Seasonal contributions of catchment weathering and eolian ... · Chemical weathering is one of the...

Seasonal contributions of catchment weathering and eolian dustto river water chemistry, northeastern Tibetan Plateau:Chemical and Sr isotopic constraints

Zhangdong Jin,1,2 Chen‐Feng You,3 Jimin Yu,4 Lingling Wu,5 Fei Zhang,1

and Hou‐Chun Liu3

Received 22 February 2011; revised 27 July 2011; accepted 29 July 2011; published 19 October 2011.

[1] River waters collected weekly over the whole year of 2007 from the Buha Riverdraining to Lake Qinghai on the northeastern Tibetan Plateau were analyzed for major ionsand Sr isotopes. Dissolved loads in the river exhibit distinct seasonal variability in majorcation ratios and Sr isotopes over the 1 year period, reflecting seasonal differences inrelative inputs from various sources and weathering reactions in the catchment. Distinctgeochemical signatures suggest that eolian dust may affect river water chemistrysignificantly, resulting in a twofold increase influx of dissolved loads during springrelative to winter. It is noticeable that both the lowest and the highest 87Sr/86Sr values ofthe Buha River waters occurred in the monsoon season, indicating a sensitive response ofcarbonate versus silicate weathering sources to hydrological forcing on a seasonal basis.A significant decrease in Na/cation, together with lower Sr isotope ratios, is consistentwith a greater proportion of carbonate weathering relative to silicate weathering in theearly monsoon season. High temperature and increased rainfall during the peak of themonsoon facilitate an increased proportion of ions derived from silicates, partly fromgroundwaters, to river water. In other seasons, elemental and 87Sr/86Sr ratios vary muchless, indicating a constant ratio of silicate to carbonate weathering, consistent with limitedvariation in discharge. Our results highlight that in a semiarid region where climaticconditions vary seasonally, in addition to silicate and carbonate contributions, supplyfrom eolian dust may also play a significant role in controlling seasonal variations inchemistry of river waters.

Citation: Jin, Z., C.‐F. You, J. Yu, L. Wu, F. Zhang, and H.‐C. Liu (2011), Seasonal contributions of catchment weathering andeolian dust to river water chemistry, northeastern Tibetan Plateau: Chemical and Sr isotopic constraints, J. Geophys. Res., 116,F04006, doi:10.1029/2011JF002002.

1. Introduction

[2] Rivers, as a vast global transportation system on theEarth’s surface, supply dissolved components and sedimentsto lakes and oceans. The dissolved components in the watersare important in the context of global geochemical cyclesand paleoenvironmental reconstruction, because dissolvedspecies respond to environmental changes and effectivelymoderate the chemical and biological processes. Knowledge

of these (bio)geochemical processes is essential to under-stand the environmental feedback mechanisms that are afundamental goal of contemporary Earth Science studies.Chemical weathering is one of the most important (bio)geochemical processes, responsible for supplying dissolvedcomponents to ecosystems, and also for regulating long‐termglobal climate via the removal of atmospheric CO2 [e.g.,Walker et al., 1981; Berner et al., 1983]. This lends partic-ular significance to understanding the role of rock weather-ing in determining chemical compositions of the waters.[3] The chemical composition of stream waters can be

affected by various natural factors including the chemicalcomposition of rocks and soils, size and shape of the catch-ment, climatic variables, and vegetation [White and Blum,1995; Drever, 1997]. The latter two factors may vary sea-sonally, so that increasing attention has been paid to thetemporal variation in weathering rates and patterns, particu-larly in response to seasonal climate [e.g., Galy and France‐Lanord, 2001; Bickle et al., 2005; Tipper et al., 2006; Rai andSingh, 2007; Gislason et al., 2009; Wolff‐Boenisch et al.,

1State Key Laboratory of Loess and Quaternary Geology, Institute ofEarth Environment, Chinese Academy of Sciences, Xi’an, China.

2Also at School of Human Settlement and Civil Engineering, Xi’anJiaotong University, Xi’an, China.

3Earth Dynamic System Research Center, National Cheng KungUniversity, Tainan, Taiwan.

4Lamont‐Doherty Earth Observatory, Earth Institute at ColumbiaUniversity, Palisades, New York, USA.

5Department of Geoscience, University of Wisconsin–Madison,Madison, Wisconsin, USA.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2011JF002002

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, F04006, doi:10.1029/2011JF002002, 2011

F04006 1 of 16

http://dx.doi.org/10.1029/2011JF002002

2009]. These studies have found large seasonal variations inthe major cation and 87Sr/86Sr ratios in the dissolved loadsindicating a climatic sensitivity of the weathering processes.Noticeably, most of these studies have focused on selectedrivers draining the Himalaya, with special interest in thisregion because of the hypothesis of global impact of largeHimalayan rivers on seawater chemistry [Richter et al., 1992;Ruddiman, 1997]. To better define mass transfer in hydro-logical cycles, a detailed understanding of the spatial andtemporal controls on the rates and processes of chemicalweathering within various catchments remains crucial.However, in other important environments such as in arid andsemiarid areas, there is less information about seasonal var-iation in river chemistry.[4] Besides catchment weathering, eolian dust has poten-

tial but uncertain impacts on dissolved load budgets of majorcations and 87Sr/86Sr ratios in river/seawater chemistry [e.g.,Campbell et al., 1995; Jacobson, 2004; Jacobson andHolmden, 2006; Pett‐Ridge et al., 2009]. Eolian dust pro-duced in arid areas has important and disparate effectsthroughout the Earth system [e.g., Arimoto, 2001; Ridgwell,2002]. Annually, major dust production occurs during springin the arid and semiarid regions of the Central Asia, and thisdust is transported long distances via midlatitude prevailingwinds to east China and beyond [Liu et al., 2008]. Recentattention has been focused on the role of eolian dust intransporting solute Fe and Al to surface oceans and inenhancing organic productivity [e.g., Measures and Vink,1999; Jickells et al., 2005; Measures et al., 2005]. Annualeolian dust input directly to the oceans is estimated to be450 Tg, amounting to ∼26% of the total dust production(1700 Tg/yr) [Jickells et al., 2005]. The dust input to rivers,especially for some large rivers (e.g., the Yellow and the NileRivers), is likely important and its role in influencing thewater chemistry needs to be quantified. To date, such eoliandust contribution to river water chemistry has rarely beentaken into account during catchment weathering evaluations[Gorham, 1961; Négrel et al., 1993; Pett‐Ridge et al., 2009].[5] In the present study, waters have been sampled from

the Buha River (draining into Lake Qinghai, northeasternTibetan Plateau) and from rain and groundwater within thecatchment. The river water samples were collected regularlythroughout 2007. Interest in this subject arose initially fromattempts to quantify the inputs to river water from rain andfrom both carbonate and silicate weathering within the LakeQinghai basin. The results suggested that dry/wet atmo-spheric input contributes 36–57% of the total dissolvedcations to the river waters [Zhang et al., 2009]. Furthermore,an elemental input‐output model of Lake Qinghai showsthat dry atmospheric input accounts for ∼65% of the totalinputs to the modern lake sediments [Jin et al., 2009a]. Ourdata presented here show that eolian dust might have asignificant contribution to both major elements and 87Sr/86Srin the riverine dissolved loads in the spring, when north-westward winds prevail. The main aims of this work are (1)to understand the geochemistry of the Buha River, (2) toillustrate how it varies seasonally, (3) to trace the nature ofthe weathering processes, and (4) to shed light on the role ofseasonal eolian dust input in river water chemistry insemiarid areas.[6] The Buha River in Lake Qinghai was chosen for the

following reasons. First, it is located in a transitional zone

between arid and semiarid climates, where seasonally alter-nating wind and rainfall processes predominate. Second,seasonal air temperature and precipitation in the area varyremarkably, providing an ideal place for evaluating the sen-sitivity of chemical weathering to climatic parameters. Forexample, the variation in daily air temperature reaches 38°C(−25 to +13°C) annually. It is predicted that future warmingand rainfall increase on the Tibetan Plateau is expected to be“much greater than average” [Intergovernmental Panel onClimate Change, 2001; Duan et al., 2006]. Third, thecatchment is characterized by high relief and low density ofvegetation. The underlying marine sedimentary rocks areeasily subjected to erosion and weathering that contributesmore than 70% of the total suspended sediment discharge(∼498 kt/yr [Colman et al., 2007]) to the lake. Fourth, LakeQinghai adjoins the Chinese Loess Plateau in the east, theQaidam Basin in the west, and the arid desert in the north andwest (Figure 1a), all being important potential sources ofAsian dust [Bowler et al., 1987; Prospero et al., 2002; Qianget al., 2007]. Dust activity is prevalent in these areas, pri-marily during the spring. Fifth, this river catchment issparsely populated, minimizing the effect of human activityon water chemistry. Last, the Ca‐(Na)‐HCO3‐type waterfrom the Buha River contributes more than 50% of the surfacerunoff and the chemical budget of Lake Qinghai [Jin et al.,2009a]. Chemical mass balance is the basis of paleoenvir-onmental reconstruction from the drilling cores of the LakeQinghai Drilling Project under the auspices of the ChineseAcademy of Sciences (CAS) and the International Conti-nental Scientific Drilling Program. Tracing water chemicalvariation and the related controlling factors on the Buha Riverhas important implications for interpreting sedimentaryrecords from Lake Qinghai.

2. Study Area

2.1. Geography and Climate

[7] As the largest river within the Lake Qinghai basin, theBuha River (the term “Buha” means wild yak in the Qinghaidialect) catchment consists of numerous minor tributaries,most originating from the Qilian Mountains (Figure 1b). Theriver source is situated at an altitude of more than 4600 mabove sea level in the Shule South Mountain (QilianMountains). The upper two major tributaries are Xiarige Quand Yangkang Qu (Qu = river) which join to form the BuhaRiver (Figure 1b). The total river length is 286 km and itflows through relatively flat fluvial plains and deltas beforefeeding into Lake Qinghai. The river is not dammed and theimpact of human influences on the catchment is limited.[8] The river drains an area of 14337 km2, nearly half of the

total Lake Qinghai basin area (29660 km2), and it contributesmore than half of annual water input to the lake [Li et al.,2007]. The water discharge in 2007 was 10.05 × 108 m3

and the average annual discharge between 1956 and 2007was9.08 × 108 m3/yr (data from the Buha River HydrologicalStation, Figure 1). The peak discharge is during the mon-soon season. In late August 2007, this reached a maximumof 220 m3/s, whereas the base flow discharge during the dryseasons was 2–10 m3/s. The Buha River supplies more than70% of the total detrital input to the lake, accompanying itshigh water discharge [Colman et al., 2007]. Vegetation atpresent is dominated by montane shrub and alpine meadow.

JIN ET AL.: SEASONAL WEATHERING, DUST TO RIVER WATER F04006F04006

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[9] Seasonal climatic variability in the Lake Qinghai basinis linked with the alternation between the monsoon season(influenced by the East Asian and the Indian Monsoons)and the Westerly Jet Stream. This alternation affects winddirection and strength, and precipitation patterns. Based onthe records from the local meteorological station (TianjunCounty), the mean annual temperature is −0.7°C and exhibitsremarkable seasonality. The mean monthly temperaturevaries from ca −11°C in winter to over +13°C in summer(mean over 1957–2007). With much sunshine (3640 h/yr)and high insolation (∼6.5 × 1015 J/m2/yr), the annual averagepotential evaporation is 1650 mm/yr. The mean annual pre-cipitation is 340 mm/yr, of which about 77% occurs betweenJune and October, resulting in 80–85% discharge during thisperiod. Averaged annual wind speed is 3.8 m/s at 10 mheight above the surface, dominated by northwestwardwinds during 1957–2007, with an average of 59 d/yr withstrong wind (≥ 18 m/s). The number of dust storm daysranged from 8 to 20 d/yr during this period.[10] Figures 2 and 3 show variations in daily precipitation,

daily air temperature, and daily water discharge in 2007 atthe Buha River Hydrological Station where the river watersamples were collected. The monsoon season is distin-guished from other periods by its high temperature, rela-

tively continuous rainfall and higher discharge. In 2007,more than 85% of precipitation and water dischargeoccurred during the monsoon season (middle June to middleOctober) and the spring (March to early June) was charac-terized by increased air temperature and sporadic rain.

2.2. Geology

[11] Situated on the southern edge of the Qilian Fold Belt,Lake Qinghai is a closed piggyback basin surrounded byseveral mountain ranges [Bian et al., 2000]. These moun-tains, with general elevations above 4000 m, consist ofUpper Paleozoic marine limestone, schist and sandstone,Triassic granite, and Mesozoic diorite and granodiorite[Zhang et al., 2009]. The distribution of these rocks isassociated closely with NWW, NNW, and N–S trendingfaults (Figure 1b) [Bian et al., 2000]. Fault escarpments andlacustrine sediment terraces have been extensively devel-oped along the southern shore.[12] The Buha River catchment comprises hummocky

terrain of predominantly Permian marine limestone andsandstones, Silurian sandstones and schist (Figure 1b).Mesozoic granites outcrop in the northern part of thecatchment near the shores of Lake Qinghai. The river hascreated fluvial plains and delta along the western shores.

Figure 1. (a) Sketch map showing the location of Lake Qinghai and the surrounding region, includingthe Tibetan Plateau, the Chinese Loess Plateau, the Qaidam and Tarim Basins, and the Gobi desert.(b) Geological map showing lithological units, the configuration of the Lake Qinghai drainage basin, andmajor rivers feeding the lake (modified after Jin et al. [2009b]). The sampling sites for groundwater (fromdrinking wells) along the Buha River are shown, with unlettered sample numbers prefixed by QH inTable 2. The Lake Qinghai catchment boundary is shown as dashed lines.


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Loess is distributed sporadically on some outwash terracesand is thought to have been deposited in the middle to lateHolocene [Porter et al., 2001]. Within the catchment, smallmodern glaciers occur on mountains in the upper BuhaRiver drainage basin.

3. Samples and Analysis

[13] A total of 53 river water samples were collectedweekly at the Buha River Hydrological Station between31 December 2006 and 30 December 2007. This station is

located in the lowest reaches of the river where waters frommost tributaries converge (Figure 1b), and so it represents theoutflow of the entire Buha River catchment. Seven rainwaterspecimens were collected at the station in 2007 and 2008. Inaddition, fifteen samples of groundwater were also collectedfrom drinking wells (with an average depth 4.2 m) along thecourse of the river during 2007 and 2009.[14] At each collection site, water temperature and pH

were measured synchronously. The water (including rain-water) samples were filtered in situ through 0.2 mm What-man nylon filters. One 30 mL filtered unacidified sample

Figure 2. Weekly variations in concentrations of major cations (Ca2+, Mg2+, Si4+, and Na+), anions(Cl−, SO4

2−, and HCO3−), total dissolved solid (TDS), pH, and calcite saturation index in the river waters

at the Buha River Hydrological Station during 2007. Daily water discharge and precipitation are shownfor correlating the seasons. The monsoon season is the period between the two dashed lines. Waterdischarge of the Buha River varies by two orders of magnitude between the monsoon season and otherperiods of the year. About 85% of the water in the Buha River flows during the monsoon season (Juneto October) in 2007. The 25 March sample (BH07‐13) is affected by melting water from a large snow-fall (see text) and has decreased concentrations of all ions but Ca2+.


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was collected for anion analysis, and one 60 mL sample forcation analysis was collected into a polyethylene bottleprecleaned with 6 M quartz distilled HNO3 and acidified topH < 2. Bottles were wrapped with a parafilm strip aroundthe closure to ensure no leakage. All samples were keptchilled until analysis.[15] Major cations and Sr2+ were analyzed by Leeman

Labs Profile inductively coupled plasma atomic emissionspectroscopy (ICP‐AES) at the State Key Laboratory of LakeScience and Environment. Major anions (F−, Cl−, and SO4

2−)were determined using a Dionex‐600 ion chromatographyat the Institute of Earth Environment, CAS. The averagereplicate sample reproducibility was 0.5–1% (2s). Alka-linity (expressed as HCO3

− in Tables 1–3) was measured byacid titration.[16] For Sr isotope analysis, water samples containing

100 ng Sr were evaporated to dryness in ultraclean Teflonvessels and redissolved in 0.1 mL 3 M HNO3. Strontium inthe solution was then separated from other ions by passingthrough an Eichrom SrSPEC exchange column (0.5 mL bedvolume each column) preconditioned with 3 M HNO3 andeluted with 4 mL UHQ (ultra high quality) deionised water.The total Sr blank level was less than 0.1 ng. The elutedsolutions were acidified by twice subboiling HNO3 to 3 M

before measuring strontium isotopic compositions. Allpreparation procedures for Sr isotopic compositions werecarried out in a Class 1000–10000 clean room. 87Sr/86Srwas measured by a ThermoFisher Neptune MulticollectorInductively Coupled Plasma Mass Spectrometer in the Iso-tope Geochemistry Lab at the National Cheng Kung Uni-versity, Taiwan. The analytical methods employed broadlyfollowed those byWang et al. [2010]. During the runs, 0.3 NHNO3 was replaced with H2O and an additional washingstep using a 0.05 N NH4OH solution was used for every5 samples to reduce memory effects. To further reduce theblank contribution, the averaged background intensitydetermined using a 0.3 N HNO3 blank was subtracted beforeprocessing the data using standard sample bracketingapproach. Before data collection, baselines were measuredwith idling and counting times of 10 and 20 s, respectively.One isotopic measurement consisted of 60 measurements in10 blocks. The measured 87Sr/86Sr ratio was normalized(assuming 86Sr/88Sr = 0.1194), and the mean 87Sr/86Sr ratioof the NBS 987 standard (recommended value = 0.710245)obtained for reproducibility during analysis was 0.710248 ±0.000007 (2s, n = 39).

4. Results

4.1. Major Ion Compositions

[17] Major ion and Sr2+ concentrations, pH, and 87Sr/86Srof the weekly samples of river waters are summarized inTable 1 and Figures 2 and 3. Total dissolved solids (TDS) ofthese samples vary from 245 to 446 mg/L (Table 1) with thehighest TDS in February and the lowest in June (Figure 2).In the ternary diagrams (Figures 4b and 4c), the dominanceof HCO3

− is shown as clustering of points near the HCO3−

apex. The Buha River waters are alkaline, characterizedby Ca‐(Na)‐HCO3‐type, reflecting the dominance of thecarbonate‐rich sedimentary rocks within the catchment. ThepH values of the river waters are uniformly high, rangingfrom 7.94 to 8.53, reflecting the dominance of limestonedissolution. Some of the pH variations might be associatedwith secondary carbonate precipitation. Saturation index (SI)calculated using Geochemist’s Workbench v.8.0 [Bethkeand Yeakel, 2009] indicates that the waters in the BuhaRiver are supersaturated with respect to both calcite (CSI)and dolomite (Table 1). The CSI is correlated positivelywith the pH values. Such a correlation can be viewed asevidence for selective removal of Ca2+ and HCO3

− by calciteprecipitation [Jacobson et al., 2002], resulting in decreasedpH and CSI in the waters during the dry seasons (Figure 2).Most Himalayan rivers are also reported to be saturated withrespect to calcite even in the monsoon [Sarin et al., 1989;Galy and France‐Lanord, 1999; Tipper et al., 2006]. Of themajor cations, all but Ca2+ display similar seasonal variationas that of the TDS. Concentrations of the major cationsdecrease to minima in the early monsoon season (middleJune to middle July) (Figure 2). The low values can beattributed to dilution by the increased discharge during themonsoon season, although the decrease is not in 1:1 pro-portion to the increase in discharge. Water discharge variesby a factor of up to 30 within the year (Figure 2). It isnoticeable that concentrations of the major cations tend toincrease gradually as the monsoon intensifies, rather thansimply reflecting dilution (Figure 2). By contrast, Ca2+

Figure 3. Weekly variations in Sr2+ concentrations and87Sr/86Sr ratio of the Buha River waters during 2007. Dailywater discharge and air temperature are shown for correlat-ing the seasons. The monsoon season is the period betweenthe two dashed lines.


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Tab

le1.

Major

Cations,Anion

s,SrIsotop

e,andSaturationIndexDataforBuh

aRiver

Feeding

LakeQingh

ai,Sam

pled

Weeklyin

2007

Sam

ple

Sam

pling

Date

Season

pHT

(°C)

Na+

(mmol/L)

K+

(mmol/L)

Ca2

+

(mmol/L)

Mg2

+

(mmol/L)

Si4+

(mmol/L)

Sr2+

(mmol/L)

F−

(mmol/L)

Cl−

(mmol/L)

NO3−

(mmol/L)

SO42−

(mmol/L)

HCO3−

(mmol/L)

TDSa

(mg/L)

NICBb

(%)

87Sr/86Sr

SaturationIndexc

Calcite

Dolom

ite

BH07‐01

31Dec

winter

8.41

0.0

733

7415

2651

392

5.15

7.72

571

–28

331

3333

212

.59

0.71

0957

0.76

81.82

2BH07‐02

7Jan

winter

8.35

0.0

938

3215

1851

191

5.15

7.03

637

–28

527

8331

620

.64

0.71

0941

0.65

71.60

0BH07‐03

14Jan

winter

8.3

0.8

721

108

1530

514

945.00

8.37

545

–27

130

5432

615

.78

0.71

0952

0.66

01.61

2BH07‐04

21Jan

winter

8.3

2.5

726

108

1552

519

935.08

9.69

700

–26

629

3932

516

.18

0.71

0971

0.67

41.64

9BH07‐05

28Jan

winter

8.15

2.8

740

4715

6652

493

5.11

7.71

540

4628

928

9831

819

.15

0.71

0999

0.52

71.35

6BH07‐06

4Feb

winter

8.01

3.8

729

4515

3551

392

5.08

7.46

553

4228

928

3631

318

.54

0.71

0917

0.38

41.07

6BH07‐07

11Feb

winter

8.00

1.9

718

4115

2651

592

5.12

9.96

385

5220

927

5029

326

.61

0.71

0914

0.33

60.97

0BH07‐08

18Feb

winter

8.13

2.1

719

4115

3651

591

5.12

7.59

523

4027

628

8031

318

.65

0.71

0942

0.48

81.27

3BH07‐09

25Feb

spring

8.12

2.5

2495

107

1303

802

139

6.16

14.17

1250

234

480

3618

445

14.45

0.71

0874

0.47

91.52

8BH07‐10

4Mar

spring

7.95

3.5

2481

9612

9980

813

76.18

10.90

1129

228

431

3677

439

16.54

0.71

0854

0.33

01.24

0BH07‐11

11Mar

spring

7.94

3.0

2502

9713

1982

113

85.96

14.16

1227

207

466

3651

446

15.54

0.71

0948

0.31

31.20

4BH07‐12

18Mar

spring

7.98

6.0

2498

9613

1581

713

76.01

14.15

1087

197

418

3528

428

20.52

0.71

0956

0.38

61.36

9BH07‐13

25Mar

spring

8.21

7.1

748

3914

8351

188

4.97

9.55

471

5724

528

9030

719

.35

0.71

0939

0.63

01.60

4BH07‐14

1Apr

spring

8.06

6.5

2134

8113

0873

612

75.75

10.34

1103

190

437

3688

429

10.12

0.71

1053

0.49

31.54

3BH07‐15

8Apr

spring

8.10

7.2

1994

8012

9470

912

05.63

11.89

1005

180

407

3576

411

11.27

0.71

1060

0.53

01.60

9BH07‐16

15Apr

spring

8.16

7.4

1762

7513

0366

011

35.43

10.53

872

161

377

3423

388

12.39

0.71

1056

0.58

21.68

1BH07‐17

22Apr

spring

8.06

10.6

2130

8213

0473

912

45.78

11.80

873

199

352

3556

404

18.50

0.71

1059

0.54

21.67

0BH07‐18

29Apr

spring

8.07

9.5

2003

8012

8571

212

05.67

10.80

1038

193

422

3571

413

10.27

0.71

1070

0.52

91.62

7BH07‐19

6May

spring

8.09

10.7

1988

7812

7870

112

05.59

10.91

1007

191

408

3523

407

11.25

0.71

1065

0.56

01.69

3BH07‐20

13May

spring

8.13

10.0

1784

7512

6066

611

15.44

10.26

954

169

412

3334

387

10.49

0.71

1076

0.56

41.67

9BH07‐21

20May

spring

8.22

12.4

1526

6612

6861

110

55.26

8.70

848

121

378

3052

356

12.97

0.71

1059

0.65

91.84

6BH07‐22

27May

spring

8.29

12.8

1243

5912

6355

897

4.97

8.47

741

9035

727

8332

514

.28

0.71

1075

0.70

01.89

1BH07‐23

3Jun

spring

8.20

11.4

1441

6412

5358

610

55.17

9.93

752

133

337

3026

344

14.10

0.71

1004

0.62

11.74

8BH07‐24

10Jun

summer

8.53

14.2

510

4312

5640

679

4.35

7.42

388

4124

228

0428

15.18

0.71

0972

0.97

02.30

5BH07‐25

17Jun

summer

8.43

15.8

484

4112

5239

281

4.27

6.67

403

4024

823

1125

215

.81

0.71

0782

0.81

71.99

4BH07‐26

24Jun

summer

8.50

13.6

503

4412

9841

383

4.23

10.95

345

4221

424

7025

918

.29

0.71

0760

0.89

72.14

6BH07‐27

1Jul

summer

8.46

15.4

428

4212

4138

089

4.07

8.40

352

4221

324

3925

513

.34

0.71

0645

0.86

92.08

5BH07‐28

8Jul

summer

8.42

15.8

484

4211

8940

385

4.26

8.75

398

3724

722

0324

516

.58

0.71

0862

0.77

41.94

3BH07‐29

15Jul

summer

8.50

16.0

420

4012

3937

091

3.99

8.23

345

3920

426

1026

48.56

0.71

0684

0.94

32.22

8BH07‐30

22Jul

summer

8.41

11.0

597

5312

8045

884

4.36

10.32

473

4431

726

5429

08.85

0.71

1169

0.79

51.97

7BH07‐31

29Jul

summer

8.30

16.3

653

4612

4147

678

4.63

10.36

505

3933

826

1529

08.15

0.71

1178

0.74

21.93

6BH07‐32

5Aug

summer

8.26

19.5

678

4513

0449

482

4.97

10.67

511

–33

927

6730

48.40

0.71

1064

0.79

02.04

8BH07‐33

12Aug

summer

8.26

20.2

668

4512

9048

582

4.97

11.13

510

1933

726

3829

410

.34

0.71

1057

0.77

72.02

2BH07‐34

19Aug

summer

8.12

11.8

674

5314

7853

285

5.25

9.37

592

6139

829

9533

47.67

0.71

1081

0.62

01.63

5BH07‐35

26Aug

summer

8.18

11.0

675

5414

7754

285

5.25

10.49

587

5639

529

4333

19.38

0.71

1082

0.66

01.71

9BH07‐36

2Sep

autumn

8.10

12.6

655

4115

7254

087

5.50

10.27

566

2439

333

1035

55.24

0.71

0962

0.67

61.73

4BH07‐37

9Sep

autumn

8.22

9.9

651

4314

3153

784

5.29

10.70

552

4839

830

9533

64.04

0.71

1082

0.69

21.78

6BH07‐38

16Sep

autumn

8.25

6.3

638

4214

3653

286

5.25

11.49

544

114

395

3100

336

3.94

0.71

1071

0.67

21.71

7BH07‐39

23Sep

autumn

8.14

6.4

645

4214

9253

184

5.34

10.77

546

3939

529

6833

09.06

0.71

1069

0.56

21.47

9BH07‐40

30Sep

autumn

8.15

7.0

628

4114

4652

182

5.19

10.27

541

5638

429

9532

86.50

0.71

0919

0.57

31.50

9BH07‐41

7Oct

autumn

8.12

6.4

654

4715

6354

586

5.61

10.37

564

3139

031

9534

87.69

0.71

0944

0.59

01.52

6BH07‐42

14Oct

autumn

8.14

5.7

638

4215

0550

883

5.35

10.70

567

3938

329

4532

89.09

0.71

0970

0.55

31.43

3BH07‐43

21Oct

autumn

8.09

7.1

657

4415

2754

086

5.52

10.32

565

2538

729

6733

210

.94

0.71

0967

0.53

11.41

9BH07‐44

28Oct

autumn

8.05

5.1

701

4715

6956

494

5.59

10.93

602

5839

031

8535

08.92

0.71

0945

0.50

01.35

1BH07‐45

4Nov

autumn

8.04

4.6

945

4714

1054

111

15.61

11.80

676

5228

430

6533

511

.95

0.71

0954

0.42

81.23

0BH07‐46

11Nov

autumn

8.11

4.4

943

4814

0054

211

15.51

11.52

679

6328

431

2333

810

.36

0.71

0982

0.50

01.37

8BH07‐47

18Nov

autumn

8.13

2.1

734

4315

4155

096

5.47

11.19

582

5733

429

3832

915

.55

0.71

0929

0.50

01.32

5


6 of 16

Tab

le1.

(con

tinued)

Sam

ple

Sam

pling

Date

Season

pHT

(°C)

Na+

(mmol/L)

K+

(mmol/L)

Ca2

+

(mmol/L)

Mg2

+

(mmol/L)

Si4+

(mmol/L)

Sr2+

(mmol/L)

F−

(mmol/L)

Cl−

(mmol/L)

NO3−

(mmol/L)

SO42−

(mmol/L)

HCO3−

(mmol/L)

TDSa

(mg/L)

NICBb

(%)

87Sr/86Sr

SaturationIndexc

Calcite

Dolom

ite

BH07‐48

25Nov

autumn

8.14

1.6

717

4315

0253

994

5.32

11.34

550

5531

630

9533

311

.67

0.71

0962

0.51

51.35

5BH07‐49

2Dec

winter

8.17

3.7

690

3914

0551

889

5.08

9.43

534

6029

828

3831

013

.27

0.71

0981

0.51

51.38

1BH07‐50

9Dec

winter

8.25

2.5

688

3914

1951

889

5.10

10.76

527

5930

029

6731

811

.02

0.71

1010

0.60

01.53

8BH07‐51

16Dec

winter

8.26

1.6

677

4014

0451

288

5.10

10.55

528

5529

930

9532

57.21

0.71

1043

0.61

01.55

1BH07‐52

23Dec

winter

8.38

2.3

679

3913

9551

086

5.05

10.76

540

6130

129

8731

88.81

0.71

0993

0.72

11.78

2BH07‐53

30Dec

winter

8.42

3.0

680

3913

9951

087

5.02

10.14

522

5329

729

2831

410

.87

0.71

1029

0.76

41.87

1

a Total

dissolvedsolid

s.bNormalized

inorganiccharge

balance(N

ICB)=(TZ+−TZ− )/TZ+,where

TZ+=Na+

+K++2M

g2++2C

a2+,TZ−=Cl−

+2S

O42−+HCO3−in

mEq.

c Saturationindexof

calcite

anddo

lomite

was

calculated

usingGeochem

ist’sWorkb

ench

v.8.0[Bethkean

dYeakel,20

09].

Tab

le2.

Major

Cations

andAnion

sandSrIsotop

eDataforGroun

dwater

Sam

ples

Collected

With

intheBuh

aRiver

Tribu

tary

Sam

ple

Lon

gitude

(E)

Latitu

de(N

)Date

Na+

(mmol/L)

K+

(mmol/L)

Ca2

+

(mmol/L)

Mg2

+

(mmol/L)

Si4+

(mmol/L)

Sr2+

(mmol/L)

F−

(mmol/L)

Cl−

(mmol/L)

NO3−

(mmol/L)

SO42−

(mmol/L)

HCO3−

(mmol/L)

NICBa

(%)

87Sr/86Sr

pH

QH08‐10

98°50′21

.5″

37°21′53

.0″

18Jul20

0886

149

1297

513

804.73

5.28

762

6849

532

33−1

0.04

0.71

1593

8.06

QH08‐13

98°30′44

.8″

37°21′38

.2″

19Jul20

0855

841

1617

551

121

5.96

8.17

478

4926

446

64−1

4.89

0.71

0706

8.54

QH07‐23‐1

99°43′30

.0″

37°01′44

.5″

28Jul20

0773

862

1536

498

122

5.21

8.48

525

2326

427

5421

.80

0.71

0984

8.25

QH‐69

99°28′30

.0″

37°08′10

.2″

25Aug

2009

707

3814

2153

893

5.11

7.38

689

6642

931

46−0

.64

0.71

1152

7.50

QH‐70

99°23′08

.4″

37°08′58

.5″

25Aug

2009

714

6814

9959

711

25.86

11.02

623

7447

433

920.22

0.71

1488

7.68

QH‐71

99°15′38

.9″

37°10′52

.2″

25Aug

2009

1451

4413

7978

513

06.66

15.31

1184

167

457

3589

2.34

0.71

0880

7.70

QH‐72

99°11′44

.5″

37°12′26

.1″

25Aug

2009

1268

4116

3964

811

36.48

8.77

930

156

425

3835

4.56

0.71

0892

7.79

QH‐73

98°58′13

.9″

37°20′03

.0″

26Aug

2009

907

4415

4062

079

5.71

7.38

1100

116

587

2950

0.89

0.71

1831

8.05

QH‐74

98°50′29

.5″

37°21′49

.0″

26Aug

2009

1394

5015

9365

492

5.70

10.29

1074

–54

633

926.40

–7.87

QH‐75

98°46′30

.8″

37°28′04

.8″

26Aug

2009

1492

6721

4088

113

66.14

22.84

1791

181

546

3933

10.33

0.71

1500

7.54

QH‐76

98°37′47

.4″

37°40′35

.8″

26Aug

2009

770

4817

9573

490

4.52

4.58

772

–54

333

4311

.49

–7.75

QH‐77

98°35′54

.7″

37°33′54

.8″

26Aug

2009

1107

3824

8766

311

64.66

13.23

918

5799

944

740.74

0.71

1764

8.43

QH‐78

99°06′11

.8″

37°23′10

.0″

26Aug

2009

1926

4817

3810

7413

27.78

25.22

1643

149

627

4670

0.41

0.71

1493

7.60

QH‐80

99°19′27

.4″

37°20′22

.2″

26Aug

2009

1083

3819

0568

511

58.18

7.89

1178

–40

841

302.81

0.71

0900

7.71

QH‐81

99°21′25

.0″

37°22′28

.6″

26Aug

2009

1615

5021

0291

613

414

.07

17.06

1500

184

652

4769

1.66

–7.63

a Normalized

inorganiccharge

balance(N


TZ+=Na+

+K++2M

g2++2C

a2+,TZ−=Cl−

+2S

O42−+HCO3−in

mEq.


7 of 16

concentrations are relatively low and show a fairly narrowrange (1.19–1.32 mmol/L) during March and mid‐August(Figure 2), with the lowest in early July. The highest value(1.57 mmol/L) in Ca2+ is found on 2 September 2007 whenthe monsoon intensity reached its maximum.[18] Anion concentrations in the waters show similar

seasonal variation to the cations and TDS (Figure 2). Alongwith the low Ca2+ during spring (March to early June),HCO3

− concentrations also decrease. The river waters havemuch higher Cl− than that in rainwater (up to 10 times).Although the rainwater may contribute evaporitic elements[Meybeck, 1987; Négrel et al., 1993], sea‐salt aerosol has aminor influence on local rainwater because Lake Qinghai isremote from the oceans. The high Cl− and SO4

2− in the rivertherefore must originate from evaporite dissolution or sul-fide oxidation [Jin et al., 2009b] under high evaporationconditions. A strong correlation and the ratio close to unitybetween Na+ and Cl− of the rainwater and the Buha Riverwaters (Figure 5, except spring samples, see discussionbelow) further indicates that Na+ and Cl− are derived fromevaporite sources. Dissolved SO4

2− can be derived fromeither dissolution of CaSO4 minerals in sedimentary eva-porites or from oxidation of sulfide minerals. A positiverelationship between Cl− and SO4

2− (not shown) of monsoonseason samples indicates an evaporitic origin of both anions[e.g., Hren et al., 2007]. The low concentration of nitrate andhence low NO3/Na further indicate very limited anthropo-genic contribution [e.g., Roy et al., 1999] within the catch-ment, consistent with the sparse population. The three tofourfold increase in NO3

− of the spring river waters (Table 1)indicates potential atmospheric input.[19] Groundwater data are given in Table 2. Although the

groundwater samples were collected from a short timeperiod (July to August), extensive samplings of groundwaterin addition to its expected slow response to changes insurface processes and conditions mean that this chemistry islikely to be representative. In the Buha River hydrosystem,as in many watersheds [e.g., Durand et al., 2005], thegeochemical characteristics of groundwater samples arequite spatially variable. The groundwaters are HCO3‐Cl‐Natype and exhibit a wide range of elemental concentrations(Table 2), probably resulting from variable weatheringintensities owing to long periods of water–rock interaction[Evans et al., 2001]. Although the ionic compositions andTDS for most of the groundwaters are higher than that ofthe river waters, groundwaters and river waters share similargeochemical characteristics, evolving from the carbonate‐and evaporite‐rich sedimentary rocks of the aquifers. Con-sequently, river and groundwaters overlap in the ternarydiagrams (Figures 4b and 4c).[20] Inspection of geochemical plots reveals that the river

waters collected in the spring (Table 1) are distinctive fromother seasons. First, concentrations of all cations show apulse‐like distribution in spring and decrease gradually afterthis (Figure 2). Second, large increases in Na+ and also Cl−

in the spring samples (red line in Figure 5) deviate from the1:1 line and the array defined by nonspring and groundwaterwaters, suggesting a different source (see discussion insection 5.1). Last, molar Na/ion ratios jump by a factor of∼2 from winter to spring, remain at relatively high ratios, andthen rapidly return to lower values with onset of monsoon(Figure 6). These distinct changes in composition suggest anT

able

3.Major

Cations

andAnion

sandSrIsotop

eDataforRainw

ater,Sam

pled

attheBuh

aRiver

Hyd

rologicalStatio

n

Sam

ple

Sam

pling

Date

Na+

(mmol/L)

K+

(mmol/L)

Ca2

+

(mmol/L)

Mg2

+

(mmol/L)

Si4+

(mmol/L)

Sr2+

(mmol/L)

F−

(mmol/L)

Cl−

(mmol/L)

SO42−

(mmol/L)

HCO3−

(mmol/L)

TDSa

(mg/L)

NICBb

(%)

87Sr/86Sr

CSIc

QH07

‐rw‐1

10Oct

2007

6712

442

4121

1.23

8.52

5874

915

86.3

−7.27

0.71

1029

−0.75

5‐F‐01

25May

2008

136

6479

083

911.76

11.59

4994

1720

157.7

−0.57

0.71

0985

−0.25

5‐F‐02

22Jun20

0851

931

326

80.52

n.a.

5757

601

59.0

−4.61

0.71

0768

−0.91

5‐F‐03

27Jun20

0813

322

761

5817

2.53

0.65

111

4918

5615

8.1

−15.17

0.71

0952

−0.09

5‐F‐04

3Jul20

0868

5790

832

0.23

n.a.

3425

265

28.6

−8.72

0.71

1216

−1.19

5‐F‐05

15Jul20

0850

2759

042

741.07

n.a.

3471

1272

114.5

−7.98

0.71

1059

−1.78

5‐F‐06

16Jul20

0838

719

621

120.52

n.a.

1331

489

43.1

−17.75

0.71

0921

−0.34

a Total

dissolvedsolid

s.bNormalized

inorganiccharge

balance(N


TZ+=Na+

+K++2M

g2++2C

a2+,TZ−=Cl−

+2S

O42−+HCO3−in

mEq.

c Calcite

saturatio

nindex.


8 of 16

addition of different sources to the spring river waters,besides weathering contributions from various lithologieswithin the tributaries.[21] Rainwater is one of the potential contributors to the

dissolved elements in river waters [e.g., Négrel et al., 1993;Gaillardet et al., 1999]. The rainwaters collected within thecatchment show significant variability in chemical compo-sitions. The TDS of these samples vary by a factor of aboutfive, from 28.6 to 158.1 mg/L (Table 3). Both anions andcations also show variations by a factor of three to nine. The

rainwater falls closer to the Ca2+ and HCO3− apexes than the

river waters (Figure 4), but all rainwater is undersaturatedwith respect to calcite (Table 3).

4.2. Sr Concentration and Sr Isotopic Ratio

[22] Sr2+ concentrations of the river water samples varyfrom 3.99 to 6.18 mmol/L. The temporal variation in the Sr2+

concentrations follows those of other cations and anions

Figure 6. Weekly variations in molar ratios (Na/Cl, Sr/Ca,Na/Ca, and Na/Sr) of the Buha River waters during 2007.The monsoon season is the period between the two dashedlines.

Figure 4. Ternary diagrams for cations and anions in river and rainwater samples from the Buha RiverHydrological Station, indicating dominance of Ca2+ and HCO3

−, especially in rainwater. (a) Mg−Ca−Na+K,(b) HCO3−SO4−Cl, and (c) HCO3−Si−SO4+Cl. Also shown are groundwater samples collected along thecourse of the river.

Figure 5. Scatter diagram of molar Na+ versus Cl− for 53river, 7 rain, and 15 groundwater samples from the BuhaRiver drainage basin. The river waters collected during thespring, with the exception of the sample BH07‐13, show astrong correlation (r2 = 0.83, red line) with high Na+ (Na/Cl = 2.01 ± 0.20) and offset from 1:1 line, indicating a dif-ferent source. This is attributed to eolian dust input (seetext). The other waters, including groundwaters, show agood correlation (r2 = 0.90, dashed black line). The ground-waters alone also correlate well (r2 = 0.79, solid blue line).Both are close to the 1:1 line, indicating evaporite inputs tononspring river waters and groundwater.


9 of 16

(Figure 3). At the beginning of the monsoon, Sr2+ shows adramatic decrease in water from 5.17 to 4.35 mmol/L, with aminimum value in the middle of July. Following theincreasing precipitation and temperature after this, Sr2+

concentrations increase gradually to high values at the end ofthe monsoon. The 87Sr/86Sr values of the samples over theyear range from 0.710645 to 0.711178. It is noticeable thatboth the lowest and the highest values of 87Sr/86Sr of theBuha water occurred in the monsoon season (Table 1 andFigure 3). The 87Sr/86Sr ratios of the samples from post-monsoonal seasons vary much less in comparison (Figure 3).The Sr isotopic compositions of the spring river waters varywithin the range of monsoon samples.[23] Sr2+ concentrations and 87Sr/86Sr of the groundwater

samples vary mostly between 4.5 and 14.1 mmol/L, andbetween 0.710706 and 0.711831, respectively (Table 2).Most of the groundwaters within the Buha River catchmenthave higher 87Sr/86Sr ratios than the river waters (Tables 1and 2), probably resulting from a long duration of rock‐water interaction [Négrel et al., 2001; Jin et al., 2009b].Meanwhile, five groundwater samples have 87Sr/86Sr ratiosclose to those of the river waters, potentially resulting fromsurface water supply.[24] Strontium isotopes of rainwater can help determine

the origin of aerosols and identify source mixing [e.g.,Dupré et al., 1994; Chabaux et al., 2005]. Sr2+ concentra-tions of the rainwater samples vary by a factor of ∼5, from0.52 to 2.53 mmol/L. 87Sr/86Sr of the rainwater samplesvaries mostly between 0.710768 and 0.711059 (Table 3),implying a mixing of different sources with different87Sr/86Sr. In the Lake Qinghai area, dissolution of mineralaerosols is suggested to be important for rainwater chem-istry, increasing the 87Sr/86Sr and Na/Cl ratios. Likewise,analyses of rainwater over Japan showed that the dissolutionof dust elevates 87Sr/86Sr ratios up to 0.711 [Nakano andTanaka, 1997; Kanayama et al., 2002].

5. Discussion

5.1. Sources of Dissolved Components to the BuhaRiver

[25] The chemical compositions of river waters are deter-mined primarily by the contributions from various lithologieswithin the tributaries [Palmer and Edmond, 1992; Gaillardetet al., 1999; Noh et al., 2009], and potentially from atmo-spheric, groundwater, hydrothermal spring, and anthropo-genic inputs [Gorham, 1961; Négrel et al., 1993; Durandet al., 2005; Jin et al., 2009b]. On the basis of seasonalvariations in weekly water chemistry of the Buha River over2007, three main observations are (1) a sudden increase indissolved components (opposite to Ca2+) at the beginning ofthe spring, with a sudden event on 25 March related to asnow storm, (2) an abrupt concentration decrease in solutesand 87Sr/86Sr at the beginning of the monsoon period fol-lowed by a sharp increase when the monsoon reaches itsmaximum, and (3) relatively little change during the rest ofthe year (Figures 2 and 3). These geochemical variationsover the seasons are proposed to reflect changes either ininput sources or in the intensity of the catchment weatheringin the context of discharge dilution.[26] Waters in the Buha River are supersaturated with

respect to calcite and dolomite, so concentrations of Ca2+,

Mg2+, and Sr2+ may not be conservative. Since both Na+

and Cl− in river water are soluble and conservative, thevariation of Na+ and Cl− is employed to assess sources andtheir mixing, because the effects of dilution/evaporation onboth Na+ and Cl− are the same. Several important obser-vations appear when comparing the Na+ and Cl− composi-tions and ratios in river waters with those of groundwaterand rainwater. First, a distinct array for the spring riverwaters is well illustrated by well correlation between Na+

versus Cl− (solid red line, r2 = 0.83; Figure 5), with higherNa/Cl ratios (2.01 ± 0.20). The array is offset from the 1:1line of Na+ and Cl−, with an exception of sample BH07‐13(Figure 5, see discussion below). The excess of Na+ relativeto Cl− is also found in some rainwater samples (Table 3) andin the water‐dissolved fraction (Na/Cl = 1.99) of one dustsample we collected within the catchment during the springof 2009. Second, Na+ versus Cl− of the rain and nonspringriver waters are close to the 1:1 line and are well correlated(solid black line, r2 = 0.90, Figure 5), with Na/Cl ratios of1.27 ± 0.09 for nonspring river waters. Third, although thegroundwaters have higher Na+ and Cl− concentrations thanthe nonspring river waters, they have Na/Cl ratios (1.12 ±0.18) close to unity and share the same linear array definedby the nonspring river waters (Figure 5). This observationsuggests that groundwaters and nonspring river waters havegeochemical signatures related to the lithological char-acteristics of the aquifers.[27] These observations indicate that a likely cause to

explain the high Na/Cl ratios in the spring river waters issupply from an additional source, because their chemistrycannot be explained by changes in weathering rates of sili-cates and/or carbonates alone given that the climatic con-dition is similar among nonmonsoonal seasons. As observedabove, groundwaters have Na/Cl ratios close to those of thenonspring river waters and share the array defined by thenonspring river and rainwaters, indicating that groundwaterinput can be excluded. Obviously, a groundwater contribu-tion to river waters only during the spring is unlikely, as isan anthropogenic input. Therefore, we suggest that theseparation of the spring river waters from other seasons andgroundwater might be controlled principally by the input ofeolian dust (see section 5.2), when the westerly winds sweepacross central Asia during the spring.[28] The sample BH07‐13, collected on 25 March 2007,

deviates from the general variations (Figures 2 and 5), withdecreased concentrations for most of the dissolved con-stituents. This sample has highest Ca2+ among the springsamples but low concentrations of other ions and TDS rela-tive to samples collected before and after (Figure 2).According to quarterly reports of local weather by the Qin-ghai Province Weather Bureau, a large‐scale severe snowfalloccurred within Qinghai and neighboring provinces(including Lake Qinghai) during 14 to 17 March 2007, withsnow accumulation of 10–16 cm thickness. After the snow,daily average air temperature increased rapidly from −17°Cto +2°C (Figure 3). This temperature increase was veryanomalous and has not occurred at this season at any timesince 1952. It would have resulted in a rapid and completemelting of snow and ice. This anomalous weather also led tothe ice out period of Lake Qinghai occurring one week earlierin 2007 than in normal years. During this snow storm event,the downstream of the Buha River showed a concomitant


10 of 16

rapid increase in water discharge by about a factor of two,from 1.40 to 2.62 m3/s. Therefore, this sample can beascribed to the snow melting event after a large‐scalesnowfall, resulting from trapped carbonate dust in the snowcoming down and dissolving in the river.

5.2. Eolian Dust Contribution During Spring

[29] Because of the extreme dryness in inland Asia,intense midaltitude westerlies sweep across the northwesternportion of China every spring, and dust is eventuallytransported by the large‐scale atmospheric circulation toeastern China and beyond [Pye, 1989; Littmann, 1991;Prospero et al., 2002]. A modern “airborne dust corridor,”across Lake Qinghai, for eastward transport of dust isdemonstrated by lidar observations, especially during thespring [Liu et al., 2008]. When northwesterly westerlywinds prevail during the spring (as shown by air mass backtrajectories, Figure 7), eolian dust entrained from aridregions would be deposited within the catchment to formloess deposits surrounding Lake Qinghai [Porter et al.,2001]. Field investigations indicate that dust events occurregularly throughout the spring. Therefore, an eolian sourcecomponent for the spring waters of the Buha River is con-sistent with its geographic and climatic settings.[30] Carbonates and evaporites are prominent components

in Asian dust [Liu, 1985; Gomes and Gillette, 1993].Atmospheric deposition of carbonate‐rich dust (with aver-age of 6.1% carbonate in eolian dust samples from Dun-huang and Aksu [e.g., Wang et al., 2005]) is commonsurrounding the Tibetan Plateau [Wake et al., 1993, 1994;Wu et al., 2005]. Loadings of carbonate have been shown tobe particularly high during the spring [Zhang et al., 1997;Kang and Cong, 2006]. Evaporites are also found com-

monly in the dust source areas. For example, halite (NaCl),mirabilite (Na2SO4·10H2O), borax (Na2B4O7·10H2O) andsylvite (KCl) are exposed widely in the Qaidam Basin[Zheng, 1997]. The excess Na+ relative to Cl− in somerainwater samples (Table 3) and the high Na/Cl ratio (1.99)in the water leaching fraction of one dust sample we col-lected within the catchment during the spring of 2009 mightoriginate from Na‐bearing salts (such as borax, mirabilite)derived from such source areas. These dusts, with highsurface areas and containing highly soluble carbonates andsalts, are prone to partial dissolution in rain and river waters,yielding high dissolved compositions of Ca2+ and associatedelements when the dust is introduced into the system. Undersuch conditions, secondary carbonate precipitation may betriggered by carbonate input from eolian dust in the super-saturated waters [Hren et al., 2007], even when the tem-perature is relatively low. It is likely that calcite precipitationis triggered when increased eolian dust is introduced into theaquatic environment during the spring [e.g., Szramek andWalter, 2004]. The secondary carbonate precipitation even-tually decreases Ca2+ andHCO3

− concentrations and increasespH in waters (Figure 2). Physiochemically, the increasedratios of both Na/Ca and Sr/Ca of the spring river watersrelative to those of other seasons (Figures 6 and 8) can beattributed to (1) increased contribution of Na+ from dust and(2) Ca2+ removal by the precipitation of secondary carbo-nates from the supersaturated water triggered by dust input.[31] Significantly, eolian dust from inland Asia is charac-

terized by homogeneous Sr isotopic composition with moreradiogenic Sr [Sun, 2002; Jacobson, 2004; Yokoo et al.,2004; Rao et al., 2009]. For example, dust particles finerthan 20 mm have 87Sr/86Sr of 0.7130 to 0.7142 in the QaidamBasin and 0.7141 to 0.7145 in the southern Tarim Basin[Sun, 2002]. Eolian dust contains abundant carbonates fromwhich Sr may be easily dissolved, resulting in uniform87Sr/86Sr ratios in the river waters in spring. The relativelylow 87Sr/86Sr but high Sr2+ concentrations at the early spring(Figure 3) might result from more limited dust dissolutionunder cold conditions competing with catchment weather-ing sources, given the high Na+ and hence high Na/Sr inFigure 6. After temperatures increased, namely after thesnow storm event, more eolian dust might have been intro-duced into the system, resulting in a high and homogeneous87Sr/86Sr in river water; slightly low Sr2+ concentrations canbe explained by dilution (Figure 3).[32] Strontium isotopes are a powerful tool for tracing

geochemical and hydrological processes [e.g., Capo et al.,1998; McNutt, 2000; Bickle et al., 2005; Blum and Erel,2004; Jin et al., 2009b]. Strontium has four naturally stableisotopes (84Sr, 86Sr, 87Sr, and 88Sr) and has similar geo-chemical behavior to Ca. However, the Sr isotopic abun-dances in rocks are variable because of different Rb and Srconcentrations and the formation of radiogenic 87Sr by theb decay of 87Rb, leading to large 87Sr/86Sr variations betweendifferent minerals, and higher 87Sr/86Sr ratios in old rockscompared to young ones. Variable 87Sr/86Sr ratios in surfacewaters can be due to different contribution from dissolution ofsilicate, carbonate, and evaporite minerals [Banner, 2004].Since the residence time of waters are sufficiently shortcompared to the half‐life of 87Rb (4.88 × 1010 a), new pro-duction of radiogenic 87Sr in the water can be ignored [Åberget al., 1989; Bickle et al., 2005; Shand et al., 2009], and the

Figure 7. Air mass HYSPLIT backward trajectory (500,1000, and 2000 m above the ground level) (from http://ready.arl.noaa.gov/HYSPLIT.php), indicating the origin ofair masses observed at Lake Qinghai in the spring of2007, with a time interval of 6 h between two marks onthe trajectory tracks, reflecting the wind sources from thedry northwestern portion of central Asia to Lake Qinghai.These winds are likely to bring significant dust to the regionin spring.


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Sr isotopic composition in surface water within continentalcatchments is therefore a function of mixing and/or exchangebetween different reservoirs. Given the effects of dilution/evaporation and carbonate precipitation on elemental con-centrations, Na normalized molar ratios are presented with87Sr/86Sr for further discussion. These tracers provide insightinto both source identification and weathering processes[e.g., Négrel et al., 1993; Stewart et al., 1998; Gaillardetet al., 1999; Roy et al., 1999; Tipper et al., 2006; Rai andSingh, 2007].[33] A distinct source with a high silicate component for

the spring river waters is also well illustrated by 87Sr/86Srversus Na/Ca (Figure 9). Several important observationsappear when comparing the Sr isotopic compositions of riverwaters with those of groundwaters, rainwater and dissolvedloess. Here, we employed the average composition of thewater and acetic acid leachable portion of typical loess as acandidate of dissolved dust end‐member, i.e., 87Sr/86Sr =0.711130 (±0.00021 SE) and Na/Ca = 0.15 (±0.02 SE)[Yokoo et al., 2004]. First, the groundwaters follow the lineararray defined by the nonspring river water samples. Second,the rainwater and the dissolved dust have similar Na/Ca andSr isotopic ratios. This observation indicates that the rain-waters have their chemical characteristics of dissolved car-bonate dust [Wang et al., 2005; Kang and Cong, 2006].Third, the spring river waters have similar Sr isotopic ratiosto the rainwaters and the dissolved component of eolian dust,but they are separated to opposite sides of the array definedby the nonspring river and groundwater samples. Theseobservations further indicate that the separation of the springriver waters from other seasons can result from (1) the pre-cipitation of secondary carbonates triggered by the eoliandust input and/or (2) a relatively high contribution fromsilicate component of the dust, rather than from groundwaterinput. A few river water samples collected during autumnand winter lie close to the spring group (Figure 9), which

indicates that eolian dust might affect episodically riverwater chemistry during other dry seasons.[34] Since the chemical signatures of the eolian dust are

identified in the river water, a number of questions arise:

Figure 9. Plot of 87Sr/86Sr versus molar Na/Ca in riverwaters from the Buha River, along with rain and ground-waters. Also shown is the chemical composition of thewater and acetic acid leachable portion of typical loess[Yokoo et al., 2004] as an end‐member of dissolved eoliandust. The spring river waters (orange shaded area) havesimilar 87Sr/86Sr to dissolved eolian dust standing out fromother seasons, with the exception of the sample BH07‐13,being attributed to eolian dust input (see text). Five summersamples (green shaded area) with much low 87Sr/86Sr arecorresponded to early period of the monsoon season (17 Juneto 15 July). The groundwaters fit well the array defined bynonspring river waters, indicating a potential contribution ofgroundwater sources to river water.

Figure 8. Correlation diagrams for river water compositions collected weekly from the Buha River.(a) Molar Mg/Ca and (b) Sr/Ca versus Na/Ca ratios. Also shown are the elemental ratios of groundwaters.The river water samples during the spring stand out from other seasons shaded in orange, with the excep-tion of the sample BH07‐13. The similar compositions of the groundwaters and the nonspring river watersindicate the potential contribution of groundwater sources to river waters, whereas some groundwatersextend to higher ions/Ca (and higher 87Sr/86Sr), resulting from calcite precipitation and/or more silicateweathering.


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(1) How much is the annual dust contribution to the rivers/lakes? (2) How and how much secondary carbonate pre-cipitation occurs after dust introduction? (3) How can thedust contribution be corrected for when the geochemicaldata of river waters are used to establish weathering massbudgets? Given that Na+ and Cl− concentrations and Na/Clratios are relatively stable during the winter, the eolian dustcontribution to the dissolved load can be roughly estimatedby comparison with the winter samples. Fluxes of Na+ andCl− increase by a factor of 4.5 and 3.0 during the spring ifthere is dust contribution, relative to fluxes in the winter.This results in 8.3% and 5.2% of annual inputs of Na+ andCl− with 2.9% of the total discharge, respectively. Despitecarbonate precipitation, the flux of dissolved Ca2+ in thespring increases 50% relative to that of in winter. The inputof eolian dust during the spring results in an increase flux ofdissolved load by factor of ∼2.2 relative to winter; this fluxis underestimated because of the strong influence of car-bonate precipitation on water chemistry.[35] The introduction of eolian dust to the surface waters

would be important in maintaining primary production andCO2 uptake, which may influence lake biogeochemicalprocesses via nutrient supply (such as iron), and may con-tribute to biogenic sediment flux. Changes in these fluxesmay be crucial to the paleoenvironmental interpretation ofsediment components. Additionally, the enrichment of dis-solved components indicates that eolian dust might be asignificant source of reactive trace elements to surface waters.

5.3. Seasonal Variation in Carbonate VersusSilicate Weathering

[36] The lithological nature of bedrock is known to be oneof key parameters controlling the chemical composition ofriver waters [e.g., Stallard and Edmond, 1987; Huh et al.,1998; Gaillardet et al., 1999; West et al., 2005]. As sug-gested above, the chemistry of Buha River waters in non-spring seasons is determined primarily by the carbonate,silicate and evaporite sources of the catchment bedrock andby varying climatic conditions. In spite of seasonal varia-tions, the major element compositions indicate that over thecourse of the year carbonate weathering is most significantand that silicate weathering is a minor source of the dissolvedloads (Figure 4). The time series information on the chemicalcompositions of dissolved load permits us to address therelative contribution of carbonate versus silicate sources, andparticularly the relationship with hydrological behavior ofthe catchment (Figures 2, 3, and 6).[37] Lake Qinghai lies at the transition from arid to

semiarid climate zones, where the climate is sensitive to theAsian summer/winter monsoons and the westerly. Theregional climate exhibits remarkably high seasonality: dur-ing winter and spring, the climate is cold and windy; duringmonsoon season, high rainfall and high temperature arecoincident. Under such distinct climatic conditions, majorelement chemistry and Sr isotope ratios in river waters showsignificant seasonal variations. These variations are relatedeither to changing sources, or to varying mineral dissolu-tion/precipitation rates under seasonal conditions [Stallardand Edmond, 1987; Négrel et al., 1993; Moon et al., 2007].[38] The seasonal variations in the 87Sr/86Sr and the ele-

mental ratios demonstrate that chemical weathering duringthe monsoon season within the Buha River catchment is

remarkable in response to hydrology. An abrupt decrease in87Sr/86Sr (Figure 3) and decreases in the Na/cation ratios(Figures 6 and 8) in the early monsoon season can beattributed to higher susceptibility to weathering of carbo-nates compared to silicates in the Buha River catchment,given that the late Paleozoic marine limestone has 87Sr/86Srequal to 0.707706 ± 0.000010 whereas the sandstones have87Sr/86Sr equal to 0.713996 ± 0.000009. At the beginning ofthe monsoon season when water discharge increases by upto a factor of 30, a higher proportion of carbonate dissolu-tion would be washed into the river because of the shortinteraction time between water and minerals, coupled withthe fast dissolution kinetics of carbonates. Given that thetemperature rose simultaneously along with water discharge,bicarbonate might be reduced by degassing of CO2, result-ing in high CSI (Figure 2). A similar scenario occurs in theheadwaters of the Ganges [Bickle et al., 2003, 2005; Tipperet al., 2006] and in the Brahmaputra [Rai and Singh, 2007]during the monsoon. On the basis of a synthesis of availabledata, Krishnaswami et al. [1999] also suggested that there isa selective increase in carbonate weathering contributionwith respect to silicates. Similar seasonal variations in dis-solved Sr2+ and its 87Sr/86Sr were observed in the southernHimalayan rivers and were attributed to a greater dissolutionof carbonate relative to silicate at monsoonal runoff becauseof its faster dissolution kinetics [Bickle et al., 2003; Tipperet al., 2006; Rai and Singh, 2007; Wolff‐Boenisch et al.,2009].[39] An abrupt increase in 87Sr/86Sr is evident (Figure 3)

at the peak of the monsoon. This notable increase indicatesthe presence of more silicate weathering sources thancarbonate‐derived ones during the monsoon maximum (atthe time of high temperature and increased runoff). Giventhe slow kinetics of silicate dissolution [e.g., Lasaga et al.,1994], these silicate sources plausibly stem from either(1) enhanced groundwater input induced by increased runofffrom monsoonal precipitation and/or (2) increased meltwaterfrom mountainous glaciers at relative high temperature. Mostof the groundwaters cluster in the same area as the nonspringsamples and the linear arrays they defined (Figures 5 and 9),which might imply a potential role of groundwater input inBuha River water chemistry. Indeed, the observation ofhigher 87Sr/86Sr and Na/Ca ratios in some groundwaters(Figure 8) favors groundwater inputs enhancing the riverinesilicate components during the monsoon peak. An alternativesourcemight be themeltwater of glaciers with higher 87Sr frompreferential weathering of biotite from high Qilian Mountainsduring summer [e.g., Anderson et al., 2000; Bickle et al.,2003].[40] After the monsoon, silicate sources contribute an

increasing fraction of ions to the river waters, resulting fromreduced carbonate dissolution in the drier climate. Thenarrow ranges of water chemistry during autumn and winterindicate a relatively constant contribution of groundwaterduring the dry seasons, if the assumption in terms of mixingholds true. The importance of groundwater in silicateweathering increases relative to carbonate during the dryseason has also been highlighted in the Himalayan rivers[e.g., Tipper et al., 2006].[41] For nonspring waters, therefore, we suggest that

(1) carbonate weathering dominates the water chemistry asprecipitation increases in the early monsoon season and


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(2) the variations of other periods are the result of variationsin contributions of different water masses associated withlocal hydrology and lithology. If the seasonal variation in87Sr/86Sr of the nonspring river waters is assumed to beattributed to the variations in groundwater input, the budgetof groundwater inputs into the Buha River varies with time.Such variation could indicate a variation of flux and/orsources of groundwater, the latter being suggested by therange of 87Sr/86Sr ratios of groundwaterswithin the catchment.In this scenario, understanding the origin and chemistry ofthe groundwater is crucial to the use of geochemical analysisof river waters to establish catchment weathering mass bud-gets [e.g., Durand et al., 2005]. However, it is difficult toquantify the flux and/or budget of groundwater inputs intothe Buha River waters at this stage, especially seasonally,because (1) surface and groundwaters have similar char-acteristics in geochemical parameters, (2) no exact data ongroundwater discharge are available, and (3) flux of thegroundwater input may vary with time.

6. Conclusions

[42] Our results on the dissolved chemistry of river watersamples from the Buha River present a good case of thepotential contribution of eolian input to the dissolved load ofsemiarid inland rivers. This eolian dust input might triggersecondary carbonate precipitation. Further work is war-ranted to constrain dust flux estimates to various ecosys-tems, especially in arid and semiarid areas.[43] Our data shed further light on processes responsible

for the mass balance of elements transported in solutionassociated with seasonal variation in carbonate and silicatesources. The data presented here, from a semiarid region,clearly demonstrate that the relative inputs from silicateversus carbonate sources are sensitive to local hydrology. Asignificant increase in the proportion of carbonate weather-ing to the dissolved load occurs in the early monsoon season,resulting from the faster dissolution kinetics of carbonates.Then, increased silicate sources respond to high temperatureand increased rainfall during the monsoon maximum, per-haps due to groundwater contributions. A constant silicatecontribution to river water chemistry is observed during thedry seasons, probably resulting from a continued contribu-tion from groundwater.

[44] Acknowledgments. The authors especially thank Zhu Yuxin inState Key Laboratory of Lake Science and Environment; Zhang Ting inthe Institute of Earth Environment, CAS; and Hazel Chapman in Depart-ment of Earth Sciences, University of Cambridge, for their kind help andsuggestions for sample analyses and laboratory works. Thanks are extendedto Yuewei Shi and Xinning Qiu at the Buha River Hydrological Station fortheir assistance with sample collection. The manuscript greatly benefitedfrom constructive comments by Domenik Wolff‐Boenisch, Joshua West,three anonymous reviewers, and the Associate Editor. This work was finan-cially supported by NSFC through grants (40873082 and 40921120406)and by National Basic Research Program of China (2010CB833404).

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Z. Jin and F. Zhang, State Key Laboratory of Loess and QuaternaryGeology, Institute of Earth Environment, Chinese Academy of Sciences,Xi’an 710075, China. ([email protected])H.‐C. Liu and C.‐F. You, Earth Dynamic System Research Center,

National Cheng Kung University, Tainan 70101, Taiwan.L. Wu, Department of Geoscience, University of Wisconsin–Madison,

1215 W. Dayton St., Madison, WI 53706, USA.J. Yu, Lamont‐Doherty Earth Observatory, Earth Institute at Columbia

University, 61 Rt. 9W, PO Box 1000, Palisades, NY 10964‐8000, USA.


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