The molybdenum isotopic composition in river water: Constraints from small catchments

Earth and Planetary Science Letters 304 (2011) 180–190

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

The molybdenum isotopic composition in river water: Constraints from small catchments N. Neubert a,b,⁎, A.R. Heri a,c, A.R. Voegelin a, T.F. Nägler a, F. Schlunegger a, I.M. Villa a,d a

Universität Bern, Institut für Geologie, Baltzerstrasse 3, 3012 Bern, Switzerland Leibniz Universität Hannover, Institut für Mineralogie, Callinstrasse 3, 30167 Hannover, Germany University of Hong Kong, Department of Earth Sciences, Pokfulam Road, Hong Kong, China d Università di Milano Bicocca, 20126 Milano, Italy b c

a r t i c l e

i n f o

Article history: Received 17 November 2009 Received in revised form 22 December 2010 Accepted 1 February 2011 Available online 25 February 2011 Editor: P. DeMenocal Keywords: Mo isotopes river water bedrock weathering fractionation anthropogenic sulfate sulfide

a b s t r a c t We report molybdenum isotope compositions and concentrations in water samples from a variety of river catchment profiles in order to investigate the influence of anthropogenic contamination, catchment geology, within-river precipitation, and seasonal river flow variations on riverine molybdenum. Our results show that the observed variations in δ98/95Mo from 0‰ to 1.9‰ are primarily controlled by catchment lithology, particularly by weathering of sulfates and sulfides. Erosion in catchments dominated by wet-based glaciers leads to very high dissolved molybdenum concentrations. In contrast, anthropogenic inputs affect neither the concentration nor the isotopic composition of dissolved molybdenum in the rivers studied here. Seasonal variations are also quite muted. The finding that catchment geology exerts the primary control on the delivery of molybdenum to seawater indicates that the flux and isotope composition of molybdenum to seawater has likely varied in the geologic past. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Molybdenum (Mo) is the most abundant transition metal in the ocean and plays an important role in the nitrogen metabolism of organisms (Dellwig et al., 2007; Mendel, 2005). Recently, Mo isotope systematics have become increasingly important as a new proxy for the redox history of the oceans and atmosphere (Arnold et al., 2004; Lehmann et al., 2007; Siebert et al., 2005; Wille et al., 2007; Wille et al., 2008). In order to realize the full potential of this proxy it is important to establish the sinks and sources of the oceanic Mo cycle in more detail. Seawater has a salinity-normalized Mo concentration of ca. 105 nmol/L (nM) (Emerson and Huested, 1991). In oxic water Mo exists as the soluble molybdate, MoO2− 4 . Its low chemical reactivity results in a comparably long residence time of ca. 800 ka (Colodner et al., 1995; Emerson and Huested, 1991). Seawater has a heavy Mo isotopic composition (hereafter denoted as δ98/95Mo) of 2.3‰ relative to our standard (for analytical methods see electronic supplement). The Mo isotopic composition of ocean water is primarily controlled by

⁎ Corresponding author. E-mail address: [email protected] (N. Neubert). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.02.001

sedimentary Mo sinks that shows distinct, redox-sensitive Mo isotopic compositions. Initially it was thought that the relative proportions of oxic and anoxic/euxinic sedimentary environments govern the Mo isotopic composition of seawater (Barling et al., 2001; Siebert et al., 2003). The subsequent discovery of variable isotope fractionation in suboxic sediments (Poulson et al., 2006; Siebert et al., 2006) and the observation that the δ98/95Mo of euxinic sediments is controlled by the concentration of dissolved H2S (Neubert et al., 2008) led to a more refined understanding of marine Mo isotope systematics. The “pre-anthropogenic” concentration of dissolved riverine Mo has been estimated at 5 nM (Martin and Meybeck, 1979). Until recently, the isotopic composition of riverine Mo was assumed to coincide with the δ98/95Mo of continental magmatic rocks, −0.1‰ to +0.3‰ (Siebert et al., 2003). Analyzes of river water, however, recently exposed much greater variations in δ98/95Mo (0.2‰ to 2.3‰) and Mo concentrations (2 nM to 511 nM; Archer and Vance, 2008). These data, together with those from Scheiderich et al. (2010), Pearce et al. (2010), and this study, require a revision of the assumption that the δ98/95Mo of continental runoff is identical to that of average basalts and granites. We endeavor to assess the importance of weathering of sedimentary and igneous rocks on the Mo isotopic composition and concentration of continental runoff. If bedrock erosion is an important factor, it is also likely that the flux and isotope composition of Mo to seawater has changed through time,

N. Neubert et al. / Earth and Planetary Science Letters 304 (2011) 180–190

because the relative proportions of exposed bedrocks have changed throughout Earth history (Bluth and Kump, 1991). Several other factors could potentially play a role for the Mo isotopic composition of dissolved river load. If chemical and physical weathering produced distinct Mo isotope effects, then changes in climate, topography and weathering style should affect the Mo isotopic composition of continental runoff. Isotope fractionation during transport from source to sink also must be evaluated (Pearce et al., 2010). Another potentially important factor affecting riverine Mo concentration is anthropogenic input. The majority of industrial Mo is used in alloys and stainless steels (Matos and Magyar, 2005). Elevated Mo concentrations are also found in lubricants, and fertilizers (Hammond, 2005). Both agricultural and industrial activities are known to increase heavy metal concentrations in river water; however, their effect on Mo isotope systematics has not been studied sufficiently. Here we present sequences of river water samples, complemented by bedrock analyzes, from a variety of catchment sizes, which allow us to assess the influence of a limited number of factors governing riverine Mo behavior. We investigate the dependence of dissolved Mo isotopic compositions and concentrations on (a) anthropogenic pollution, (b) the bedrock lithology in catchments, (c) fractionation processes during river transport, and (d) seasonal changes in rainfall, river discharge and erosion. 2. Geographical settings The samples were taken from four different locations: the Tista River (Sikkim, India), the Entlebuch catchment and the Aare River (Switzerland), and the Chang Jiang (Yangtze River, China). These catchments were chosen for their different dominant land covers (forest, pasture land, agricultural fields, cities and industry) and bedrock lithology (silicate rocks, marine sediments and terrestrial sediments). Their approximate sizes are given in Table 1. The river course of the Aare further traverses two oligotrophic lakes. Complementary bedrock samples were also included. In particular, marine lithologies were sampled to test their potential to impose heavy isotopic composition on the respective rivers, as marine sediments can be isotopically heavy (e.g. carbonates, Voegelin et al., 2009, suboxic and anoxic sediments, Siebert et al., 2006; Poulson et al., 2006) and can even reach seawater values (e.g., euxinic sediments: Neubert et al., 2008; some non-skeletal carbonates: Voegelin et al., 2009). 2.1. Tista River (Sikkim, India) The Tista (Fig. 1), the largest river in Sikkim (India), originates from the wet-based Zemu glacier at the base of the Kangchendzönga massif and flows through regions with extremely contrasted population densities. The glacier overlies gneisses and leucogranites of the High Himalayan Crystallines (e.g. Dasgupta et al., 2004). Sample Sik-1 was collected at the outflow of the Zemu Glacier at an elevation of ca. 4350 m where glacial meltwater drains the basal moraine. Sik-2 was taken in a swamp in the uninhabited Hemaghati rhododendron forest (3400 m), which derives its water from rain and glacial meltwater and drains into the Tista. Sik-3 was collected from the Tista downstream of the village of Lachèn (elevation 2510 m; ca. 500 inhabitants; heavy vehicle repair facilities). Sik-4 was taken at Rangpo (elevation 310 m) where the Tista integrates runoff of most of Sikkim's ca. 540,000 population. Anthropogenic contribution is thus expected to be most pronounced here. 2.2. Entlebuch catchment (Switzerland) We focused on the four principal small river systems in the Entlebuch region of Switzerland: Kleine Emme, Grosse Entlen, Kleine

181

Table 1 Mo isotopic composition and concentration. Sample

Mo (nM)

δ98/95Mo⁎ 2 σ River (‰) error#

1. Water samples Entlebuch catchment (Switzerland, size = 0.5†) KF 1-1 1.79 0.46 0.09 Kleine Fontanne KF 2-1 1.11 0.36 0.05 Kleine Fontanne KF 1-2 1.65 0.35 0.04 Kleine Fontanne TR 1-1 0.90 0.16 0.12 Trueb TR 2-1 0.71 0.14 0.14 Trueb TR 2-2 0.53 0.27 0.20 Trueb KEM 1-1 2.95 1.25 0.04 Kleine Emme KEM 1-2 2.45 1.50 0.05 Kleine Emme KEM 2-2 2.71 1.29 0.06 Kleine Emme KE 1 2.69 1.60 0.06 Kleine Emme KE 2 1.97 1.09 0.12 Kleine Emme GE 1 2.34 1.01 0.10 Grosse Entlen GE 2 2.49 1.21 0.07 Grosse Entlen Aare catchment (Switzerland, size = 3.5†) Aa 1-1 5.09 1.22 0.06 Aare Berne Aa 2-1 5.69 1.13 0.04 Aare Berne Aa (ARA) 5.46 1.07 0.04 Treatment plant Aa 4-2 5.82 1.34 0.01 Aare Thun Aa 4-3 6.94 1.04 0.02 Aare Spiez Aa 4-4 7.18 1.05 0.01 Aare Interlaken Aa 4-5 9.12 0.90 0.02 Aare Fritz 4-1 2.08 1.90 0.02 Fritzenbach † Sikkim catchment (India, size = 7 ) SIK-1 139 0.59 0.05 Tista SIK-2 43 0.56 0.24 Tista SIK-3 90 0.57 0.07 Tista SIK-4 44 0.66 0.06 Tista Chang Jiang (China, size = 1200†) Jin-1 9 1.22 0.03 Chang Jiang YCh4 13 1.11 0.11 Chang Jiang at Nanjing Sample

Mo (μg/g)

δ98/95Mo⁎ 2 σ Rock type (‰) error#

2. Bedrock samples Entlebuch catchment (Switzerland) SF 0.27 0.90 0.04 SK 0.48 1.09 0.03 KK 0.48 0.62 0.04 UMM 0.16 1.01 0.10 OMM 0.13 0.88 0.05 Aare catchment (Switzerland) gy 0.06 1.72 0.07 Sikkim catchment (India) SK05-29 1.08 0.63 0.09

Sampling period

1 2 1 1 2 2 1 1 2 3 3 3 3 1 2 4 4 4 4 4 4 5 5 5 5 6 7 Tectonic unit (in Figs.)

Flysch Limestone Black siliceous limestone Marine molasse Marine molasse

Sch Wi Wi SM M

gypsum

Ni

leucogranite

HHC

Sampling period 1 = winter (01/2006); 2 = summer (09/2006;) 3 = summer (07/ 2007); 4 = summer (09/2007); 5 = fall (10/2005); 6 = summer (08-09/2007); 7 = fall (10/2007). ⁎ Relative to standard solution (Siebert et al., 2001). # In run precision of analyzes (2 standard error of the mean). † Catchment size is given as 103 km3 upstream of lowest sample locality.

Fontanne and Trueb (Fig. 2). Their total ca. 500 km2 drainage area is dominated by agriculture and forest, and only 3% is covered by small villages and minor industry, particularly along the lower Kleine Emme (villages of Schüpfheim and Entlebuch with 3700 and 3400 inhabitants, respectively). The Entlebuch catchment is located in the Swiss Molasse basin in the northern foothills of the Alps. This inverted basin was filled with erosional debris of fluvial and marine origin during the Tertiary Alpine orogeny (Labhart, 1995). The catchment lithology of the Kleine Fontanne and the Trueb consist of freshwater molasse sediments (Matter, 1964). The Trueb River was sampled upstream (TR2-2) and downstream (TR1-1, 2 –1) of a wood processing plant. The Kleine Fontanne River drains sandstones, clays, red marls and conglomerates of the NW Entlebuch region and is largely unaffected by human activity; it was sampled near its source (KF1-2) and near its mouth (KF 1–1; 2–1). The two other rivers, the Kleine Emme and Grosse Entlen, drain a tectonically more complex area (Matter, 1964; Mollet, 1921), predominantly

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investigations by, e.g., Diem (1986) and Schlunegger et al. (1996) revealed that disseminated diagenetic sulfides are ubiquitous in the Entlebuch UMM unit of the subalpine Molasse. Water sample KE1 was taken at the source, draining sediments and evaporites of the Klippen Nappe. Sample KE2 was collected from a small tributary that drains marine sediments of the Wildhorn Nappe. Farther downstream the Kleine Emme River was sampled above (KEM 1–2; 2–2) and below (KEM 1–1) the villages of Schüpfheim and Entlebuch. One sample (GE1) of the Grosse Entlen River was taken close to its source in the marine Schlieren-Flysch. GE2 was taken ca. 15 km downstream, after passing large swampy area, and the same lithologies as in case of the Kleine Emme. The single exception is the evaporites of the Klippen Nappe, which are absent in the drainage area of the Grosse Entlen. Representative samples of the above mentioned marine lithologies were also sampled (Table 1, Fig.2). The Entlebuch river samples were first sampled on January 19, 2006 and a second time on September 2, 2006. These dates correspond to lowest and highest cumulative rainfall during the preceding months (MeteoSchweiz, 2010). The September sampling allows us to investigate the effects of enhanced agriculture (fertilizer usage) in summer. 2.3. Aare catchment (Switzerland) Fig. 1. Geological sketch map of the Tista river catchment in Sikkim, India, modified after Dasgupta et al. (2004). HHC = Higher Himalayan Crystallines (sillimanite paragneisses and leucogranites), LH = Lesser Himalayas (phyllites and schists), MCTZ = Main Central Thrust Zone (sheared rocks of HHC and LH). Sampling sites are denoted by stars.

consisting of marine sediments: marine molasse, and in the upper reaches siliceous black limestone and limestone of the Wildhorn Nappe and the Schlieren-Flysch. Localized evaporite deposits occur in this river catchment, particularly at the base of the Klippen nappe that consists of Triassic evaporites and dolomites. Gypsum also locally occurs in easily weathered Rauwacke (cellular dolomite). The

The second study area in Switzerland is the catchment of the River Aare, sourced in the Aar Massif gneisses, amphibolites and granites (Fig. 3). These lithologies contain disseminated high-temperature pyrite (Hofmann et al., 2004). The Aare River flows through two oligotrophic lakes, Lake Brienz and Lake Thun. Their presence offers the possibility to investigate whether or not the passage through lakes has an effect on dissolved Mo. Samples were taken from the Aare River before and after each lake (Aa4-5 to Aa4-3). In addition, we sampled a small brook (Fritzenbach, Fritz 4–1) that drains into Lake Thun downstream of the Krattigen gypsum quarry (Fig. 3). The gypsum

Fig. 2. Geological sketch map showing the catchments in the Entlebuch area in Switzerland, based on the tectonic map of Switzerland (Swisstopo, 2005). Sampling sites are denoted by stars (filled: water samples, open: rock samples).

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Fig. 3. Map of river Aare passing Lake Thun and Lake Brienz. The city of Bern is also traversed by the Aare, ca. 30 km downstream of Thun. Aare samples Aa 4–1 and ARA were collected in Bern.

deposit is stratigraphically equivalent to the evaporite horizon in the Entlebuch River catchment (“KEl” in Fig. 2). The Aare River was also sampled in the city of Bern and downstream of the local wastewater treatment plant, to quantify anthropogenic input.

rivers in the Entlebuch catchment are very low (up to 5 times below world average). While large differences are observed among all river systems, the variations within each river are small. 3.2. Tista River (Sikkim, India)

2.4. Chang Jiang (China) Two samples of the Chang Jiang (Yangtze River) were collected about 4500 km apart (Fig. 4). Sample Jin1 was taken near Batang in an area of low population density. Sample YCh4 was collected in Nanjing city, a region of more intensive heavy industry and higher population density. Archer and Vance (2008) analyzed five Chang Jiang samples near Wuhan, ca. 530 km upstream of Nanjing, finding no significant variations in Mo concentration and isotope composition. Comparing the Mo data of the upper and lower Chang Jiang River may reveal anthropogenic sources. Moreover, the Chang Jiang River system drains black shale deposits of the Yangtze Platform, which include sulfide deposits with Mo concentrations in the percent range; these black shales and sulfides have an average δ98/95Mo of 1.1‰ (Lehmann et al., 2007). 3. Results 3.1. Mo concentration and isotopic composition The investigated river systems show remarkable differences in their Mo isotopic compositions and concentrations (Fig. 5, Table 1). Most samples have a heavier Mo isotopic composition than the juvenile crustal igneous rocks reported by Siebert et al. (2003), δ98/95Mo = −0.1 to +0.3‰. The Tista shows a constant δ98/95Mo associated with Mo concentrations 9 to 28 times above the estimated world average (Martin and Meybeck, 1979); the Chang Jiang also shows a constant Mo isotopic composition and concentration (about twice world average). The Entlebuch and Aare catchments in Switzerland have highly variable δ98/95Mo ratios and concentrations. The Mo concentrations of some

The most striking result from the Tista catchment is the very high Mo concentration (139 nM) at the Zemu glacier outflow (Sik-1). All downstream samples have lower concentrations (40 to 90 nM). The δ98/95Mo values are constant along the course of the river. The rocks exposed along the upper reaches of the Tista in northern and central Sikkim are paragneisses and leucogranites (Dasgupta et al., 2004). The measured leucogranite from the Zemu Glacier has δ98/95Mo = 0.6‰. 3.3. Entlebuch catchment (Switzerland) We analyzed ten samples from the Entlebuch catchment (Fig. 2). The Kleine Fontanne and Trueb draining the Swiss Molasse basin have the lowest Mo concentrations (b2 nM) and the lightest Mo isotopic compositions ranging from δ98/95Mo = 0.14 to 0.46‰. We observed no significant fractionation between upstream and downstream samples. The Kleine Emme and its tributary Grosse Entlen have slightly higher Mo concentrations (2 to 3 nM) and δ98/95Mo ratios ranging from 1 to 1.6‰. The source of the Kleine Emme (KE1) is 0.4‰ heavier than sample KEM 1–1 downstream. Its small nameless tributary (KE2) close to the source has a δ98/95Mo ratio of 1.1‰ similar to the large downstream tributary Grosse Entlen. High δ98/95Mo values were found in the exposed marine sediments (0.6 to 1.1‰, Table 1). Mo isotopic composition and concentration variations within the Entlebuch rivers mirror significant variations in dissolved sulfate concentration (Table 2). Sample KE1 (Kleine Emme source) has up to 7 times higher sulfate concentration than Kleine Fontanne and Trueb, as well as the tributaries Grosse Entlen and sample KE2. The average world river sulfate concentration is 0.1 mM (Huang et al., 2009; Wetzel,

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Fig. 4. Geological sketch map of the upper and lower reaches of the Chiang Jiang. a: upper reaches, 1: clastic rocks, 2: Quaternary fluvial deposits, 3: carbonate rocks, 4: metamorphic rocks, 5: ophiolitic mélanges, 6: granitoids, modified from Wu et al. (2008); 7 area rich in evaporites according to data by Noh et al. (2009). b: Lower reaches: Simplified paleogeographic map of the Yangtze platform during the Precambrian-Cambrian boundary interval (modified from Zhu et al., 2003). I: platform interior, II: transitional zone, III: slope and deep basin. Mo labels indicate black shale hosted occurrences or mines of Ni- Mo-PGE-Au, after Mao et al. (2002). Samples were taken in Batang (a) and Nanjing (b), Archer and Vance (2008) sampled near Wuhan (b).

1975). Our Entlebuch samples thus range between 0.2 times to 4 times the world average. We observe small or negligible seasonal rainfall effects on Mo concentration and isotopic composition (Fig. 5, Table 1). The largest seasonal variation in Mo concentrations is 0.6 nM in the Kleine

Fontanne, whereas the largest isotopic variation in δ98/95Mo is 0.2‰ (upper Kleine Emme). 3.4. Aare catchment (Switzerland) The course of the Aare catchment is interrupted by two lakes (Fig. 3). A total of 8 samples of the Aare were measured before and after each lake, as well as further downstream in the city of Bern. The Mo concentration in the Aare decreases from 9 to 7 nM downstream. The Mo isotopic composition in the Aare is constant in the upper reaches (samples Aa4-5 to Aa4-3), with δ98/95Mo≈ 1‰, and becomes heavier (δ98/95Mo= 1.3 ± 0.1‰) within Lake Thun. The Fritzenbach (Fritz 4–1), which drains into Lake Thun (Fig. 3), shows a heavy Mo isotopic signature (δ98/95Mo = 1.9± 0.1‰). A complementary gypsum sample from the Krattigen gypsum quarry drained by the Fritzenbach shows a Mo concentration of 0.06 μg/g and a heavy δ98/95Mo= 1.7 ± 0.1‰. The Aare sample Aa(ARA) was taken downstream of the wastewater treatment plant in Bern. Its Mo concentration (5.5 nM) and isotopic composition (δ98/95Mo = 1.1 ± 0.1‰) are identical to those upstream of the plant. Sulfate concentrations are high in the Fritzenbach and in the Aare downstream of Lake Thun (Aa4-2) and in the city of Bern (Aa1-1, Aa2-1). The Aare shows almost no seasonal variation in either Mo concentration or isotopic composition. 3.5. Chang Jiang (China)

Fig. 5. The Mo isotopic composition is shown against reciprocal Mo concentration (nM−1) of the studied catchment areas (Table 1). The inset expands the Tista data. The Entlebuch data indicate a steep negative trend for 1/Mob 0.6 and a different, shallow negative trend for 1/Mo N 0.6. Simple binary mixing, or a single fractionation process, would result in a single linear array. Seasonal changes do not change the Mo isotopic composition significantly. Error bars represent 2 σ errors as outlined in the caption of Table 1. MOMo (Mean Ocean Molybdenum) refers to the seawater Mo isotopic composition after Siebert et al. (2003).

Our two samples from the Chang Jiang, Jin-1 and YCh4, have indistinguishable δ98/95Mo values (1.2 ± 0.1 and 1.1 ± 0.1‰) and high Mo concentrations (9 nM and 13 nM, respectively). These values are identical to those measured by Archer and Vance (2008) in Wuhan, a locality intermediate between our samples. The Mo isotopic composition shows no change downstream. The Chang Jiang catchment

N. Neubert et al. / Earth and Planetary Science Letters 304 (2011) 180–190

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Table 2 Major ions and field parameters. Sample

mM

pH

Eh (mV)

EC (μS/cm)

O2 (mg/L)

T (°C)

0.11 0.10 0.08 0.09 0.09 0.07 0.11 0.09 0.04 0.01 0.01 0.02 0.05

8.4 8.6 8.2 8.2 7.9 8.2 8.4 8.3 8.5 8.7 8.3 8.2 8.5

255 226 247 216 307 277 197 211 228 390 270 650 220

155 298 145 224 229 255 162 238 257 311 226 171 295

12.7 9.7 12.0 11.6 9.8 10.6 12.0 11.3 10.0 8.0 9.0 7.5 8.5

0.4 15.3 0.8 5.0 9.1 10.0 2.4 2.1 11.8 16.1 4.8 4.7 17.1

n.d. 0.05 0.08 0.03 0.01 0.01 0.01 0.01

n.d. 0.04 0.05 0.03 0.02 0.02 0.02 0.04

8.2 8.6 7.9 8.0 8.5 8.6 8.5 8.4

230 225 127 165 240 415 235 440

243 200 969 248 149 147 66 387

12.13 9.7 7.7 n.d. n.d. n.d. n.d. n.d.

6.4 16.6 17.3 14.4 14.2 16.3 9.7 9.4

K+

Ca2+

Mg2+

F−

Cl−

NO3

Entlebuch catchment (Switzerland) KF 1-1 0.05 0.19 KF 2-1 0.03 0.14 KF 1-2 0.04 0.08 TR 1-1 0.03 0.16 TR 2-1 0.03 0.10 TR 2-2 0.02 0.09 KEM 1-1 0.15 0.27 KEM 1-2 0.31 0.39 KEM 2-2 0.26 0.12 KE 1 0.34 0.06 KE 2 0.09 0.03 GE 1 0.07 0.10 GE 2 0.11 0.12

0.03 0.03 0.04 0.03 0.00 0.00 0.03 0.03 0.00 0.01 0.01 0.01 0.02

1.87 1.87 1.57 1.91 1.89 1.86 1.66 1.65 1.58 1.32 1.13 0.79 1.37

0.23 0.24 0.14 0.19 0.13 0.03 0.24 0.19 0.16 0.37 0.14 0.08 0.18

0.05 0.02 0.05 0.05 0.02 0.02 0.05 0.06 0.02 0.01 0.01 0.01 0.01

0.18 0.09 0.07 0.16 0.06 0.05 0.21 0.28 0.04 0.01 0.00 0.00 0.03

Aare catchment Aa 1-1 Aa 2-1 Aa (ARA) Aa 4-2 Aa 4-3 Aa 4-4 Aa 4-5 Fritz 4-1

n.d. 0.00 0.03 0.02 0.02 0.03 0.03 0.04

n.d. 1.10 1.23 1.08 0.66 0.66 0.27 1.66

n.d. 0.16 0.17 0.19 0.09 0.09 0.04 0.20

n.d. 0.02 0.02 0.01 0.01 0.01 0.01 0.01

SO2− 4

Na+

(Switzerland) n.d. n.d. 0.26 0.08 0.26 0.11 0.31 0.06 0.10 0.04 0.10 0.05 0.06 0.04 0.41 0.49

Sikkim catchment (India) SIK-1 0.07 SIK-2 0.06 SIK-3 0.08 SIK-4 0.04

b0.04 b0.04 b0.04 b0.04

0.03 b0.03 b0.03 b0.03

0.05 0.05 0.04 0.03

b 0.04 b 0.04 b 0.04 b 0.04

b0.03 0.05 0.05 0.04

0.03 0.02 0.01 0.01

b 0.01 b 0.01 b 0.01 b 0.01

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

Chang Jiang (China) Jin-1⁎ 0.97 YCh4 0.31

3.04 0.06

0.09 0.06

1.30 0.90

0.74 0.31

n.d. 0.03

3.50 0.32

0.01 0.01

8.5 n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d.

13.5 n.d.

⁎ Data are taken from Huang et al. (2009).

upstream of Wuhan and Nanjing (Fig. 4) includes black shales and sulfide deposits of the Lower Cambrian Niutitang Formation of the Yangtze Platform (Chen et al., 2009). The Mo isotopic composition of these rocks is δ98/95Mo = 1.1‰ (Lehmann et al., 2007), indistinguishable from that of the Wuhan and Nanjing water samples. The most upstream sample of the Chang Jiang, Jin-1, has the highest sulfate concentration amongst the rivers in this study (0.77 mM, Huang et al., 2009). 4. Discussion The questions we intend to address are whether the Mo data define systematic patterns that can constrain the processes controlling the Mo behavior in rivers, and whether such processes differ among various rivers. The observation that Mo concentration and δ98/ 95 Mo do not define a linear trend in Fig. 5 indicates that the data are neither explained by mixing of two end-members, nor by a single fractionation process. The Entlebuch data show a clear negative correlation: low δ98/95Mo ratios are associated with low concentrations, and vice versa. This trend, however, is non-linear and therefore requires mixing of more than two reservoirs, and/or fractionation by more than one process. 4.1. Anthropogenic Mo contribution In the catchments we analyzed, small and large settlements neither increase the Mo concentration nor modify the Mo isotopic composition. The heavy isotopic signatures of several rivers like the Kleine Emme and Grosse Entlen (Switzerland) and the Tista downstream of Gangtok cannot be caused by anthropogenic input because they appear in the headwaters, upstream of any possible

anthropogenic source. Similarly, Mo concentrations and isotope ratios from the Aare upstream and downstream of Bern and the wastewater treatment plant are indistinguishable. An additional argument against agriculture as the predominant control on Mo concentrations and isotopic compositions is that these two parameters vary in the Entlebuch catchment despite the very similar land-use practices in each tributary, and are unchanged in the Tista catchment despite the transition from high-mountain desert to intensive lowlands agriculture with a high population density. The Chang Jiang data are more difficult to interpret, because our sample spacing is insufficient to discern local sources. Major anthropogenic contributions to sample Jin-1 are unlikely, but there is a large potential for industrial and agricultural Mo input between Batang and Nanjing. However, we observe no significant variation in Mo isotopic composition. The anthropogenic input could coincidentally have a δ98/95Mo of ca. 1‰, identical to Jin-1 and to the Mo-rich rocks of the Yangtze Platform (Lehmann et al., 2007). Closer-spaced sampling could resolve the issue in future. 4.2. Catchment rock weathering The Tista catchment (Fig. 4, grey inset) combines the highest dissolved Mo concentrations and intermediate isotopic compositions. The good agreement between the isotopic composition of the leucogranite from the Zemu Glacier (δ98/95Mo = 0.6 ± 0.1, Table 1) and the Tista River indicates that weathering of High Himalayan Crystallines dominates the input of Mo into the Tista. We hypothesize that the high Mo concentration at the outflow of the wet-based glacier reflects release of Mo from fine rock flour created by glacial erosion. Leaching experiments on crushed rocks (Siebert et al., 2003) show that release of relatively mobile Mo occurs immediately after

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crushing. Downstream of Sik-1, non-glacial tributaries dilute the Mo concentration. The Entlebuch and Aare catchments are not as strongly dominated by physical weathering as the Tista catchment, and Mo is lost from the host rocks less efficiently. Moreover, the marine and fluvial sediments of the Entlebuch Molasse have been extensively leached prior to their final deposition. The bedrock of the Trueb and Kleine Fontanne catchments consist of silicate clasts without evaporitic cement, and are therefore depleted in easily leachable Mo. We consider the low Mo concentrations (b2 nM) and the light Mo isotopic composition (δ98/95Mo b 0.4‰) of these two rivers a consequence of the lithological characteristics of their drainage area, namely continental silicate rocks. The Kleine Emme and Grosse Entlen have Mo concentrations between 2 and 3 nM and δ98/95Mo = 1 to 1.6‰. The catchments of these rivers include marine sediments (marine molasse, sulfate of the Klippen Nappe, as well as carbonates and black siliceous limestone of the Wildhorn Nappe), whose measured δ98/95Mo is 0.6 to 1.1‰, and evaporites, with δ98/95Mo = 1.7‰. The whole δ98/95Mo range of these two rivers is encompassed by the bedrock lithology. Bedrock weathering is reflected by the Mo/SO2− ratios, as discussed in 4 Section 4.2.1. 4.2.1. Sulfate weathering versus sulfide oxidation Our most prominent finding is the positive correlation between Mo isotopic composition and sulfate concentration in Kleine Emme and Aare (Fig. 6). Three possible sources contribute dissolved sulfate to river water: sulfate weathering, sulfide oxidation, and acid rain. The latter source is likely least important, as the neighboring Entlebuch and Aare catchments receive similar amounts of precipitation, which has traveled the same distance from heavily industrialized regions. This precludes significant inter-river sulfate variations due to acid rain, and limits the discussion of the Entlebuch and Aare catchments to the relative importance of sulfate and sulfide weathering. The correlation of the Mo isotopic composition with the high sulfate concentration can be used to identify the potential source rocks (Fig. 6). The most upstream sample (KE 1) of the Kleine Emme

Fig. 6. Mo isotopic composition against dissolved sulfate concentration of the Entlebuch and Aare catchments. The filled circles represent the Entlebuch catchment, the open circles the Aare. Samples in the upper right are dominated by sulfate weathering, as indicated by sample Fritz4-1 from a brook downstream of a gypsum quarry (Fig. 3). Samples in the upper left are interpreted as dominated by sulfide weathering (see Fig. 7).

originates in the Klippen nappe (Fig. 2), whose base consists of Triassic evaporites and dolomites (Wissing and Pfiffner, 2002). Sulfates occur in very easily weathered cellular dolomite (Rauwacke). The gypsum deposit of the Krattigen gypsum quarry drained by the Fritzenbach (Fig. 3) is stratigraphically equivalent to the evaporite horizon of the Klippen nappe (Fig. 2). The similarity of the Mo isotopic composition between the gypsum sample (gy), the Kleine Emme (KE1) and the Fritzenbach (Fritz4-1) waters strongly supports the assumption that sulfate weathering provides both high SO2− and 4 isotopically heavy Mo. Because gypsum originates from the evaporation of seawater, and because sulfate and molybdate anions are stereochemical, it is most likely that Mo is incorporated into the gypsum lattice without large fractionation relative to the coeval seawater, and is therefore isotopically heavy. Sulfate concentrations are plotted against Mo concentrations in Fig. 7. The Fritz 4–1 Mo and sulfate concentrations, 2 nM and 0.4 mM, −6 correspond to a (Mo/SO2− . This is a similar order of 4 ) ratio of 5 × 10 magnitude as the Krattigen quarry gypsum, whose Mo concentration −7 of 0.06 μg/g corresponds to an even lower (Mo/SO2− . 4 ) ratio of 10 The correlation of Mo isotope composition with sulfate in catchments with known evaporite outcrops is strong evidence that evaporite weathering is an important source of isotopically heavy Mo. Kleine Emme samples downstream of KE1 are progressively more diluted by runoff from sulfate poor rocks (Figs. 2 and 6). Three rivers draining gypsum-free bedrock (Trueb, Kleine Fontanne, and Aare upstream of Lake Thun) define a line with a steeper −4 slope (Fig. 7), corresponding to a (Mo/SO2− . It is 4 ) ratio of 1 × 10 likely that the sulfate in these river systems derives from oxidative weathering of disseminated sulfides. This is in line with the findings of Malinovsky et al. (2007). These authors proposed that the Mo isotopic composition is influenced by catchment rock composition, based on their work on molybdenites (ranging from δ98/95Mo = −0.38 to +2.3‰) and lake sediments in the Kalix river catchment. While the Entlebuch and Aare catchments lack large-scale sulfide mineralizations, different modal abundances of μm-sized disseminated sulfides are ubiquitous, with high-temperature pyrites in the Aar Massif crystalline rocks (Hofmann et al., 2004). Pyrites may contain tens to hundreds of μg Mo/g (Wedepohl, 1987), which corresponds to

Fig. 7. The Mo concentration against the sulfate concentration of the Entlebuch and Aare catchments (filled circles = Entlebuch catchment, open circles = Aare). The two grey bands mark a shallow slope indicating the gypsum weathering and a steep slope suggesting pyrite weathering. The dashed line connecting the open circles (Aare catchment profile) is interpreted as representing mixing between the gypsumdominated and the pyrite-dominated end member. Errors are smaller than symbol size.

N. Neubert et al. / Earth and Planetary Science Letters 304 (2011) 180–190 −4 (Mo/SO2− . In the drainage area of the Kleine 4 ) ratios as high as 10 Emme and Grosse Entlen, diagenetic pyrite occurs in the Subalpine Molasse and the Schlieren-Flysch (Diem, 1986; Schlunegger et al., 1996; Winkler, 1983). Further, sulfides are a conceivable Mo host in the black siliceous limestone, sedimented under anoxic conditions. Water samples GE1, GE2 and KE2, related to limestones (SK, KK) and flysch (SF), but not to sulfate sources, plot close to the Trueb and Kleine Fontanne in Fig. 6, i.e. at low concentrations, and therefore do not discriminate between the ‘sulfide’ trend defined by Aa 4–5 and the ‘sulfate’ trend defined by Fritz 4–1. We conclude that weathering of evaporitic gypsum leads to a low (Mo/SO2− 4 ) ratio, i.e. a shallow positive slope in Fig. 7, while oxidative pyrite weathering is indicated by a steeper positive slope. The observed data points of the Aare river profile define a negative slope instead (dashed line, Fig. 7), which we interpret as water mixing. Upstream of Lake Thun, Mo is high and SO2− 4 low (Aa 4–5, upper left of Fig. 7). The Fritzenbach with low Mo and high SO2− 4 (lower right of Fig. 7) enters the Aare at Lake Thun. All Aare samples downstream of Lake Thun lie on a mixing trend between these two end-members. The break in slope of Fig. 6 can be understood when taking into account the distinct behavior of sulfide and sulfate weathering. Trueb, Kleine Fontanne and Aare upstream of Lake Thun define a steep slope with δ98/95Mo≤ 1‰. This trend can be viewed as a mixing between pure silicates (plotting near the origin) and an increasing amount of pyrite, which contributes both sulfate ions and Mo having δ98/95Mo ≈ 1‰. Additional input from gypsum weathering (Fritz 4–1, KE1) raises the sulfate concentration substantially, and increases the δ98/95Mo to up to δ98/95Mo≈ 1.8‰. Fig. 6 solves the ambiguity regarding the Kleine Emme and Grosse Entlen samples GE1, GE2 and KE2, draining marine sediments. In Fig. 7, the low Mo and SO2− concentrations of these 4 samples did not allow to assign them to the sulfide or sulfate trend. In Fig. 6, these samples lie on the steep trend, corresponding to the weathering of disseminated pyrite. The Chang Jiang, with its extremely diverse catchment, records several Mo sources. At Batang, sulfate and Mo concentrations were determined by Huang et al. (2009, their sample 14) as 0.77 mM and 17 nM, respectively. The Mo concentration in our sample Jin-1, collected after the monsoon season and measured in Bern (9 nM), is about half of that reported by Huang et al. (2009). Similarly, Noh et al. (2009, their sample CJ215) determined a significantly lower sulfate concentration of 0.48 mM and lower pH at the same locality. The similar concentration differences between Huang et al. (2009) and both Noh et al. (2009) and our sample Jin-1 are likely due to rainfall differences at the respective sampling periods. In its uppermost reaches Noh et al. (2009) found sulfate concentrations up to 6 mM. These very high sulfate concentrations point to dissolution of the evaporites of the Lhasa block exposed in the upper parts of the catchment (Fig. 4; Huang et al., 2009; Noh et al., 2009). This would also explain the isotopically heavy Mo by analogy with the Aare catchment gypsum. The sulfate concentration in Nanjing shows a clear downstream decrease by a factor of 3 relative to sample Jin-1. This finding is consistent with Huang et al. (2009) who observed a lower concentration 2+ of dissolved salts (SO2− , Mg2+, Na+ and Cl−) 800 km downstream 4 , Ca due to dilution by tributaries. The Mo concentration increases downstream (9–13 nM at Batang; 17 nM at Wuhan and 13 nM at Nanjing), but does not correlate with sulfate concentrations. The higher Mo/SO2− ratio suggests oxidative sulfide weathering. Indeed, black 4 shales and sulfide deposits of the Yangtze Platform (with δ98/ 95 Mo= 1.1‰: Lehmann et al., 2007) are located between Batang and Wuhan. Our sampling density is however insufficient to unequivocally connect higher Mo concentrations to black shale weathering.

4.3. Mo fractionation during river transport Mo undergoes isotope fractionation during adsorption onto oxyhydroxide particles (Barling and Anbar, 2004; Goldberg et al.,

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2009). Within river catchments Mo isotopic fractionation by adsorption could occur, in principle, in soils as well as during river transport. In both cases, Mo trapping by organic matter could potentially cause additional isotope fractionation. In soils, Mo occurs as dissolved Mo, as adsorbate on Al–Fe–Mn oxides, as discrete minerals (molybdenite, powellite, wulfenite, etc.) and in organic compounds (Reddy et al., 1997). The leachability of these reservoirs is different (Wichard et al., 2009), which probably entails a variable isotopic fractionation. Unfortunately, no published data exist on Mo isotope fractionation among these soil components. In river water samples, a net effect of Mo isotope fractionation due to soil processes would only be observable if the catchment rocks and river water had different Mo isotopic compositions; this is not the case for the Aare, Entlebuch, Tista and probably Chang Jiang. In order for soils to achieve any isotopic fractionation, it is necessary to have at least two reservoirs that can no longer exchange. While adsorption of isotopically light Mo to particles of well-developed soils is conceivable, it only results in a net fractionation if the isotopically light Mo is permanently sequestered and never finds its way into the river. The observations from our catchments do not favor long-term storage of isotopically light Mo in soils. Either soil formation does not detectably fractionate Mo isotopes in these catchments or the fractionation is subsequently balanced out, when the erosion of the topsoil layers recycles the older soil particles and mixes their isotopically light Mo with the isotopically heavy Mo escaping adsorption right at that time. All in all, the net soil effect is zero. Concerning Mo isotope fractionation during river transport, swamps near river sources provide an environment where dissolved Mo could potentially be fractionated and trapped in suboxic conditions. However, sample Sik-2 collected in a swamp of a tributary to the Tista River gives no indication of an altered Mo isotope signal. Similarly, the Grosse Entlen shows no increase of heavy Mo isotopes between source (GE1) and downstream sample (GE2) in spite of extended swampy areas along the river and its tributary Kleine Entlen (Fig. 2). Thus, swamps do not appear to significantly influence the isotopic composition of dissolved river Mo. Another possible cause of fractionation is adsorption on Fe/Mn oxyhydroxide particles suspended in the water. Pearce et al. (2010) reported light Mo isotopic compositions (δ98/95Mo between −0.65 and 0.07‰) in Fe-precipitates. However, they concluded (page 10) that “… the adsorption of isotopically light Mo to particulate phases within the river cannot be driving the dissolved load to heavier δ98/95Mo…”. The filtered particle load contributes insignificantly to the Mo budget (see electronic supplement). Furthermore, no significant downstream fractionation was observed in the rivers we analyzed. Both observations fully agree with Pearce et al. (2010). Two isolated instances of significant downstream variation are likely due to admixture of tributaries. The Kleine Emme at location KE2 has acquired a lower δ98/95Mo relative to upstream location KE1 due to a lighter input by its tributary Grosse Entlen (Fig. 2). Even for longer transit times, such an effect is not significant, as shown by our alongriver profiles of the Tista, Chang Jiang and the Aare upstream and downstream of Lake Brienz. The Tista has a very high Mo concentration at the source; downstream the concentration is more than 3 times lower. If this extreme Mo depletion were an effect of precipitation along the river course, the isotopic composition should be shifted towards much heavier values. However, we observe that it is unchanged. Similarly, if downstream fractionation were a major factor, our sample YCh-4 at Nanjing should be isotopically much heavier and have a lower Mo concentration than both Archer and Vance's (2008) sample at Wuhan and especially our own sample Jin-1. Finally, the passage of the Aare through Lake Brienz causes no variation of either the Mo concentration or the isotopic composition. In summary, our observations fail to support significant Mo isotopic fractionation due to secondary precipitation of isotopically light Mo in either lakes or along the river bed.

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4.4. Continental runoff through time We have shown above that the isotopically heavy dissolved Mo of the studied rivers derives from sedimentary catchment rocks. To the extent that the isotopic composition of Mo dissolved in rivers depends on the mass balance of the Mo made available by the different rock types exposed, δ98/95Mo is highly variable and dependent of the continents' configuration and of climate. It should therefore not be assumed to have been a constant throughout the post-Archean oxic Earth history. In this respect, a useful analogy can be made with the oceanic Sr isotope curve, well known to be variable because it represents the interplay between erosion of radiogenic continental rocks during orogenies and the release of Sr from oceanic basalts during rifting. The Mo system is similarly controlled by the timevariable proportion of two end-members: light igneous rocks (i.e. basalts and granites: Siebert et al., 2003) and isotopically heavy sources: marine sediments (and possibly their higher- and highestgrade metamorphic derivatives: metasediments, paragneisses, and anatectic granites); and sulfides. 4.5. Consequences for the global Mo isotope budget As most Mo isotopic compositions of dissolved river load (Fig. 8) are heavier than the silicate magmatic rocks analyzed so far (whose average δ98/95Mo is ca. 0.2‰), the fate of the light Mo isotopes missing in the balance needs to be evaluated. In the light of the importance of the source rock Mo isotopic composition put forward here, we should take into account the dualism between pelagic and shallow water sedimentary rocks. Deep open ocean sediments are a known sink for isotopically light Mo: pelagic sediments and Fe–Mn crusts have δ98/95Mo = −0.5 to − 0.7‰ (Siebert et al., 2003). In contrast, shallow marine and restricted basin sediments have been reported to have δ98/95Mo N 0: euxinic black shales (δ98/95Mo = 2.3‰, Siebert et al., 2003); anoxic sediments (δ98/95Mo = 1.6‰; Poulson et al., 2006); suboxic sediments (δ98/95Mo = − 0.5 to 1.3‰; Siebert et al., 2006); shallow water carbonates (δ98/95Mo = 0 to 2.2‰; Voegelin et al., 2009); evaporites (δ98/95Mo = 1.7‰; this study). The relative proportion of pelagic and shallow water sedimentary rocks now exposed on continents is not well known; the Mn concentrations given by Ronov (1968) hint that

Fig. 8. River water Mo isotopic composition against the Mo concentration based on Fig. 5 of this study (black circles), compared with Archer and Vance (2008) (grey circles), Pearce et al. (2010) (open circles), Scheiderich et al. (2010) (black framed circle). Circle size given in the legend reflects the size of the catchment area. Large rivers smooth out a portion of the scatter of their smaller tributaries. The high, non linear variability indicates that several mixing and/or fractionation processes operate. The ± 0.1‰ error bar is ≥ the typical uncertainty of most analyzes in the cited references.

the proportion of fossil pelagic sediments exposed on continents relative to shelf and epeiric sea sediments under-represents the mass balance in the oceans. It follows that Mo from pelagic sediments may be preferentially recycled in subduction zones, while the isotopically heavier shallow sea sediments make their way to the subaerial environment and re-enter the erosion — river transport subcycle. Recycling of isotopically lighter Mo from pelagic sediments may occur (i) via dissolution within the forearc seafloor hydrothermal systems or (ii) subduction and incorporation in the arc volcanic magmas. In case (i) the total Mo input to the oceans would be the sum of the forearc seafloor hydrothermal systems and the dissolved river load. Because the sum would be isotopically lighter than continental runoff alone, this would balance the system. In case (ii), arc magmas are predicted to be isotopically lighter than non-arc magmas. The hypothesis of a decoupled Mo cycle for shallow and deep sediments has the testable implication that the sediments exposed to weathering would be isotopically heavier than the true average total crust. Our data so far support this. A further testable prediction is that an isotopically light Mo source should exist, either in white smokers (case i) or in juvenile arc magmas (case ii). In both cases, the true average continental Mo isotopic composition would be between that of observed rivers and that of observed igneous silicates. An alternative possibility to balance the light and heavy Mo isotopes would be soil processes (Archer and Vance, 2008), even though our data imply that these process are subordinate in the catchments studied here when compared to catchment rock lithology. On a global scale, the total volume of soil at any given time is limited. If light Mo isotopes are preferentially retained in soil, erosion will liberate them again. As soon as the integrated volume of soil formation is equal to integrated soil erosion, the net Mo isotope effect will be zero. Thus, in order to bias the total continental runoff towards an isotopic composition heavier than the continental crust, a continuous net soil volume increase on a global scale would be required, which, however, cannot be maintained over long geological times. 5. Conclusions (1) Our results on four rather diverse river catchment profiles show a broad variation in the Mo isotopic composition. Large rivers smooth out a portion of the scatter of their smaller tributaries. Thus, smaller catchments provide higher clarity and resolution for studies on the underlying processes. (2) Our new data vastly extend the field of previously published data in the δ98/95Mo vs. 1/Mo space. Some of our samples show light isotopic compositions in the range of the juvenile crustal magmas, and much lower concentrations than average rivers. The Entlebuch river system clearly indicates a positive correlation between increasing Mo concentrations and heavier Mo isotopic compositions. (3) Our Mo isotopic data on bedrock lithologies overlap with values found in the respective rivers. In particular, observed marine sediments have δ98/95Mo around 1‰ with evaporite values being even higher. Thus, the rivers investigated here strongly support the predominant control of catchment outcrop weathering on the Mo budget in river water. (4) Along-stream profiles show that rivers studied here do not significantly modify their Mo isotope signature downstream, unless major tributaries modify the Mo mass balance. Subsequent processes such as precipitation of secondary minerals during river transport and in lakes are of minor influence. (5) Sulfide and sulfate weathering give distinguishable geochemical patterns. Incongruent weathering of disseminated sulfides is likely to be a very significant process. Given that silicate rocks contain sulfides with variable δ98/95Mo, their weathering could result in anomalous Mo isotopic composition in the runoff.

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(6) The glacier-dominated Tista catchment in the Sikkim Himalaya demonstrates that the weathering style controls the Mo concentration. (7) An anthropogenic influence of the Mo budget in rivers was not resolvable in this study. No variations in the isotopic composition or significant changes in the concentration downstream of human activity have been positively identified. (8) The Mo isotopic composition of continental runoff depends on the relative weathered masses of magmatic and sedimentary rocks exposed in river catchments. As such, its variations are analogous to those of the marine Sr isotopic composition through Earth history. The imbalance of light and heavy Mo isotopes suggests that improved models of the global Mo cycle should take into account the recycling of pelagic sediments in subduction zones in addition to the continental runoff.

Acknowledgments This work was supported by Swiss National Science Foundation grants 200020–113658 (to J.D. Kramers) and 200021–116486 (to I.M. Villa). The 2005 Sikkim field work was supported by Italian Ministry of University grant 2003045427 (to E. Garzanti). We thank Florian Eichinger, Ruth Mäder and Christine Lemp for help with anion and cation determinations. Kevin Norton is thanked for correcting the English. We also thank Nick Waber for the always fruitful discussions concerning the major element data interpretation. Special thanks to Bernhard Peucker-Ehrenbrink who improved the manuscript by commenting on language and outline. Thanks to Giovanni Vezzoli and Dario Visonà for help in and around Sikkim, to Mika Sillanpää and Dekey Huang for discussions in Tibet, and to Yue Li for support in Nanjing. Thanks are due to two anonymous referees and to the editorial work by M.L. Delaney and P. DeMenocal. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.epsl.2011.02.001. References Archer, C., Vance, D., 2008. The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans. Nat. Geosci. 1, 597–600. Arnold, G.L., Anbar, A.D., Barling, J., Lyons, T.W., 2004. Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans. Science 304, 87–90. Barling, J., Arnold, G.L., Anbar, A.D., 2001. Naturel mass-dependent variations in the isotopic composition of molybdenum. Earth Planet. Sci. Lett. 193, 447–457. Barling, J., Anbar, A.D., 2004. Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett. 217, 315–329. Bluth, G.J.S., Kump, L.R., 1991. Phanerozoic paleogeology. Am. J. Sci. 291, 284–308. Chen, Y.Q., Jiang, S.Y., Ling, H.F., Yang, J.H., 2009. Pb–Pb dating of black shales from the Lower Cambrian and Neoproterozoic strata, South China. Chem. Erde Geochem. 69, 183–189. Colodner, D., Edmond, J., Boyle, E., 1995. Rhenium in the Black Sea: comparison with molybdenum and uranium. Earth Planet. Sci. Lett. 131, 1–15. Dasgupta, S., Ganguly, J., Neogi, S., 2004. Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P–T gradient and implications. J. Metamorph. Geol. 22, 395–412. Dellwig, O., Beck, M., Lemke, A., Lunau, M., Kolditz, K., Schnetger, B., Brumsack, H.-J., 2007. Non-conservative behavior of molybdenum in coastal waters: coupling geochemical, biological, and sedimentological processes. Geochim. Cosmochim. Acta 71, 2745–2761. Diem, B., 1986. Die Untere Meeresmolasse zwischen der Saane (Westschweiz) und der Ammer (Oberbayern). Eclogae Geol. Helv. 9, 493–559. Emerson, S.R., Huested, S.S., 1991. Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Mar. Chem. 34, 177–196. Goldberg, T., Archer, C., Vance, D., Poulton, S.W., 2009. Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides. Geochim. Cosmochim. Acta 73, 6502–6516. Hammond, C.R., 2005. The Elements. In: Lide, D.R. (Ed.), CRC Handbook of Chemistry and Physics. CRC Press, Boca Raton, FL, pp. 4.19–4.20. Internet Version 2005 http:// www.hbcpnetbase.com. Hofmann, B.A., Helfer, M., Diamond, L.W., Villa, I., Frei, R., Eikenberg, J., 2004. Topography-driven hydrothermal breccia mineralization of Pliocene age at

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