until analysis. All of the discharge data was obtained on line from the Ministry of Water Resources of the People's Republic of China (http://xxfb. hydroinfo.gov.cn/ssIndex.html).

3.2. Laboratory analyses Sr was analyzed in filtered and acidified water by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a precision better than 5 % at the Center of Modern Analysis, Nanjing University. The detection level for Sr is 10 ppb and blanks were beneath the detection limits of ICP-AES. For Sr isotope analysis, about 150 ml of water sample were evaporated to dryness in clean Teflon vessels and re-dissolved with 1 ml 3 M HNO3. Then Sr was separated from other ions using standard ion exchange techniques. 87Sr/86Sr was determined using a Finnigan Triton thermal ionization mass spectrometer at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Reproducibility and accuracy of Sr isotope analyses were checked periodically by running the Standard Reference Material NBS 987. The certified value of 87 Sr/ 86 Sr ratio for the NBS 987 strontium standard was 0.710250; the average measured value was 0.710231 (2σ external standard

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C. Luo et al. / Chemical Geology 388 (2014) 59–70

deviation = 0.000004, n = 50). The measured 87Sr/86Sr ratio was normalized to 87Sr/86Sr = 0.1194. The analytical blank was b 1 ng for Sr. 4. Results 4.1. Sr isotopic composition versus water depth The water depth at Zhutuo is around 15 m and 11 m during summer and winter respectively; and around 15 m at Datong during winter and 26.5 m at Yichang during summer (Tables 1 and 2). The depth variations of pH and electrical conductivity (EC) among different depths are around 0.19 and 42, respectively, which are small enough to be ignored. Sr concentrations and Sr isotopic compositions of the water samples from the depth profile are presented in Table 1 and Fig. 3. Among those samples collected during summer, 87Sr/86Sr ratios of the surface, middle and bottom water at Zhutuo are 0.710542, 0.710567 and 0.710555, respectively. 87Sr/86Sr ratio of surface water is 0.00013 lower than that of the bottom water. At the Yichang station, depth variation of dissolved Sr isotopes is around 0.00008 with lower values in the surface water. With regard to the winter sample, 87Sr/86Sr values range from 0.710504 to 0.710516- at Zhutuo with the lowest value in the bottom water and from 0.71065 to 0.71069 at Datongwith the smallest value in the water from the middle of the river depth. The largest variation of 87Sr/86Sr ratio (around 8 × 10−5) occurs within samples collected at Yichang station during summer. 4.2. Spatial variation in Sr concentration and 87Sr/86Sr ratio The dissolved Sr concentrations and 87Sr/86Sr ratios of the basin-wide samples vary significantly (Table 1 and Fig. 4). In the summer samples,

the concentration of dissolved Sr in the mainstream water decreases dramatically from 8.03 to 3.10 μmol/L within the Jinshajiang segment (from the headwaters to Yibin), increases slowly towards the three Gorges Dam (TGD), and then decreases downstream of Yichang. The Sr concentrations of the tributaries in the upper reaches are systematically higher than those of the tributaries in the middle-lower reaches. The variation of Sr concentration in the samples collected during winter in the downstream transect exhibit the same trend as the samples collected in summer. However, due to monsoon dilution, the Sr concentrations of samples collected in winter (0.72-5.91 μmol/L) are higher than those collected in summer (0.75-4.14 μmol/L, excluding the data from above Panzhihua). The water samples collected during summer have slightly higher 87 Sr/86Sr ratios compared to samples collected during winter, with the exception of the Gan Jiang which has higher values in winter. Low 87 Sr/86Sr values occur in the upper reaches, especially in the TGD area, while the middle and lower reaches exhibit higher 87Sr/86Sr values. The Sr isotopic compositions exhibit a large range of variation within the entire basin, with a minimum value for Wu Jiang (0.708509) and a maximum value for Gan Jiang (0.715456). 87Sr/86Sr ratios in the Yangtze River water range from 0.708834 to 0.713286 in summer and from 0.708509 to 0.715456 in winter. 4.3. Temporal variations in Sr concentration and 87Sr/86Sr ratio at Datong The 87Sr/86Sr ratios and the Sr concentrations, together with water temperature and river discharge. at Datong station of the Yangtze River are given in Table 3 and Fig. 5. The discharge at Datong varies significantly, from 12850 m3/s in winter to 62850 m3/s during the peak flow period, while the water temperature varies from 7.25 °C in

Table 1 Sr isotopic composition of water samples from the Yangtze River in summer 2011 (1–22 July, 24–28 August and 8–11 September). Sample No.

River location

Date

Latitude

Longitude

Tributaries YLJ DDH MJ TJ JLJ WJ YJ XJ DTH HS GJ PYH

Yalongjiang, Panzhihua Daduhe, Leshan Minjiang, Leshan Tuojiang, Luzhou Jialingjiang, Chongqing Wujiang, Chongqing Yuanjiang, Changde Xiangjiang, Changsha Dongting Lake, Yueyang Hanshui, Wuhan Ganjiang, Nanchang Poyang Lake

8-Sep 1-Jul 30-Jun 1-Jul 3-Jul 3-Jul 26-Aug 26-Aug 24-Aug 8-Jul 27-Aug 28-Aug

26.7128° 29.5783° 29.7933° 28.9128° 29.6764° 29.7469° 28.9680° 28.1628° 29.4333° 30.6964° 28.5684° 29.8078°

101.9253° 103.6967° 103.8183° 105.4844° 106.5522° 107.48819° 111.7066° 113.0697° 113.1175° 114.0842° 115.8227° 116.0422°

Mainstream JSJ-GLS JSJ-SG JSJ-PZH-Q JSJ-PZH-H JSJ110701 YZ110701 YZ-ZT-S* YZ-ZT-M* YZ-ZT-B* YZ110703 YZ-ZX-04 YZ-BD-05 YZ-TPX-06 YZ-YC-S* YZ-YC-M* YZ-YC-B* YZ-YY-Q YZ-YY-H YZ-YL-08 YZ-JJ-Q YZ-JJ-H YZ-NJ-09 YZ-SH-10

Jinshajiang, Benzilan Jinshajiang, Shigu Jinshajiang, Panzhihua Jinshajiang, Panzhihua Jinsshajiang, Yibin Yangtze,Yibin Yangtze, Zhutuo Yangtze, Zhutuo Yangtze, Zhutuo Yangtze, Chongqing Yangtze, Zhongxian Yangtze, Badong Yangtze, Taipingxi Yangtze, Yichang Yangtze, Yichang Yangtze, Yichang Yangtze, Yueyang Yangtze, Yueyang Yangtze, Yangluo Yangtze, Jiujiang Yangtze, Jiujiang Yangtze, Nanjing Yangtze, Shanghai

11-Sep 10-Sep 9-Sep 8-Sep 1-Jul 1-Jul 2-Jul 2-Jul 2-Jul 3-Jul 5-Jul 6-Jul 6-Jul 7-Jul 7-Jul 7-Jul 25-Aug 25-Aug 8-Jul 27-Aug 28-Aug 9-Jul 22-Jul

28.2089° 26.9528° 26.6806° 26.6067° 28.74° 28.8708° 29.0115° 29.0115° 29.0115° 29.6042° 30.3517° 31.1330° 30.74° 30.74° 30.74° 29.5542° 29.6258° 30.6524° 28.576083° 29.7914° 32.0061° -

99.405° 100.0786° 101.8664° 101.8427° 104.7008° 104.9417° 105.8529° 105.8529° 105.8529° 106.6347° 108.1764° 110.3983° 111.3814° 111.3814° 111.3814° 112.9298° 113.2533° 114.5286° 115.8164° 116.3747° 118.701° -

H/depth (m) 967 382 355 226 162 143 40 23 64 32 14

2011 1823 983 983 255 258 1 7.5 14 171 257 1 13 25.5 25 47 41 11 20 12 -

T (°C)

pH

25 21 26.1 29.66 29.5 22 28.1 28.1 27 27.9 22

8.3 8.2 7.2 7.68 8.14 8.4 8.18 7.92 8.06 8.12 7.95 7.7

87.4 227 355 534 337 318 292 329 327 392 214 53

16.7 18 20 24 24.2 24.8 25 24.8 25.3 26.3 -

8.27 8.43 8.64 8.22 8.49 8.33 8.2 8.37 8.37 8.15 8.10 8.02 8.37 8.34 8.33 8.07 8.08 8.26 7.7 7.75 -

1133 785 677 196 403 350 354 352 346 330 350 352 351 335 344 402 416 357 97 83.8 -

24.3 25.4 27 22 -

EC

Discharge (m3/s)

Sr (μmol/L)

87

Sr/86Sr

2σ

970 2250 3250 442 1790 1290 4320 2540 8170 3300 3000 3480

2.05 2.58 1.29 4.09 3.02 3.76 1.07 0.92 1.70 2.31 0.98 0.75

0.710905 0.711774 0.711694 0.710996 0.710952 0.708834 0.711561 0.712405 0.710876 0.711349 0.713286 0.713901

7 4 6 6 8 7 4 3 6 4 3 4

1810 2270 3280 5470 8800 10600 10600 10600 11900 12300 19800 19800 19800 13500 21000 30200 21800 25200 40500

8.03 6.38 5.02 4.31 3.85 3.10 3.45 3.40 3.25 3.22 3.27 3.63 4.05 3.92 4.08 3.89 3.20 2.41 2.97 2.50 1.48 2.57 2.82

0.710132 0.710290 0.710144 0.710305 0.710449 0.710646 0.710542 0.710567 0.710555 0.710678 0.710447 0.710024 0.709983 0.709985 0.710050 0.710067 0.710228 0.710511 0.710261 0.710342 0.711647 0.710383 0.710347

5 3 5 5 4 4 7 4 5 5 4 8 4 14 6 7 4 3 3 2 5 3 -

-

C. Luo et al. / Chemical Geology 388 (2014) 59–70

63

Table 2 Sr isotopic composition of water samples from the Yangtze River in winter 2012 (8–20 February). Sample No.

River location

Date

Latitude

Longitude

H/depth (m)

T (°C)

Tributaries DDH MJ TJ JLJ WJ SZ LS YJ ZJ XJ DTH HS GJ PYH

Daduhe, Leshan Minjiang, Leshan Tuojiang, Luzhou Jialingjiang, Chongqing Wujiang, Fuling Songzijiang Lishui,Changde Yuanjiang, Changde Zijiang Xiangjiang, Changsha Dongting Lake, Yueyang Hanshui, Wuhan Ganjiang, Nanchang Poyang Lake

8-Feb 8-Feb 9-Feb 11-Feb 11-Feb 14-Feb 14-Feb 15-Feb 15-Feb 16-Feb 16-Feb 18-Feb 20-Feb 19-Feb

29.5981° 29.6075° 28.9278° 29.7003° 29.5842° 30.2397° 29.7067° 29.0956° 28.6436° 28.255° 29.5514° 30.7142° 28.6833° 29.8603°

103.6589° 103.7889° 105.4981° 106.5411° 106.6925° 111.8497° 111.9114° 111.9031° 112.3811° 113.0867° 113.1839° 114.3033° 115.8761° 116.3508°

377 340 222 161 168 48 39 40 36 9 -

9.7 9.6 9.9 9.3 11.1 9.1 8.7 7.2 8.4 8.08 6.4 7 8.4 -

-

253 548 772 424 408 407 267 274 242 292 285 322 169 -

Jinsshajiang, Yibin Yangtze,Yibin Zhutuo, Surface Zhutuo, Middle Zhutuo, Bottom Yangtze, Chongqing Yangtze, Chongqing Yangtze, Yichang Yangtze, Jinzhou Yangtze, Yueyang Yangtze, Wuhan Yangtze, Wuhan Yangtze, Jiujiang Yangtze, Pengze Datong, Surface Datong, Middle Datong, Bottom Yangtze, Nanjing

9-Feb 9-Feb 10-Feb 10-Feb 10-Feb 11-Feb 12-Feb 13-Feb 17-Feb 17-Feb 18-Feb 18-Feb 19-Feb 20-Feb 21-Feb 21-Feb 21-Feb 22-Feb

28.7394° 28.8675° 29.1178° 29.1178° 29.1178° 29.5842° 29.9033° 30.7461° 29.6153° 29.6336° 30.6406° 30.7403° 29.8267° 29.9975° 30.8406° 30.8406° 30.8406° 29.9653°

104.7011° 104.9553° 105.8817° 105.8817° 105.8817° 106.6931° 107.7297° 111.2903° 113.0753° 113.2383° 114.3072° 114.3364° 116.0464° 116.6675° 117.7311° 117.7311° 117.7311° 116.6728°

258 255 0.5 5.6 10.2 171 173 50 15 1 7.5 14 19

12.1 11.1 11.5 11.4 11.2 11 10.2 12.5 11.3 8.2 9.4 7.3 8.9 7.8 8.3 8.3 8.3 -

8.22 8026 8.16 8.14 8.12 7.94 8.05 7.95 8.04 8.18 7.99 8.10 8.10 8.12 8.08 8.11 7.92 -

351 363 378 375 376 386 413 396 399 322 364 339 361 276 276 318 318 -

Mainstream JSJ-YB YZ-YB YZ-ZT-S⁎ YZ-ZT-M⁎ YZ-ZT-B⁎ YZ-CQ-1 YZ-CQ-2 YZ-YC YZ-JZ YZ-YY YZ-WH-Q YZ-WH-H YZ-JJ YZ-PZ YZ-DT-S⁎ YZ-DT-M⁎ YZ-DT-B⁎ YZ-NJ

pH

EC 8.38 7.93 8.01 8.46 8.26 8.40 8.18 8.18 7.88 6.9

8.24 8.38

discharge (m3/s) 661 647 184 515 360 97.9 130 474 406 1450 3070 1500 1200 3880

992 2200 3120 3120 3120 3110 4156 6440 6550 9000 10000 10900 11200 15000 14846 14846 14846 16000

Sr (μmol/L)

87

Sr/86Sr

2σ

2.26 4.13 5.91 5.49 4.82 4.23 2.82 1.42 1.15 1.00 1.33 1.94 0.72 0.74

0.711642 0.711525 0.711111 0.710475 0.708509 0.710179 0.709525 0.711521 0.711974 0.712297 0.711820 0.711556 0.715456 0.714391

5 5 9 5 5 5 5 3 3 5 4 4 5 5

3.28 3.51 3.58 3.58 3.59 3.88 4.39 4.10 4.13 2.10 3.18 2.30 3.06 1.97 2.58 2.55 2.52 2.59

0.710485 0.710500 0.710516 0.710506 0.710504 0.710394 0.710177 0.710225 0.710226 0.710866 0.710382 0.711092 0.710409 0.711119 0.710654 0.710650 0.710659 0.710557

7 8 6 5 6 5 4 4 4 5 3 6 8 5 4 5 3 4

⁎ indicate samples collected at station with different depth.

January 2011 to 29.66 °C in August 2010. The Sr concentration and 87 Sr/86Sr ratios at Datong also vary greatly, with lower Sr and 87Sr/86Sr values during the monsoon. The Sr concentration in the Yangtze River lies within the range of 1.74 to 2.92 μmol/L, whereas 87Sr/86Sr ranges from 0.710125 to 0.710965 which is slightly lower than the average 87 Sr/86Sr value of the major rivers of the world (0.71171) (Tripathy et al., 2011). The highest 87Sr/86Sr value occurs in the sample collected on 27 December 2010 when there was a small peak in discharge at Datong, while the lowest value occurs in the sample collected on 21 May 2011. Major ions composition of samples collected in Datong was described in Wang et al. (in preparation).

0 5 Summer-ZT

Depth (m)

10

Winter-ZT

15

Summer-YC Winter-DT

20 25 30 0.7098

0.71

0.7102

0.7104

0.7106

0.7108

87Sr/86Sr

Fig. 3. Depth variation of 87Sr/86Sr in water samples collected at different hydrological stations of the Yangtze River. ZT, YC and DT stand for Zhutuo station, Yichang station, and Datong station respectively. Error bars are 2 standard deviations for the 87Sr/86Sr measurements.

5. Discussion 5.1. Vertical and spatial variations of Sr isotopic composition Stratification of the Sr isotopic composition of suspended matter is observed in the Yangtze River, which indicates that suspended sediments collected at the water surface are not representative of the average sediment transported by the river (Luo et al., 2012). It is important to investigate whether or not a similar stratification also occurs in the dissolved load since almost all the previous research is based on water samples collected from the surface of the river channel. The results for samples collected from various depths in the different reaches all exhibit a very small variation in dissolved Sr isotope composition with water depth (Fig. 3). The largest variation of 87Sr/86Sr is at Yichang station, ranging from 0.709985 ~ 0.710067, which is small compare to the variation of the 87Sr/86Sr ratios of different sources. Therefore unlike in the case of suspended matter, the 87Sr/86Sr ratios of water samples collected from different depths in the Yangtze River are depth independent, and thus measurements of samples collected from the surface are representative of the entire water column. On a basin-wide scale, 87Sr/86Sr ratios in river systems are primarily controlled by the lithology of the drainage basin and are defined by the mixing of Sr from silicate and carbonate and/or evaporitic rocks (Krishnaswami et al., 1992; Palmer and Edmond, 1992; Bickle et al., 2001; Douglas et al., 2002). The typical 87Sr/86Sr ratios of carbonate/ evaporites and silicates in the Yangtze River range from 0.708 to 0.709 and from 0.72 to 0.73, respectively (Chetelat et al., 2008). The 87 Sr/86Sr versus 1/Sr mixing diagram (Fig. 6) for our samples clearly demonstrates that the 87Sr/86Sr isotopic values of the Yangtze River water are dominated mainly by the weathering of carbonates and evaporites in the upper stream region, with an increasing contribution

64

C. Luo et al. / Chemical Geology 388 (2014) 59–70

Summer main stem

Summer tributries

Winter main stem

Winter tributries

0.7160 Poyang Lake

0.7150

Dongting Lake

Dissolved 87Sr/86Sr

0.7140 0.7130 Three Georges Dam Yalong Jiang

0.7120

Jialing Jiang

0.7110 0.7100 Wu Jiang

0.7090 0.7080 4500

4000

3500

3000

2500

2000

1500

1000

500

0

Distance from estuary (km) Fig. 4. Basin-transect variation of dissolved 87Sr/86Sr in the Yangtze River.

from silicate weathering downstream. The Sr values for carbonate and silicate end-members in this graph are derived from Zhang et al. (1996) and Han and Liu (2004), respectively. 87Sr/86Sr ratios of samples from Wu Jiang are close to the carbonate end-member while the samples from Gan Jiang are relatively close to the silicate end-member. Wu Jiang and Gan Jiang are two tributaries of the Yangtze River which are well known for their distinct karstic and granitic lithologies. The significant spatial variation in Sr concentration and isotopic composition of the samples collected from the Yangtze River are consistent with the complex range of strata from Archean to Quaternary age present within the basin. Since the variation of Sr isotopic composition between summer and winter is small (except for the case of GJ), samples from the summer campaign are used to discuss the spatial variations of Sr concentration and isotopic composition (the issue of seasonal variability is discussed in Section 5.2, below). 87Sr/86Sr ratios in the mainstream water are low in Jinsha Jiang (0.7102) and increase slowly with the contribution of tributaries in the upper reaches (0.7104-0.7106). They

then, decrease significantly after entering the Yangtze Gorge area and reach the lowest value (0.7099) at Yichang. In the middle and lower reaches, 87Sr/86Sr values are increased mainly by the contribution of Dongting Lake and Poyang Lake. The 87Sr/86Sr ratio of Poyang Lake is extremely high (0.7139) and raises the mainstream 87Sr/86Sr ratio from 0.7103 to 0.7116 (Fig. 4). The average 87 Sr/ 86Sr ratio for the tributaries in the upper reaches and the middle-lower reaches are 0.7107 and 0.7122, respectively. The low 87 Sr/86Sr ratios in the upper reaches are related to the wide spread occurrence of carbonate rocks, while higher 87Sr/86Sr ratios in the middle-lower reaches result from weathering of silicate rocks. In addition, the clear fall of 87Sr/86Sr ratio where the river enters the Yangtze Gorge area may be related to the abundant carbonate rocks distributed within the area. Na/Ca ratios Sr

87Sr/86Sr

0.7110

3.2

0.7108

Sr (μmol/L)

2010-06-12 2010-07-08 2010-07-26 2010-08-11 2010-08-29 2010-09-12 2010-09-29 2010-10-18 2010-11-06 2010-11-22 2010-12-10 2010-12-27 2011-01-21 2011-02-23 2011-03-11 2011-03-28 2011-04-13 2011-04-29 2011-05-21 2011-06-09 2011-06-28 Mean⁎

23 27.4 27.23 29.66 28.29 26.46 24.3 21.56 17.2 16.74 14.79 9.32 7.25 9.05 10.23 12.27 14.43 19.15 22.77 Nd 26

9.09 8 8.51 7.68 7.79 7.75 7.79 7.82 7.85 7.79 7.71 8.12 7.9 8.12 8.01 8.13 7.99 7.93 7.75 Nd 7.58

46300 56200 62850 56425 43325 42775 38375 28050 22950 15225 12850 18125 14200 13925 13225 18125 14125 14875 19275 19700 45375

2.23 1.74 2.28 2.10 2.24 2.40 2.16 1.92 2.21 2.44 2.62 1.95 2.25 2.67 2.48 2.42 2.57 2.50 2.48 2.92 2.17 2.32

0.710453 0.710747 0.710506 0.710622 0.710718 0.710727 0.710769 0.710886 0.710838 0.710763 0.710719 0.710965 0.710822 0.710640 0.710593 0.710580 0.710522 0.710433 0.710296 0.710125 0.710397 0.710625

Sr/ Sr

2σ

2.4

7 4 7 7 5 6 4 7 4 7 8 5 5 5 4 3 2 2 6 6 4

0.7104

Sr (µmol/L)

discharge (m /s)

2

0.7102

0.7100

1.6

30

80000

25 60000 20

15

40000

10

3

pH

0.7106

Discharge (m /s)

T (°C)

⁎ discharge weighted (2010-2011).

86

/

Sample No.

87

Temperature (C ° )

3

87Sr 86Sr

2.8 Table 3 Sr isotopic composition of water samples from Datong station of the Yangtze River.

20000

5 0

0

Month (2010.6-2011.6) Fig. 5. Temporal variation of temperature, discharge, Sr concentration and 87Sr/86Sr of the river water collected at Datong from June 2010 to June 2011.

C. Luo et al. / Chemical Geology 388 (2014) 59–70 Summer

Winter

0.726

Silicate

Dissolved 87Sr/ 86Sr

0.722

0.718 Ganjiang

0.714

Downstream

0.710 Wujiang

Carbonate

0.706

0

0.5

1

1.5

2

2.5

3

1/[Sr] umol/l Fig. 6. An 87Sr/86Sr versus 1/Sr mixing diagram for the dissolved loads from basin wide sampling in the Yangtze River. Red and blue circles represent samples collected in summer and winter, respectively. The data of Sr concentrations and 87Sr/86Sr ratios of carbonate and silicate end-members are taken from Zhang et al. (1996) and Han and Liu (2004), respectively.

also show a decreasing trend after entering the Yangtze Gorge area (Luo et al., in preparation). Therefore, the variation of 87Sr/86Sr values in the Yangtze River primarily reflects the controls of source rock composition in the drainage basins (Wang et al., 2007; Chetelat et al., 2008; Mao et al., 2010; Jiang and Ji, 2011). Consequently, the distinct Sr isotopic characteristics of water from different areas can be used as a tracer for provenance studies. According to the 87Sr/86Sr ratios and lithology over the basin, the Yangtze River can be divided roughly into two end-members: the upper reaches, dominated by carbonate lithologies with low 87Sr/86Sr ratios, and the middle-lower reaches, dominated by silicate lithologies with high 87Sr/86Sr values. 5.2. Temporal variation of Sr concentration and isotopic composition and its implications The 87Sr/86Sr ratios of the Yangtze River at Datong station exhibit a distinct pattern of variation, with values ranging from 0.710125 to 0.710965, and a general increase in the 87Sr/86Sr ratio from summer to winter, with some fluctuations during July and December, followed by a gradual decrease until the next monsoon season (Fig. 5). This pattern of variation differs from that of other Tibetan rivers which exhibit lower 87 Sr/86Sr values during the monsoon season. A relative decrease in silicate weathering compared to carbonate weathering during the monsoon season may result from several factors, including the faster dissolution kinetics of carbonate, lower water-rock interaction time and the availability of a large area for weathering (Tipper et al., 2006; Rai and Singh, 2007; Tripathy et al., 2010). However, Bickle et al. (2003) and Voss et al. (2014) suggest that changing inputs of weathered material and solutes from different lithotectonic units, because of spatially uneven rainfall, contributes to seasonal variations in 87Sr/86Sr ratio. Wei et al. (2013) attributed the higher 87Sr/86Sr ratios in the Xijiang River during intervals of high discharge to the enhanced weathering of silicates during rainy seasons. With no clear differences in 87Sr/86Sr between wet and dry season, the data in the present study raise a series of questions about the mechanisms controlling the temporal variation of 87Sr/86Sr ratios in the Yangtze River. In general, temporal variations in water geochemistry can result from changes in the temperature and the mixing proportion of different water sources. The dissolution rate of carbonate and silicate rocks can be enhanced or decreased by variations in temperature which can therefore lead to temporal variation in the water geochemistry. The

65

water temperature at Datong station exhibits a large seasonal variation from 7.25 to 29.66 °C. However, the 87Sr/86Sr composition of samples collected during July and January is similar, while the water temperatures at the time of collection were 27°C and 7.5°C, respectively. In addition, the 87 Sr/86Sr values of samples collected at individual sites over the Yangtze River drainage do not vary significantly between summer and winter (Fig. 5); therefore, temperature is not the major control on the Sr isotopic composition of the Yangtze River, since the observed temperature variation is insufficient to explain the variation of 87Sr/86Sr ratios at Datong. As mentioned above, the Sr concentrations and 87Sr/86Sr values of the dissolved load exhibit significant variability within the Yangtze River basin as a result of the dissolution of the abundant carbonate/ evaporitic rocks with less radiogenic Sr in the upper reaches and widely distributed silicate rocks with high radiogenic Sr in the middle-lower reaches. Influenced by the subtropical monsoon climate, the uneven spatial and temporal distribution of rainfall in the Yangtze River is likely to cause variable erosion rates across the basin. An important question to be answered is whether the uneven spatial and temporal distribution of rainfall over the Yangtze River catchment can lead to the variable 87Sr/86Sr signal at Datong. At first sight, the 87 Sr/86Sr compositions shift from a lower radiogenic contribution during summer to a greater radiogenic contribution during winter. The peak flow period in the Yangtze River is from July to September when the rain belt is located in the upper reaches. The heavy rainfall in the upper reaches will increase the contribution from carbonate at Datong and thus decrease the 87Sr/86Sr composition. On a finer temporal scale, an abrupt increase and subsequent fall in 87Sr/86Sr ratios is observed during July which may be related to the occurrence of floods in the middle-lower and upper reaches at that time. Since several large floods within the Yangtze drainage basin are documented during the peak flow period in 2010: In the middle-lower reaches floods occurred during late June and in the upper reaches, especially in the Jialing Jiang basin, they occurred in late July and were the largest ever recorded (Wang et al., 2011). The increasing contribution from the middle-lower basin with high yield of radiogenic Sr raises the 87Sr/86Sr composition observed at Datong, while the 87Sr/86Sr composition decreases when flooding occurs in the upper reaches. A minor peak occurs at Datong on December 27th when the 87Sr/86Sr ratio reached its highest values for the year. Singnificantly, both Gan Jiang and Xiang Jiang recorded an increase in discharge at that time. Gan Jiang is a typical silicate basin, and its runoff is characterized by a high 87Sr/86Sr ratio. Therefore an increased fluvial contribution from Gan Jiang would be expected to raise the 87Sr/86Sr composition of the mainstream, as observed in our data. After December, the 87Sr/86Sr ratios decrease gradually until the next freshet of 2011 which may result from the severe drought around the middle-lower reaches from January. From March to May in 2011, the average precipitation in the middle-lower reaches was 196 mm, about half of the typical amount in the same period over the past, and the lowest value for 60 years (Sun, 2012). The drought became increasingly severe towards the end of May resulting in the almost complete drying up of Poyang and Dongting Lakes. The low precipitation in the middle-lower reaches, with a silicate lithology, would have slowed the weathering process and reduced their chemical contribution to the mainstream. In turn this would reduce the 87Sr/86Sr ratios at Datong, as observed in our data. However, the subsequent heavy rainfall in early June in the middle-lower area reaches raised the 87Sr/86Sr composition almost immediately. Based on the foregoing, the variations of the 87Sr/86Sr ratios in the Yangtze River are convincingly explained by varying contributions from the upper and middle-lower reaches, driven by changes in rainfall across the basin. Furthermore, our study indicates that the abrupt changes in the 87Sr/86Sr ratios correspond well to flood events within the drainage basin and thus results point to the possibility of using 87 Sr/86Sr ratios to reconstruct past climatic changes. Based on a seasonal investigation of the Brahmaputra, Rai and Singh (2007) also highlighted the use of Sr isotopic composition of water samples to track a flash flood

66

C. Luo et al. / Chemical Geology 388 (2014) 59–70

event. In addition to flood events, the effects of drought in the Yangtze basin will be reflected by variations of 87Sr/86Sr ratios downstream.

with the Dongting and Poyang Lake as well as Han Jiang (Fig. 7). These discrepancies are resulted from two reasons. For those samples collected after the junction with lakes, the discrepancy is due to the complicated water exchange between lake and the mainstream. Water from the mainstream will come into the lake and at the same time water from lake goes to the mainstream (Zhao et al., 2013). However, the exact amount of exchanged water and their Sr flux cannot be quantified which will lead to the discrepancy in the calculation. This may also be a reason for the observed divergence between the model and measured values for the seasonal variation of Sr isotope at Datong which will be discussed in the next part. With regard to the inconsistent of the point after Han Jiang, the reason is related to the sampling site. It turns out that the sampling sites after the confluence with the Han Jiang may be too close to the confluence (5 km) which means the water from two different sources are not well mixed. Bouchez et al. (2010) demonstrated that the homogeneous mixing of two tributaries in large rivers only occurs at least several tens of kilometers downstream from the confluence. This factor should be carefully considered in future studies when taking samples from sites near confluences. According to our calculation, in the Yangtze River the Sr concentration downstream from a confluence can be estimated by the simple mixing of the two sources. The distribution of Sr over the entire basin was investigated by calculating the Sr fluxes in the mainstream and in each major tributary with the discharge data and Sr concentrations at each sampling sites. The results indicate that the Sr flux from the upper reaches is about 3 times greater than that from the middle-lower reaches, thus emphasizing the importance of the upper reaches on the Yangtze River Sr isotopic composition (Fig. 8).The high Sr flux from the upper reaches is resulted from several reasons. Firstly, the upper reaches, especially the Jinsha Jiang section is mostly carbonate and evaporates with higher Sr concentration while the middle-lower reaches is silicate with low Sr concentration. Secondly, the drainage area of the upper reaches (100 km 2 ) is slightly higher than the middle-lower reaches (80 km2). A notable increase in Sr flux is evident in the mainstream from the sites Fengdu (point A) to Yichang (point B) despite the absence of any major contribution from the tributaries between these two points (Fig. 8). The area between Fengdu and Yichang is called the Yangtze Gorge area where outcrops consist mainly of carbonate rocks (Fig. 2). Approximately 35 km upstream of Yichang is the Three Gorges Dam (TGD), the world’s biggest dam, with a storage capacity of 3.93 × 1010 m3. The coverage area of the water within the dam is

5.3. Modeling the temporal variability of different Sr sources The discussion in Section 5.2 indicates that the seasonal trend in 87Sr/86Sr ratios at Datong is the result of changes in the relative contribution from the upper and middle-lower reaches. In order to verify our explanation, we devised a simple numerical model to predict seasonal variations in the Sr isotopic composition at Datong using all of the basin-wide data. The model requires several assumptions regarding Sr in the Yangtze River, namely that the variations at Datong are explained solely by varying contributions from the different sources, and that no additional chemical or physical reactions take place during the mixing process. On this basis, a simple binary mixture calculation was performed on the data from samples that were collected from the mainstream and the confluences of the major tributaries. The Sr concentration and the 87Sr/86Sr ratios at the point immediately downstream of a confluence can be calculated using Eqs. (1) and (2): ½Sr lower

½Rlower

stem

stem


 ð1Þ ¼ Q upper stem  Srupper stem þ Q tributary  Sr tributary
 = Q upper stem þ Q tributary


¼ Rupper

stem


= Sr upper

 Q upper

stem

stem

 Q upper

 Sr upper

stem

stem

þ Rtributary  Q tributary  Srtributary

þ Srtributary  Q tributary





ð2Þ where [Sr]lower stem and [R]lower stem are the modeled Sr concentration and 87Sr/86Sr ratio in the mainstream after the confluence, respectively; Srupper stem and Rupper stem is the Sr concentration and 87Sr/86Sr ratio in the mainstream before the confluence, respectively; Qupper stem and Qtributary are the discharges in the mainstream upstream of the confluence and the discharge of the tributary; Srtributary is the Sr concentration of the tributary. Comparison of the modeled results with the measured data indicates that almost all of the points are distributed along the 1:1 line, with the exception of samples collected downstream of the junction

Winter

Summer

Summer

Winter

600

0.7120

B

A 500

Modeled 87Sr/86Sr

Modeled Sr (ug/L)

0.7115

400

DTL

300

PYL

0.7110

PYL

0.7105

HJ

200

DTL 100 100

200

300

400

Measured Sr (ug/l)

500

600

0.7100 0.7100

0.7105

0.7110

Measured

0.7115

0.7120

87Sr/86Sr

Fig. 7. Modeled Sr concentration and 87Sr/86Sr ratio based on mixing of the upper mainstrem and tributary compared with the measured values in the lower mainstream. DTL and PYL stand for Dongting Lake and Poyang Lake, respectively.

C. Luo et al. / Chemical Geology 388 (2014) 59–70

67

Sr Flux of theYangtze River (mol/s) Upper reaches 77.63 \26.42

Middle-lower 20.75 \11.88 77.63 \26.42

40.25 \18.23 TGD

8.38 \2.73

5.61 \2.83 1.83 \1.09

B A

3.43 \1.46

Yibin 11.54 \3.03

7.61 \3.52

Yic

Wuhan

han

g

Hukou

0.37

4.85 \1.73

2.93 \1.48

0.47 4.61 \0.67

98.38 \38.3

Datong

2.34 \1.42

Fig. 8. Sr flux of the Yangtze River. Red and black numbers represent fluxes in summer and winter, respectively. The orange shading represents an area of the mainstream that may be an important source of Sr.

almost the same as the Yangtze Gorge area. Hence, the dissolution of carbonate within this area by the large amount of water within the dam results in the supply of a large quantity of dissolved Sr. The huge amount of dissolved Sr flux within the TGD is called ‘TGD source of Sr’ and can be seen as an important source of Sr in the Yangtze river. Therefore, future provenance studies of the Yangtze River should pay attention to the ‘TGD source of Sr’ in the Yangtze Gorge area. Since large dams can profoundly alter river flow regime and with a series of significant consequences (Stanley and Warne, 1993), future geochemical investigations in the middle-lower part of the Yangtze River should pay special attention to the influence of the Three Gorges Dam. The existence of a TGD source of Sr in the Yangtze Gorge area and the absence of time series of discharge data from all the major tributaries make it impossible to conduct the model with all the major tributaries. Therefore, the model assumes that the isotopic composition at Datong is a binary mixture of water from the upper reaches and middle-lower reaches. Sr concentration and the 87Sr/86Sr composition at Yichang (YC) were used to represent the upper reaches, and the Sr concentration and 87Sr/86Sr composition for the middle-lower reaches (ML) were calculated using the isotope mixing Eqs. ((3) and (4)) which sums the contributions of the tributaries in the middle-lower reaches (Yuan Jiang, Xiang Jiang, Han Jiang, Gan Jiang in summer plus Li shui and Zi Shui in winter). The equations are as follows: ½SrML ¼ ½RML ¼

X i

X i

X ðQ i  Sr i Þ= i Q i

X ðQ i  Sr i  Ri Þ= i ðQ i  Sr i Þ

ð3Þ ð4Þ

where [Sr]ML and [R]ML are modeled Sr concentration and the 87Sr/86Sr ratio of the middle-lower reaches; Sri is the Sr concentration; R is the 87 Sr/86Sr composition; and Qi is the water discharge of tributary i on the sampling day. From the Sr concentration and 87Sr/86Sr composition data for the upper and middle-lower reaches, the 87Sr/86Sr ratio at Datong can be calculated by the following equations: RDT ¼ RYC  f YC þ RML  f ML

ð5Þ

f YC ¼ ðQ YC  Sr YC Þ=½Q YC  SrYC þ ðQ DT −Q YC Þ  SrML 

ð6Þ

f ML ¼ ½ðQ DT −Q YC Þ  Sr ML =½Q YC  Sr YC þ ðQ DT −Q YC Þ  Sr ML 

ð7Þ

where ‘DT’ and ‘YC’ represent Datong and Yichang Station; fYC and fML are the proportions of Sr flux from the upper and middle-lower reaches, respectively; SrYC and SrML are the Sr concentrations of the upper and middle-lower end member, respectively; and QYC and QML are water discharges at Yichang and Datong hydrological station on the sampling date (available from online real time records). Although results show that variation in the 87Sr/86Sr composition of samples collected across the basin during summer and winter does not change significantly on a seasonal basis (Fig. 4), this is not the case for the variation of Sr concentration. Therefore, in order to better constrain the model, Sr concentrations and 87Sr/86Sr ratios calculated using the data from samples collected during summer were used for the peak flow period, and data from samples collected during winter were used for the dry season. There are several potential sources of error in our model: Firstly, interannual variations are observed in Sr concentration and 87Sr/86Sr composition of rivers (Tripathy et al., 2010; Voss et al., 2014). However, our basin-wide sampling was carried out in 2011 and 2012 while the time series sampling was carried from 2010 to 2011, and therefore, this interannual variation could lead to potential errors in model results. Secondly, the Sr concentration and 87Sr/86Sr composition of the two end-members are not time series data but rather a particular value for the peak flow period and a particular value for a dry season. Though no significant seasonal variation was found in the 87Sr/86Sr composition of samples, changes in Sr concentration will also lead to errors in the results. The model results demonstrate similar seasonal shifts, both in timing and magnitude, with an increasing trend from June to December in 2010 followed by a more gently decreasing trend to June 2011 (Fig. 9). However, there are divergences between the model and measured 87Sr/86Sr ratio during the beginning and at the end of the sampling period when a large flood and drought occurred in the basin, respectively. This is because the Sr concentration and 87Sr/86Sr composition we used in the model were deduced from normal conditions which will differ from the values that would be produced during either flooding or a drought. According to our analyses, the dissolved Sr isotopic composition in the lower Yangtze River can be modeled by the mixing of water from the upper reach with that from the middle-lower reaches. Overall, the model demonstrates that changes in the contribution from different sources, driven by seasonally and spatially variable rainfall, can lead to variations in the 87Sr/86Sr composition of samples collected

68

C. Luo et al. / Chemical Geology 388 (2014) 59–70 Sr Measured

Sr Modeled

0.7112

Dissolved

87Sr/86Sr

0.7110 0.7108 0.7106 0.7104 0.7102 0.7100

Month (2010.6-2011.6) Fig. 9. The modeled temporal variation of dissolved 87Sr/86Sr at Datong station based on Sr flux and 87Sr/86Sr composition of the upper reaches and middle-lower reaches. Note the similarity of the model estimates with the seasonal shifts of the measured time series values.

from the downstream sections of rivers. A similar explanation can be applied to the seasonal variation of dissolved Sr isotopic composition of the Fraser River (Voss et al., 2014). According to our model, it may be possible to estimate past discharges of the Yangtze River from the 87 Sr/ 86 Sr ratios recorded in shells of the delta sediments using end member values of the upper and middle-lower reaches. 5.4. Flux of dissolved Sr and the impact on the oceanic 87Sr/86Sr ratio Accurate calculations of Sr fluxes transported by rivers are important, not only for elucidating the weathering intensity of river basins, but also for estimating their contributions to the ocean. Sr fluxes of the Yangtze River reported previously are based mainly on samples collected from the wet or dry seasons (Wang et al., 2007; Chetelat et al., 2008; Wu et al., 2009a,b). However, as indicated in this study, seasonal variations in Sr concentration and isotopic composition in the Yangtze River can be large. Therefore, previously estimated fluxes that are based sampling at a single date may be unrepresentative, and thus a discharge-weighted average 87Sr/86Sr and annual Sr flux of the Yangtze River calculated using the time series data is necessary to provide a more accurate estimate of the oceanic Sr isotopic variation. The dischargeweighted dissolved 87Sr/86Sr value for the Yangtze River is 0.7106, which is slightly lower than the 87Sr/86Sr value of 0.7107 reported by Gaillardet et al. (1999) and Wang et al. (2007), and much lower than the value of ~0.7111 published by Chetelat et al. (2008). The discharge-weighted annual Sr flux is 1.9 × 109 mol · a−1, which is lower than the values of Gaillardet et al. (1999) and Wang et al. (2007) (Table 4). This low value is resulted from the serious drought happened in the middle-lower reaches during the sampling time by decreasing the annual water discharge. And it can provide scientists a better understand on the variation of Sr flux under extream climate.

Given the evidence for year to year variation, a variation of annual Sr flux range from 1.54 × 109 to 3.11 × 109 mol · a−1 is estimated based on the discharge record during 1946 ~ 2009 at Datong (Fang et al., 2011). In addition to the year to year variation, annual Sr flux based on each single sample were also calculated to understand the divergence between fluxes calculated from a single sample and the time series samples. The results show that the dissolved annual Sr fluxes based on a single sample at Datong station vary from 1.47 × 109 to 2.46 × 109 mol · a−1, leading to an uncertainty of 29.45 ~ −22.9% in annual Sr flux with respect to the discharge-weighted annual Sr flux. Fluxes calculated using a single sample, collected during peak flow period, are close to the dischargeweighted annual flux. Large uncertainties are evident in samples collected during a major flood (−22.9%) and during a period with alternating drought and flood (29.45%). According to our analyses, when time series sampling is not feasible, sampling should be carried out during the monsoon season and extreme weather climatic events should be avoided. The variation of oceanic Sr isotopic composition depends not only on the Sr flux of rivers but also on their 87Sr/86Sr ratios. Hence, in order to better understand the significance of rivers in regulating seawater 87 Sr/86Sr ratio, excess 87Sr flux (87Srex) can be defined (Bickle et al., 2003). 87

Sr ex ¼ ðR−0:709176Þ  F

ð8Þ

where F and R are the Sr flux and 87Sr/86Sr ratio of the rivers, respectively, and 0.709176 represents the 87Sr/86Sr ratio in modern seawater (Allegre et al., 2010). The estimated Sr flux in our study is 1.9 × 109 mol · a−1 and represents 40.43% of the total Sr flux transported by the eleven rivers that originate from Tibet (Wu et al., 2009b), and 5.59% of the Sr fluxes transported to the ocean by the total global rivers (Palmer and Edmond, 1989). The excess 87 Sr flux calculated in the Yangtze River is 3.1 × 106 mol · a− 1, and accounts for 3.73% of the 87Srex transported by the global rivers. In order to evaluate the significance of Tibetan rivers in regulating the seawater 87Sr/86Sr composition, Sr data from our study are compared with the results of other research (Table 4). It can be observed that, although the Sr flux of the Yangtze River is one order of magnitude higher than that of the Brahmaputra and Ganges, which are 0.44 × 109 mol · a− 1 and 0.47 × 109 mol · a− 1, respectively (Krishnaswami et al., 1992), its excess 87Sr flux is lower than those of these two rivers. 6. Conclusions We report the results of a systematic investigation of the Sr isotopic characteristics of dissolved loads in the Yangtze River. The results demonstrate that: (1) The depth variation of dissolved 87Sr/86Sr ratios in the Yangtze River is small enough so that water from the channel surface is representative of the total dissolved load of the entire water

Table 4 Composition of dissolved Sr concentration, 87Sr/86Sr, Sr flux and excess 87Srex in the Yangtze River estimated in this study with the results of other studies. River Basins

Date

Sr

87

Sr/86Sr

(μmol/L) Yangtze River⁎ Yangtze River Yangtze River Yangtze River Brahmaputra Brahmaputra Ganges Ganges Total global rivers

Time series Octerber 1997 August 2006 -

2.32 2.42 2.28 0.44

0.7106 0.71071 0.7108 0.7115 0.7192

-

1.2

0.7239

Sr flux

87

mol a − 1

mol a − 1

1.9 × 109 2.45 × 109 2.12 × 109

3.1 × 106 4.19 × 106 3.82 × 106

0.44 × 109 0.47 × 109

3.64 × 106 5.27 × 106 8 × 106 5.55 × 106 83 × 106

33.3 × 109

Srex

Data sources

This study (Gaillardet et al., 1999) (Wang et al., 2007) (Chetelat et al., 2008) (Krishnaswami et al., 1992) (Galy et al., 1999) (Krishnaswami et al., 1992) (Galy et al., 1999) (Palmer and Edmond, 1989)

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column. The variations in 87Sr/86Sr composition of the Yangtze River basin primarily demonstrate the control of the composition of the source rocks. Isotopic compositions of basin-wide samples do not change significantly on a seasonal scale, with slightly higher 87Sr/86Sr values in summer than in winter, while the Sr concentration is higher in winter. (2) The temporal variation of dissolved 87Sr/86Sr at Datong station is driven by the changing contribution from the upper and middlelower reaches as a result of the uneven spatial and seasonal distribution of rainfall across the basin. This explanation is supported by a mixing model based on the Sr concentration, 87 Sr/86Sr composition and discharge of the upper and middlelower reaches. (3) With regard to provenance studies, our results demonstrate that under the dissolution of rocks with large amount of water within TGD, the Yangtze Gorge area becomes an important source of dissolved Sr in the Yangtze River. Its large contribution to Sr flux has not previously been observed since the TGD is finished in 2009, but it needs to be considered in future attempts to apply measurements of Sr isotopic composition to chemical weathering and provenance studies. In addition, Sr flux from the upper reaches is 3 times higher than the Sr flux from the middle-lower reaches, indicating the significant control of the former on the Sr budget of the Yangtze River. (4) The discharge weighted annual Sr flux and the 87Sr/86Sr ratio of the Yangtze River are 1.9 × 109 mol · a−1 and 0.7106, respectively, slightly lower than values reported in previous studies. Finally, our study has important implications for assessing the representativeness of river water sampling and provides a framework for analyzing the Sr isotopic characteristics of the Yangtze River. Only with a systematic recognition of Sr isotopes in the Yangtze River can we accurately elucidate the provenance and weathering intensity information with 87Sr/86Sr values of sediments from the Yangtze delta. Acknowledgments We thank Yoshiaku Suzuki (University of Tokyo), Yingmeng He (Nanjing University) and the staff of the Zhutuo, Yichang and Datong hydrological stations for their help during the sampling campaigns. We also thank Pu Wei from the State Key Laboratory for Mineral Deposits Research for her help with laboratory analyses. We thank Dr. Jan Bloemendal for constructive review of the manuscript, and Daniel Rits for checking the grammar and spelling of this paper. The manuscript benefited from comments by two anonymous reviewers. This work was supported by the "Strategic Priority Research Program" of the Chinese Academy of Sciences (Grant No. XDB03020300) and the China Geological Survey Projects (Grant No. 12120113005400). References Allegre, C.J., Louvat, P., Gaillardet, J., Meynadier, L., Rad, S., Capmas, F., 2010. The fundamental role of island arc weathering in the oceanic Sr isotope budget. Earth Planet. Sci. Lett. 292 (1–2), 51–56. Bickle, M.J., Bunbury, J.M., Fairchild, I.J., 2001. Controls on the 87Sr/86Sr ratio of carbonates in the Garhwal Himalaya, headwaters of the Ganges. J. Geol. 109 (6), 737–753. Bickle, M.J., Bunbury, J., Chapman, H.J., Harris, N.B., Fairchild, I.J., Ahmad, T., 2003. Fluxes of Sr into the headwaters of the Ganges. Geochim. Cosmochim. Acta 67 (14), 2567–2584. Bickle, M.J., Chapman, H.J., Bunbury, J., Harris, N.B.W., Fairchild, I.J., Ahmad, T., Pomies, C., 2005. Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges. Geochim. Cosmochim. Acta. 69 (9), 2221–2240. Bouchez, J., Lajeunesse, E., Gaillarde, J., France-Lanord, C., Dutra-Maia, P., Maurice, L., et al., 2010. Turbulent mixing in the Amazon River: The isotopic memory of confluences. Earth Planet. Sci. Lett. 290, 37–43. Bouchez, J., Gaillardet, J., France-Lanord, C., Maurice, L., Dutra-Maia, P., 2011. Grain size control of river suspended sediment geochemistry: Clues from Amazon River depth profiles. Geochem. Geophys. Geosyst. 12 (Q03008). Changjiang Water Resource Commission, Ministry of Water Resources (CWRC), 2002. Floods and Droughts in the Yangtze River Catchment. Water Conservancy and Water Electricity Publication House, Beijing (in Chinese).

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