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Geochimica et Cosmochimica Acta 74 (2010) 907–927

Seasonal variations of physical and chemical erosion: Athree-year survey of the Rhone River (France)

P. Ollivier *, B. Hamelin, O. Radakovitch

CEREGE, Aix-Marseille Universite, CNRS, BP80, 13545 Aix-en-Provence cedex, France

Received 17 September 2008; accepted in revised form 13 October 2009; available online 3 November 2009

Abstract

Numerous studies of weathering fluxes have been carried out on major world rivers during the last decade, to estimate CO2

consumption rates, landscape evolution and global erosion rates. For obvious logistical reasons, most of these studies werebased on large scale investigations carried out on short timescales. By comparison, much less effort has been devoted to longterm monitoring, as a means to verify the temporal variability of the average characteristics, their trends, and the represen-tativeness of short-term investigations. Here we report the results of a three-year survey (November 2000 to December 2003)of the major and trace element composition of dissolved and suspended matter in the lower Rhone River (France), the largestriver of the Mediterranean area. Subsurface water samples were collected in Arles, about 48 km upstream of the estuary, twicea month routinely, and at higher frequency during flood events.

During each flood event, the suspended particulate matter (SPM) show the usual trend of clockwise hysteresis with higherSPM concentrations on the rising limb of the flood than at the same discharge on the falling limb. We show that the annualaverage SPM flux of the Rhone River to the Mediterranean Sea (7.3 ± 0.6 � 106 tons yr�1) was largely controlled by the floodevents (83% of the solid discharge occurred in less than 12% of the time), and that the precision on the total output fluxdepends strongly on the precise monitoring of SPM variations during the floods.

The chemical composition of water and SPM are characterized by the predominance of Ca2+ due to the abundance of car-bonate rocks in the Rhone watershed. Chemical budgets have been calculated to derive the contributions of atmosphericdeposition, carbonate, silicate and evaporite weathering, and anthropogenic inputs. The chemical weathering rate of carbon-ates is estimated to be 89 ± 5 t km�2 yr�1 compared to 14.4 ± 3 t km�2 yr�1 from silicates. By contrast, the physical erosionrate of silicates is about 51 t km�2 yr�1 against 19 t km�2 yr�1 for carbonates.

The steady-state model of Gaillardet et al. (1995) has been applied to the chemical composition of dissolved and solid prod-ucts. The results show that the Rhone River currently exports much less material than produced at steady-state by weatheringin its watershed. The sediment flux inferred from the steady-state calculation (21–56 � 106 t yr�1) is on the same order as thatestimated in literature for the 19th and the beginning of the 20th centuries. This imbalance may suggest that the Rhone isunder a transient erosion regime following climate change (i.e. significant decrease of the flooding frequency since the begin-ning of the 19th century). On the other hand, the imbalance may also be due to the trapping of alluvion by the numerous damson the river and its tributaries.

Our data corroborate with previous studies that suggest a strong coupling between chemical and physical erosion fluxes,during the hydrological seasonal cycle of the Rhone River. The correlation between physical and chemical transport rates is,however, clearly different from that reported for global annual averages in large world rivers.� 2009 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2009.10.037

Abbreviations: riv, river; carb, carbonates; sil, silicates; eva, evaporites.* Corresponding author. Present address: CEREGE, UMR 6635, Europole de l’arbois, BP 80, 13545 Aix-en-Provence cedex, France.

E-mail address: patrick.ollivier@gmx.fr (P. Ollivier).

908 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

1. INTRODUCTION

Numerous studies of weathering fluxes have been carriedout on major world rivers during the last decade, to esti-mate CO2 consumption rates, landscape evolution and glo-bal erosion rates. The main focus has been to establishgeneral erosion laws and discriminate between the variousforcing factors, climate, tectonics and bedrock lithology(Stallard and Edmond, 1983; Sarin et al., 1989; Amiotte-Su-chet and Probst, 1993; White and Blum, 1995; Louvat andAllegre, 1997; Gaillardet et al., 1997, 1999a; Dessert et al.,2001, 2003; Dalai et al., 2002; Dupre et al., 2003; Jacobsonet al., 2003; West et al., 2005; Wu et al., 2005). Large effortshave been devoted to understand water chemistry and todelineate anthropogenic perturbations from natural contri-butions from different lithologies in the watersheds (e.g. Ne-grel et al., 1993; Meybeck, 1998; Roy et al., 1999).

For obvious logistical reasons, most of these studies ofthe large world rivers have been based on investigationscarried out on short timescales. Field expeditions includednumerous sampling stations over the basin, in order toget an insight on the spatial variability of the tributaries,and their contribution to the integrated chemical and phys-ical features of the main river. By comparison, only fewworks have been devoted to long term monitoring, as ameans to verify the temporal variability of the average char-acteristics, their trends, and the representativeness of short-term investigations (Bluth and Kump, 1994; Tipper et al.,2006). Time series are thus often incomplete, of short dura-tion, or sampled at an insufficient frequency to capture bothlong term and episodic fluxes.

Moreover, much more attention has been paid in thesestudies on the silicate rock weathering than on carbonateweathering, since only silicate alteration consumes atmo-spheric CO2 on long time scales (Berner et al., 1983).

Here we report the results of a three-year survey of themajor and trace elements in dissolved and suspended phasesof the lower Rhone River (France), one of the largest riversof the Mediterranean area (46th in the world, Meybeck andRagu, 1996). Its catchment is characterized by large cli-matic and geological heterogeneity, and the river has beensubject to major human impacts through damming and riv-er embankment since the Second World War. Annual fluxesof suspended particulate matter have been determined, witha special attention to the consequences of Mediterraneanflood events on the erosion regime and annual SPM fluxes.Chemical and physical denudation rates have been calcu-lated, in order to compare silicate and carbonate erosionrates in this mixed watershed, as well as fluxes of atmo-spheric/soil CO2 consumption.

Further, we used these data as a case study for the appli-cation of the “steady-state erosion” model developed byGaillardet et al. (1995). The concept of steady-state assumesthat the amount of bedrock which is eroded in a givendrainage basin per unit of time equals the fluxes of solidand dissolved material (corrected for non-weathering in-puts) leaving the drainage basin. Thus, a balance should ex-ist between solutes (produced continuously by rockchemical weathering) and solids (released physically fromsoils especially during periods of high floods). Generally,

this calculation has been applied to average annual values(Gaillardet et al., 1999b). Our data allow to check the appli-cability of the model on a suite of samples collected over thehydrological cycle of a river highly impacted by humanactivities through damming and river embankment.

2. STUDY AREA

The Rhone River is 816 km long and has a drainage areaof 98,800 km2 (Fig. 1). Since the construction of the Aswandam in 1968 and the consecutive reduction of the Nile river,the Rhone is the major river of Mediterranean sea, with anaverage water discharge of 1700 m3 s�1 at Beaucaire(60.5 km from the mouth) and an annual solid dischargefluctuating between 1.2 and 22.7 million tons (Pont, 1997;Sempere et al., 2000; Pont et al., 2002).

The Rhone has its source at 1850 m in Switzerland. Itshydrographic basin is characterized by mountainous catch-ments in the Alps, Cevennes, Jura and Vosges, inducingstrong climatic and geological heterogeneity. The strongtopographic gradients and the huge annual discharge com-pared to the river length led to the construction of 66hydroelectric dams on the river and its tributaries. About7% of the annual runoff is stored in these reservoirs (Pontet al., 2002). The alpine mountains (Isere and Durancewatersheds) consist mainly of sedimentary rocks (alluvium,sands, sandstones, conglomerates, schists, limestones, dolo-mite and shales) with some siliceous crystalline and meta-morphic rocks (granites and gneiss). The Jura and Vosgesmountains on the north (Ain and Saone watershed) are alsomainly calcareous, whereas crystalline siliceous rocks dom-inate in the old Cevennes mountains on the western side ofthe catchment (Ardeche, Ceze and Gard watershed), withsome sedimentary deposits on its borders. Evaporite out-crops (Triassic formations of Halite and Gypsum) arescarce and essentially concentrated in the Jura and Vosgesmountains (Saone watershed). Fluvio-glacial deposits arewidespread in the Alps, Jura, and Saone rivers. Therefore,the Rhone River transports in summer of lot of fine-grainedmaterial that has been eroded from glaciers.

Fifty kilometers up from its mouth, the Rhone Riversplits into two unequal branches. Our sampling station inArles is located on the “Grand Rhone” which drains 85%of the water discharge (Ibanez et al., 1997). All fluxes re-ported below are corrected for this partition, and corre-spond thus to the total output of the Rhone.

The strong climatologic variations observed over thecatchment encompass oceanic, continental and Mediterra-nean rainfall regimes. High variations of discharge resultfrom localized storms on the lower part of the watershed,Southern Alps or Cevennes mountains, while the seasonalmelting of glaciers and snow is responsible for high riverflow in spring (Vivian, 1989).

3. MATERIALS AND METHODS

The water discharge data is measured routinely by theCompagnie Nationale du Rhone (CNR, http://www.cnr.tm.fr/fr/) at the gauging station in Arles. The hydrological cycle of the Rhone is characterized by very large

Fig. 1. Geological map of the Rhone River watershed.

Physical and chemical erosion in the Rhone basin 909

variations. The daily discharge ranged from 247 to10600 m3 s�1 during our sampling period (Fig. 2). Themean water discharge was 1554 m3 s�1 over the 3 years.The annual runoff, obtained from the sum of the dailywater discharges, was 58.2 � 109 m3, 50.2 � 109 m3 and38.8 � 109 m3 in 2001, 2002 and 2003, respectively. Fourflood events (discharge larger than 3000 m3 s�1) occurredduring this period, in March–April 2001 (peak dischargeat 5868 m3 s�1), September 2002 (9034 m3 s�1), November2002 (with two successive peaks of 8250 and 9250 m3 s�1)and December 2003 (10600 m3 s�1). The last three eventswere the 4th, 5th and 1st largest floods in Arles since1856, with return periods of 55, 50 and 100 years. The floodof March–April 2001 was related to steady rainfall comingfrom Atlantic depressions affecting the northern part of thecatchments. The flood of September 2002 was related to alocal torrential rainfall affecting mainly two southern tribu-taries from the west bank, the Gard and Ceze rivers, result-ing in a discharge increase of 7000 m3 s�1 in 24 h. The flood

of November 2002 and December 2003 affected all thesouthern part of the Rhone basin, up to the lower part ofthe Saone watershed. In November 2002, the flood was re-lated to major depressions from the Mediterranean Seawhereas in December 2003 resulted from a steady rainfallcoming from Atlantic depressions.

Two rainwater samples were collected, at Vaison la Ro-maine in the Ouveze watershed (southeastern tributary ofthe Rhone) and in Camargue (Rhone delta) (Fig. 1). Thesampling sites were in open areas far from dust roads orother sources of contamination.

Subsurface water samples of the Rhone River were col-lected about twice a month from November 2000 toDecember 2003 at the town of Arles where the river flowswithin embankments (about 48 km upstream from the rivermouth, Fig. 1). The cross section is almost rectangular andthere is no overflow, even during severe floods. Further-more the pronounced narrowing of the river channel in-duces high water velocities and enhanced turbulence that

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Fig. 2. Water discharge, Suspended Particulate Matter (SPM) concentrations and cumulated fluxes of water and Suspended ParticulateMatter (on the top) in the Rhone River (at Arles) for the period 2001–2003. Flood events are specifically indicated.

910 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

ensures efficient homogenization of the suspended matteracross the section (Pont, 1992; Pont et al., 2002). A depthprofile of suspended sediment has been collected during aflood event in 2004, to assess the homogeneity of suspendedmatter concentration and composition across the section.Daily to hourly samples were taken during the floods.Water was collected manually from the bank (about40 cm below the surface) in 8 l polypropylene bottles. Fourliter bottles and a polypropylene funnel 26 cm in diameterwere used to collect the rain samples. pH was determinedon the field immediately after collection. Water was filteredrapidly (within 1 h after sampling) in the laboratorythrough pre-weighed 0.2 lm cellulose acetate filters. The fil-ters with suspended particulate matter (SPM) were weighedto determine SPM concentration before acid digestion. Twoaliquots of 30 ml of filtered water were collected for majorand trace elements analysis. Suboiled nitric acid (600 ll)was added to each aliquot to make to a pH of �1. Thirtymilliliters of filtered water without acidification was usedfor anion analysis.

Alkalinity was measured by HCl titration and Granmethod. Dissolved major cations (Ca2+, Mg2+, Na+ andK+) were determined by ICP (Jobin Ybon ULTIMA C).A home-made standard was used to check the validity ofthe analyses. The concentrations of SO4

2�, NO3� and Cl�

were determined by capillary electrophoresis (WatersCIA). Dissolved trace elements were measured by ICPMS(Perkin-Elmer, Elan 5000 and 6000 at LMTG, Toulouse).An indium internal standard solution was added to eachsample before analysis. Replicates of the international geo-standard SLRS-3 (Riverine Water Reference Material forTrace Metals certified by the National Research Council

of Canada) were performed to check the accuracy and thereproducibility of the analyses. Our data agreed with certi-fied values within 5%, for major and trace elements andanions.

Suspended particulate matter filters were dissolved insealed Teflon beakers with suboiled HNO3–HF–HClO4.Boric acid (H3BO3) was added following digestion to com-plex F in solution. Some filters were dissolved after alkalinefusion (LiO2) for Si analysis. Analyses of the STSD-3 river-ine particles standard (National Research Council of Can-ada) under the same conditions as samples yielded resultswithin 5% of the certified values. The external reproducibil-ity, including both chemical preparation and ICP measure-ments, was also performed on replicate samples resulting inprecisions of less than 5%. The contamination resultingfrom the acid digestion procedure was less than 2% of thesample concentrations.

The mineralogical composition of each sample wasdetermined by X-ray diffraction (Philips 1729 diffractome-ter, cobalt anticathode, 1600 W, 40 mA, 40 kV). The semiquantitative mineralogical composition was obtained bymeasuring the area of the principal mineral peak (0 0 1).

4. RESULTS

The suspended particulate matter (SPM) concentrationsand chemical analyses of major and trace elements are listedin Tables 1 and 2. Dissolved compositions were obtained in85 samples for major elements, and 52 for trace elements.Eighty-five SPM concentrations and 63 particulate majorand trace elements concentration were determined, princi-pally during the flood events.

Table 1Water discharge (Q, m3 s�1), Suspended Particulate Matter concentrations (SPM, mg l�1), dissolved major and trace elements (in mmol l�1, except for Sr and Ba in lmol l�1 and Rb in nmol l�1) inthe Rhone River during the period 2001–2003.

Date Q (m3 s�1) SPM (mg l�1) pH HCO3� Cl� SO4

2� NO3� Ca2+ Mg2+ Na+ K+ Sr Ba Rb

28/11/00 3847 223 0.343 0.309 0.111 1.83 0.232 0.345 0.06112/12/00 2940 61.5 2.48 0.320 0.316 1.60 0.204 0.310 0.04804/01/01 1991 30.1 2.82 0.462 0.438 1.78 0.253 0.421 0.05318/01/01 2049 31.6 3.15 0.373 0.328 1.81 0.250 0.341 0.05007/02/01 2493 28.6 3.15 0.423 0.382 1.89 0.258 0.395 0.05215/02/01 1663 15.5 7.85 0.410 0.418 0.12301/03/01 1038 5.1 7.89 3.28 0.520 0.523 0.125 1.87 0.266 0.455 0.0562/3/01 11:00 1370 4.7 7.83 3.02 0.545 0.530 0.126 1.92 0.271 0.475 0.0582/3/01 15:00 1770 6.4 7.85 2.95 0.538 0.528 0.129 1.88 0.263 0.464 0.05703/03/01 2100 11.0 3.12 0.532 0.541 0.127 1.89 0.265 0.460 0.05505/03/01 3001 90.1 7.96 3.18 0.494 0.513 0.115 1.80 0.267 0.436 0.05306/03/01 3945 129 7.98 3.22 0.522 0.443 0.123 1.81 0.253 0.446 0.0559/3/01 11:40 5110 583 3.02 0.323 0.320 0.104 1.51 0.201 0.286 0.0529/3/01 15:30 5140 685 3.61 0.329 0.304 0.108 1.60 0.210 0.306 0.05612/03/01 4612 246 7.88 3.06 0.283 0.308 0.096 1.72 0.209 0.263 0.04814/03/01 5402 483 7.98 2.88 0.292 0.315 0.094 1.65 0.214 0.274 0.05016/03/01 4951 240 7.98 3.05 0.270 0.298 0.098 1.73 0.206 0.255 0.04920/03/01 5056 191 7.60 3.16 0.236 0.269 0.082 1.75 0.203 0.250 0.04822/3/01 10:00 5047 125 7.98 3.28 0.250 0.300 0.082 1.76 0.208 0.251 0.04822/3/01 15:00 5159 136 7.98 3.03 0.247 0.301 0.081 1.74 0.209 0.239 0.04826/03/01 4757 434 8.19 3.12 0.232 0.304 0.085 1.75 0.197 0.231 0.04829/03/01 3959 149 7.98 3.03 0.263 0.285 0.090 1.77 0.204 0.262 0.05102/04/01 3182 65.1 8.06 3.27 0.284 0.328 0.082 1.84 0.230 0.293 0.05303/04/01 3115 49.8 8.30 3.34 0.298 0.321 0.094 1.84 0.223 0.302 0.05320/04/01 2715 24.2 8.00 3.32 0.328 0.364 0.093 1.84 0.225 0.301 0.04816/05/01 2085 45.1 8.15 2.98 0.320 0.544 0.140 1.72 0.269 0.317 0.046 4.95 0.212 13.010/07/01 1350 15.1 7.98 0.331 0.526 0.054 5.59 0.188 17.013/09/01 1081 6.2 7.92 2.52 0.415 0.507 0.068 1.60 0.261 0.463 0.055 4.81 0.170 20.405/10/01 1445 19.9 8.04 3.13 0.403 0.427 0.084 1.76 0.244 0.427 0.057 4.80 0.181 16.706/12/01 1408 28.3 7.60 3.42 0.565 0.336 0.134 1.94 0.226 0.537 0.079 3.69 0.213 24.607/01/02 1105 35.4 7.90 3.06 0.495 0.380 0.149 1.88 0.225 0.487 0.074 3.77 0.181 18.230/01/02 1854 49.4 7.86 0.548 0.400 0.124 1.92 0.252 0.565 0.070 4.61 0.224 24.218/02/02 1105 14.4 7.91 3.29 0.536 0.452 0.122 2.11 0.279 0.550 0.066 5.07 0.191 15.202/03/02 2604 82.7 7.87 3.15 0.339 0.289 0.119 1.80 0.214 0.350 0.057 3.33 0.180 14.328/03/02 1189 13.9 7.97 3.25 0.419 0.344 0.102 1.76 0.246 0.434 0.057 3.75 0.203 16.125/04/02 537 9.8 8.10 3.04 0.502 0.572 0.095 1.78 0.293 0.521 0.058 5.56 0.205 16.707/05/02 2470 56.6 8.05 2.94 0.471 0.376 0.10508/05/02 2488 51.7 7.60 3.09 0.542 0.366 0.105 1.82 0.234 0.505 0.065 3.95 0.181 18.322/05/02 1330 19.4 7.95 2.73 0.389 0.526 0.089 1.76 0.268 0.407 0.050 5.54 0.187 14.506/06/02 1483 13.0 7.70 2.62 0.508 0.598 0.098 1.67 0.268 0.458 0.056 4.80 0.189 16.716/08/02 904 12.3 7.95 2.48 0.415 0.455 0.053 1.64 0.251 0.452 0.057 4.14 0.149 19.69/9/02 22:00 7384 1874 7.65 1.83 0.237 0.314 0.052 1.21 0.168 0.277 0.072 2.90 0.236 13.9

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Table 1 (continued)

Date Q (m3 s�1) SPM (mg l�1) pH HCO3� Cl� SO4

2� NO3� Ca2+ Mg2+ Na+ K+ Sr Ba Rb

10/9/02 7:00 8927 1144 7.74 1.91 0.149 0.209 0.050 1.22 0.132 0.195 0.087 2.47 0.221 13.010/9/02 12:00 7135 1068 7.59 2.05 0.161 0.218 0.048 1.32 0.140 0.200 0.081 2.65 0.264 12.210/9/02 16:30 4597 1241 7.78 2.32 0.189 0.258 0.056 1.46 0.162 0.236 0.079 2.97 0.293 12.011/9/02 14:00 2143 207 7.91 2.80 0.313 0.390 0.074 2.09 0.261 0.412 0.093 5.15 0.287 16.913/9/02 14:00 1600 45.8 7.75 3.05 0.386 0.454 0.076 2.03 0.266 0.457 0.075 4.67 0.242 16.927/09/02 849 7.7 8.03 2.77 0.480 0.649 0.082 1.95 0.320 0.556 0.067 5.42 0.216 17.609/10/02 1006 3.0 8.11 2.83 0.606 0.654 0.085 1.96 0.325 0.652 0.06908/11/02 2619 114 7.90 3.12 0.367 0.301 0.100 1.85 0.208 0.346 0.063 3.07 0.156 16.416/11/02 6400 1534 7.90 2.75 0.240 0.301 0.089 1.81 0.224 0.268 0.06317/11/02 8100 2912 7.90 2.68 0.239 0.333 0.088 1.62 0.216 0.236 0.06418/11/02 11:00 8140 877 7.88 2.65 0.203 0.259 0.079 1.70 0.208 0.236 0.066 3.94 0.280 13.418/11/02 17:00 8096 720 7.59 2.55 0.192 0.244 0.079 1.72 0.203 0.236 0.066 3.79 0.272 12.619/11/02 10:00 7532 502 7.93 3.07 0.206 0.248 0.085 1.83 0.207 0.244 0.066 3.70 0.239 11.819/11/02 16:00 7041 421 7.93 2.83 0.212 0.255 0.084 1.81 0.205 0.237 0.063 3.69 0.238 12.320/11/02 9:00 5475 327 7.94 3.25 0.217 0.281 0.082 1.83 0.220 0.250 0.062 4.00 0.232 11.920/11/02 17:00 5067 266 8.17 2.77 0.217 0.286 0.085 1.82 0.217 0.253 0.06121/11/02 4702 228 7.85 2.59 0.228 0.308 0.090 1.95 0.237 0.275 0.064 4.35 0.226 12.022/11/02 9:00 6301 225 7.95 3.19 0.234 0.310 0.092 1.91 0.225 0.266 0.061 3.97 0.216 12.222/11/02 17:00 6300 389 7.90 3.14 0.197 0.266 0.076 1.71 0.202 0.232 0.061 3.69 0.234 12.525/11/02 8554 691 8.01 2.50 0.192 0.260 0.066 1.48 0.196 0.232 0.057 3.58 0.261 13.126/11/02 9209 641 7.95 2.51 0.192 0.226 0.077 1.54 0.189 0.240 0.061 3.37 0.244 12.327/11/02 8398 521 7.95 2.95 0.214 0.249 0.082 1.68 0.211 0.253 0.061 3.76 0.243 11.228/11/02 6340 314 8.10 3.04 0.245 0.264 0.085 1.79 0.216 0.278 0.060 3.86 0.239 11.129/11/02 5320 198 8.00 3.13 0.311 0.313 0.118 1.77 0.214 0.288 0.058 3.85 0.255 11.002/12/02 4126 94.7 7.95 3.52 0.308 0.313 0.112 1.84 0.211 0.288 0.054 3.84 0.219 11.319/02/03 1281 7.9 7.98 3.01 0.504 0.562 0.127 2.00 0.282 0.486 0.053 4.82 0.195 13.301/04/03 900 11.1 8.18 3.38 0.630 0.647 0.103 2.23 0.335 0.597 0.055 5.24 0.207 14.223/05/03 1073 10.9 7.92 2.40 0.503 0.572 0.085 1.69 0.284 0.573 0.056 5.04 0.184 18.817/06/03 1052 7.6 7.94 2.16 0.439 0.567 0.072 1.50 0.266 0.522 0.052 4.94 0.182 19.915/07/03 650 5.1 7.95 2.30 0.535 0.734 0.047 1.55 0.317 0.658 0.064 5.69 0.197 26.913/08/03 820 4.2 7.50 1.89 0.518 0.678 0.056 1.42 0.286 0.573 0.05929/08/03 1235 8.9 7.65 1.72 0.461 0.637 0.054 1.36 0.277 0.527 0.02112/11/03 1030 114 7.80 3.17 0.602 0.531 0.150 1.99 0.271 0.535 0.0641/12/03 13:00 2297 42.4 7.92 3.67 0.602 0.543 0.132 1.86 0.286 0.557 0.061 5.33 0.237 17.52/12/03 9:00 5886 592 7.80 3.02 0.410 0.348 0.095 1.50 0.220 0.417 0.056 4.04 0.260 15.92/12/03 12:35 6902 1250 7.80 2.91 0.297 0.277 0.077 1.37 0.177 0.312 0.055 3.37 0.245 16.83/12/03 10:30 10013 1402 8.01 2.20 0.212 0.210 0.095 1.28 0.153 0.244 0.061 3.16 0.350 13.43/12/03 17:30 10600 1704 7.73 2.33 0.231 0.224 0.110 1.31 0.162 0.249 0.062 3.29 0.366 11.34/12/03 12:15 9997 897 7.93 2.24 0.206 0.202 0.099 1.24 0.161 0.228 0.056 2.73 0.268 11.04/12/03 19:45 7964 631 7.99 2.23 0.218 0.217 0.099 1.31 0.181 0.280 0.055 2.85 0.284 11.25/12/03 8:00 4806 617 8.00 2.40 0.280 0.282 0.130 1.52 0.212 0.342 0.061 3.78 0.314 12.35/12/03 14:30 4411 531 8.20 2.37 0.279 0.288 0.128 1.49 0.215 0.341 0.062 3.85 0.311 12.06/12/03 7:00 3561 351 7.90 2.58 0.305 0.327 0.139 1.56 0.234 0.367 0.063 4.27 0.312 12.0

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Table 2Concentrations of major (in mol kg�1 except for P, Mn and Ti which are in mmol kg�1) and trace elements (in mmol kg�1 except for Sm, andTh in lmol kg�1) in the suspended particulate matter of the Rhone River during the period 2001–2003.

Date Si Ca Al Mg Na K Fe P Mn Ti Rb Sr Ba Nd Sm Th

12/12/00 8.76 1.43 1.93 0.285 0.150 0.326 0.489 26.0 11.5 58.018/01/01 6.75 3.27 2.39 0.430 0.161 0.440 0.594 39.7 15.7 68.0 1.33 4.65 2.62 0.17 32 3807/02/01 7.20 2.90 2.37 0.447 0.166 0.432 0.624 43.7 15.5 68.4 1.30 4.39 2.79 0.17 32 3806/03/01 6.92 3.07 1.85 0.390 0.189 0.354 0.444 32.8 12.2 58.69/3/01 11:40 6.02 4.05 1.66 0.279 0.110 0.367 0.378 21.3 7.57 50.1 0.88 5.11 2.22 0.13 24 299/3/01 15:30 5.42 3.99 1.79 0.291 0.098 0.390 0.403 26.5 7.44 55.0 0.97 5.12 2.25 0.14 25 3012/03/01 6.27 3.23 2.11 0.412 0.186 0.464 0.484 29.6 12.0 50.9 1.23 4.43 2.62 0.15 28 3616/03/01 6.45 3.04 2.02 0.412 0.200 0.383 0.489 29.9 12.5 61.720/03/01 6.37 3.30 1.99 0.348 0.186 0.463 0.443 30.2 11.1 57.8 1.18 4.62 2.70 0.16 30 3922/3/01 10:00 7.29 3.21 2.11 0.380 0.199 0.444 0.492 39.0 12.4 62.2 1.23 4.55 2.99 0.17 32 4022/3/01 15:00 6.86 3.04 2.12 0.369 0.200 0.445 0.472 34.9 11.8 61.8 1.20 4.32 2.95 0.17 31 3626/03/01 6.99 2.52 2.37 0.478 0.254 0.535 0.517 29.0 14.2 70.7 1.29 4.66 3.97 0.17 30 4029/03/01 7.38 2.58 2.37 0.447 0.242 0.539 0.522 32.4 14.4 69.9 1.31 4.67 4.39 0.16 32 4002/04/01 7.10 2.57 2.19 0.450 0.214 0.410 0.525 31.3 14.8 59.5 1.22 3.73 2.86 0.14 29 3503/04/01 5.46 2.69 2.32 0.447 0.222 0.505 0.544 46.4 16.3 63.9 1.35 4.11 2.96 0.17 33 4420/04/01 6.61 2.43 2.35 0.390 0.179 0.468 0.567 48.0 15.3 63.3 1.42 3.53 2.72 0.18 34 4416/05/01 6.09 2.56 2.13 0.375 0.197 0.487 0.439 29.3 13.4 36.4 1.35 5.12 3.02 0.12 25 3310/07/01 7.08 4.54 3.55 0.728 0.365 0.661 0.810 72.0 29.4 100 1.87 6.89 4.55 0.25 47 6413/09/01 7.77 4.17 3.39 0.712 0.368 0.682 0.788 91.0 28.8 91.3 1.82 6.28 4.61 0.22 41 5805/10/01 4.10 3.18 2.77 0.511 0.217 0.530 0.624 45.0 19.7 62.7 1.58 4.99 3.04 0.15 30 4106/12/01 7.44 1.80 3.06 0.414 0.146 0.461 0.834 73.6 24.4 88.4 1.61 2.33 3.28 0.25 48 6607/01/02 1.75 3.13 0.457 0.130 0.497 0.868 73.6 19.7 88.1 1.65 2.10 3.03 0.27 50 6430/01/02 7.19 2.52 2.40 0.440 0.197 0.422 0.619 56.4 21.8 73.4 1.39 3.58 3.06 0.20 39 5018/02/02 2.19 2.62 0.431 0.171 0.455 0.705 71.2 30.7 75.6 1.51 3.46 3.13 0.22 42 5902/03/02 7.99 2.09 2.50 0.427 0.166 0.436 0.647 48.1 16.2 74.1 1.62 3.36 2.72 0.19 35 3928/03/02 2.11 2.57 0.410 0.167 0.463 0.609 55.0 17.0 74.2 1.43 3.14 2.76 0.17 32 4425/04/02 2.65 1.97 0.384 0.163 0.358 0.495 70.2 21.5 54.0 1.06 3.72 2.71 0.15 29 3607/05/02 2.79 2.18 0.387 0.166 0.393 0.527 44.0 18.2 66.508/05/02 2.76 2.24 0.402 0.186 0.402 0.551 44.1 19.3 63.4 1.29 3.68 2.65 0.18 33 4322/05/02 2.79 2.46 0.447 0.169 0.446 0.614 53.8 21.2 52.206/06/02 6.53 2.80 2.64 0.458 0.195 0.472 0.628 60.4 20.7 75.616/08/02 7.78 2.62 3.06 0.552 0.253 0.554 0.682 44.9 24.0 82.99/9/02 22:00 2.14 2.49 0.382 0.156 0.455 0.587 23.0 12.5 67.810/9/02 7:00 7.42 2.27 2.47 0.336 0.099 0.421 0.588 24.3 11.0 65.0 1.45 3.10 3.15 0.21 38 4910/9/02 12:00 7.07 2.66 2.35 0.297 0.086 0.496 0.519 25.6 9.42 60.9 1.34 3.33 2.81 0.19 35 4610/9/02 16:30 6.45 3.31 2.20 0.279 0.075 0.475 0.461 23.6 8.10 59.4 1.24 4.04 2.54 0.17 31 4111/9/02 14:00 7.05 3.28 2.23 0.313 0.094 0.465 0.490 29.5 9.63 61.6 1.28 4.31 2.48 0.17 31 3813/9/02 14:00 6.90 3.71 2.55 0.383 0.133 0.517 0.573 41.1 14.9 63.0 1.48 4.99 2.85 0.18 33 3927/09/02 4.17 2.99 0.504 0.201 0.546 0.691 66.8 19.8 80.6 1.64 6.03 3.14 0.20 36 4608/11/02 2.78 2.20 0.383 0.196 0.490 0.521 34.3 16.4 65.6 1.30 3.59 2.45 0.17 33 4518/11/02 11:00 6.54 3.41 2.09 0.359 0.158 0.488 0.450 23.8 11.7 60.9 1.17 4.72 2.64 0.15 27 3519/11/02 10:00 6.76 3.43 1.99 0.353 0.168 0.467 0.441 26.4 11.2 60.2 1.12 4.58 2.79 0.16 29 3619/11/02 16:00 6.04 3.46 1.97 0.354 0.168 0.454 0.438 25.7 11.3 58.2 1.09 4.62 2.82 0.15 28 3220/11/02 9:00 6.52 3.80 1.96 0.342 0.150 0.451 0.431 26.5 12.0 57.9 1.06 5.00 2.41 0.15 28 3321/11/02 7.04 3.74 1.94 0.326 0.137 0.440 0.424 25.4 11.1 56.7 1.06 4.85 2.25 0.16 28 3522/11/02 9:00 3.41 1.93 0.349 0.178 0.431 0.447 29.9 11.6 58.6 1.03 4.63 2.92 0.16 30 3625/11/02 3.51 1.95 0.313 0.139 0.433 0.439 26.9 10.9 62.6 1.06 4.57 2.62 0.16 29 3526/11/02 3.50 1.97 0.319 0.152 0.444 0.441 28.6 11.1 58.9 1.09 4.57 2.98 0.16 29 3527/11/02 3.93 1.81 0.312 0.143 0.424 0.400 26.0 10.4 53.9 0.98 5.13 2.80 0.15 28 3228/11/02 4.11 1.81 0.312 0.140 0.411 0.398 27.2 10.4 51.2 0.97 524 2.58 0.14 26 3029/11/02 4.07 1.82 0.301 0.127 0.398 0.406 25.6 10.7 51.9 0.96 5.58 2.19 0.14 27 3402/12/02 4.00 1.88 0.303 0.130 0.403 0.424 28.9 11.8 51.8 0.99 4.20 2.09 0.15 28 3623/05/03 1.96 1.65 0.304 0.144 0.340 0.373 91.4 26.3 46.0 0.94 3.55 1.96 0.10 20 2517/06/03 3.43 3.07 0.561 0.286 0.577 0.676 104 43.2 86.91/12/03 13:00 3.94 2.14 0.360 0.132 0.391 0.457 26.2 13.6 61.8 1.24 5.27 2.12 0.15 26 282/12/03 9:00 3.30 2.11 0.352 0.167 0.406 0.484 30.0 12.7 63.4 1.19 4.39 2.80 0.17 32 432/12/03 12:35 3.47 1.83 0.307 0.134 0.335 0.415 23.3 9.80 57.1 1.03 4.53 2.35 0.16 29 363/12/03 10:30 3.75 1.89 0.308 0.114 0.356 0.437 20.3 9.05 56.7 1.06 4.69 2.46 0.14 26 313/12/03 17:30 3.97 1.87 0.318 0.114 0.357 0.430 21.1 10.0 56.9 1.03 4.90 2.36 0.15 27 324/12/03 12:15 3.71 2.06 0.329 0.120 0.397 0.475 18.5 10.6 58.2 1.14 4.61 2.52 0.17 30 40

(continued on next page)

Physical and chemical erosion in the Rhone basin 913

Table 2 (continued)

Date Si Ca Al Mg Na K Fe P Mn Ti Rb Sr Ba Nd Sm Th

4/12/03 19:45 3.73 2.02 0.318 0.117 0.383 0.467 21.1 10.3 56.6 1.16 4.67 2.46 0.16 29 365/12/03 8:00 4.04 1.84 0.303 0.115 0.342 0.431 17.6 10.0 53.9 1.05 5.37 2.08 0.14 25 296/12/03 7:00 4.00 1.90 0.318 0.124 0.330 0.443 19.2 10.4 57.1 1.13 5.10 2.21 0.15 27 30

914 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

4.1. Suspended sediments concentration and flux

Over the period of study, the suspended load rangedfrom 3 to 2911 mg l�1, with very strong peaks during theflood events (Figs. 2 and 3). The maximum concentrationwas observed during the flood of November 2002.

The overall correlation (Fig. 3) between water dischargeand SPM concentration can be fitted with an empiricalpower-law (Walling, 1974, 1978; Sempere et al., 2000; Pontet al., 2002):

September 2002

water disch0 2000 4000 60

SPM

con

cent

ratio

ns (

mg.

l-1)

0

500

1000

1500

2000

2500

3000

350010000

1000

100

10

1100 1000 10000

mg.l-1

m3.s-1

water discha

0 2000 4000 60

SPM

con

cent

ratio

ns (

mg.

l-1)

0

200

400

600

800

1000Flood of March-April 2001

Fig. 3. Relations between water discharge and Suspended Particulate MMarch–April 2001 ( , Fig 3b), September 2002 ( ), November 2002 (jarrows indicate the time sequence. The inset (in Fig 3a) shows the overall2003.

log CSPM ¼ 2:149 log Q� 5:489

ðN ¼ 85; r2 ¼ 0:851; p < 0:0001Þ ð1Þ

However, a large scatter in SPM concentration is observedat high flow during the floods. During each flood event, thedata show the classical trend of clockwise hysteresis, withhigher SPM concentrations on the rising limb of the floodthan at the same discharge on the falling limb (Fig. 3)(Schumm, 1977; Meybeck, 1985). The hysteresis factor(maximum ratio between SPM concentrations at the same

1st November 2002

2nd November 2002

December 2003

arge (m3.s-1)00 8000 10000 12000

rge (m3.s-1)

00 8000 10000 12000

a

b

atter concentrations (SPM) in the Rhone River (d). Floods of1st, 2nd), and December 2003 (s) are specially indicated. The

relation (log–log scale) including all samples collected from 2001 to

Physical and chemical erosion in the Rhone basin 915

discharge) is variable between the different flood events: fac-tor 5 during the first flood of November 2002, factor 1.3during the second flood of November 2002. We will showbelow that this high variability of SPM concentration mustbe taken into account for the precise calculation of erosionfluxes.

4.2. Chemical composition of the dissolved and particulate

phases

4.2.1. Dissolved composition

The Rhone River is characterized by high pH valuesranging from 7.5 to 8.3. Total concentrations of major cat-ion (R+ = K+ + Na+ + Ca2+ + Mg2+) range from 3.0 to5.8 meq l�1, with a mean value (4.41 meq l�1) higher thanthe average for world rivers (1.47 meq l�1, Meybeck andRagu, 1996; Gaillardet et al., 1999a). Prevalent cationsare Ca > Na �Mg > K. These chemical features (Ca2+

dominance and R+ > 4 meq l�1) are generally found in riv-ers draining carbonate bedrocks (Meybeck, 1986; Han andLiu, 2004). The data are illustrated in Fig. 4 as Na/Ca ver-sus Mg/Ca molar ratios. In this plot, the Rhone composi-tion appears quite homogeneous (0.1 < Mg/Ca < 0.2 and0.09 < Na/Ca < 0.35), and defines a triangular array wherethe high Ca end-member corresponds mostly to the samplescollected during the floods. In this figure, the Rhone is alsoclearly distinct from the other large French rivers (Loireand Saone). The calcite saturation index (CSI) calculatedwith PHREEQC shows that most of our samples areslightly supersaturated with respect to calcite (CSI rangingfrom �0.08 to 0.81, average = 0.36). Jacobson et al. (2002)and Wu et al. (2005) have shown that the correlation of theCa/Sr ratio with the CSI index can be considered as evi-dence of in situ precipitation of CaCO3. We do not observethis correlation in our data and therefore we assume that

0.01

0.1

1

10

Mg/C0.01 0.1

Na/

Ca

Rhône River - Diss. (this study)

Saône River - Diss. (RMC)Loire River - Diss.(Grosbois et al., 2000)

Rhône River - Part. (this study)Gard River - Part. (this study)Ardeche River - Part. (this study)Durance River - Part. (this study)Ouveze River - Part. (this study)

Dissolved load(Rhone River)

Fig. 4. Mixing diagram of the dissolved and particulate load in the RhoSaone rivers), and in the Loire River. Silicates, carbonates and evaporiteGaillardet et al. (1997), Grosbois et al. (2000), Han and Liu (2004) and frorivers are corrected from atmospheric inputs using Cl as a reference (seesilicate end-members by considering the uncertainties on these end-mem

calcite precipitation is insignificant in the samples. As aconsequence, the carbonate contributions calculated fromour data could be minimum estimates in the dissolvedphase, and upper estimates in the particles, due to this ef-fect. But, this has no implications on the main conclusionsof this study.

4.2.2. Chloride concentration and correction of the

atmospheric input

Rainwater has been recognised as a major source of dis-solved species in surface waters (Gibbs, 1970; Stallard andEdmond, 1981; Meybeck, 1983; Negrel et al., 1993). Thecontribution from atmospheric deposition is generally esti-mated from chloride ions, which behave conservativelythrough the hydrological cycle (Stallard, 1980; Meybeck,1983, 1986). Only a few data of Cl� concentrations havebeen measured in rainwater in the Rhone watershed. Negreland Roy (1998) reported an average of 19 lmol l�1 (13 sam-ples) in the north of Massif Central. We measured152 lmol l�1 (1 sample) in the Ouveze watershed (south-eastern tributary of the Rhone), and as high as 300 lmol l�1

(1 sample) in Camargue close to the Mediterranean Sea.Obviously, this input is strongly dependent on the specificorigin of the floods, since the chloride concentration in rain-water decreases with increasing distance from the coast, dueto progressive rainout along the air mass circulation.Assuming that the rain over Massif Central is representa-tive of the whole northern part of the Rhone basin, whichreceives 60% of the total precipitation (Cl� = 19 lmol l�1),and that rain over Ouveze is typical of the 40% precipita-tion (Cl� = 152 lmol l�1) falling on central and southernsections, we obtain an upper limit of Cl� concentration inthe rain of about 72 lmol l�1. This value has to be en-hanced by an average concentration factor close to 2, sincetotal precipitation is 1124 kg m�2 yr� and runoff 580

a1

Particulate load(Rhone River)

ne River and its tributaries (Ardeche, Durance, Gard, Ouveze andend-members are constrained using the data from Meybeck (1986),m RMC’s network (see text). Dissolved load in the Rhone and Loiretext). The gray area shows the mixing line between carbonate and

bers.

Rhône RiverSaône River

Gard River

Cl / Ca0 0.1 0.2 0.3 0.4 0.5 0.6

0

0.1

0.2

0.3

0.05

0.15

0.25

Mg

/ Ca

Diss. Sil.

Eva.

0 2.521.510.50

0.8

0.6

0.4

0.2

Carb.

(RMC)(this study)

(RMC)

Fig. 5. Relationship between Mg/Ca and Cl/Ca molar ratios in the dissolved load of the Rhone, Gard and Saone rivers. Data from the RhoneRiver are corrected from atmospheric inputs using Cl as a reference (see text).

916 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

kg m�2 yr�1 for the whole watershed of the Rhone (Etche-vers et al. 2001), except for samples collected during floodevents in the massif central for which a concentration factorof 1.2 is applied (Grosbois et al., 2000). The average Cl�

concentration from the atmosphere (cyclic Cl) in the RhoneRiver ([Cl]cyclic) is thus of about 145 lmol l�1 except forsamples collected during flood events for which it is of87 lmol l�1.

Nevertheless, the chloride concentrations measured inthe Rhone River are clearly higher than this value (�150–640 lmol l�1). High concentrations are contributed by theSaone (data from Reseau Rhone–Mediterranee–Corse(RMC), http://www.rhone-mediterranee.eaufrance.fr, Figs.

Cl /

Na

/ Ca

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.

Rhône River

Gard River

(this study)

(RMC)

Fig. 6. Relationship between Na/Ca and Cl/Ca molar ratios in the dissolvRiver are corrected from atmospheric inputs using Cl as a reference (see

5 and 6), and by the Durance (Alary, 1998), due to the pres-ence of evaporites in the drainage basin of these two rivers(Fig. 1). Fertilizers, as well as industrial and urban waste-waters may also represent additional sources (Meybeckand Helmer, 1989; Flintrop et al., 1996). Indeed, the in-crease in NO3/Na with water discharge in the Seine andLoire Rivers has been interpreted as evidence of anthropo-genic input (Roy et al., 1999; Grosbois et al. (2000)). Theseare also consistent with the steady increase in NO3/Na withCl/Na in these rivers.

Each element is then corrected for the rainwater input(Xa), using the average value of the element to chloriderainwater ratio (X/Cl)rw:

Ca

00 1 2 3

1

2

Diss. Sil.

3 0.4 0.5 0.6

Carb.

Saône River(RMC)

Evap.

ed load of the Rhone, Gard and Saone rivers. Data from the Rhonetext).

Table 3Proportions of Ca2+, Mg2+, Na+ and K+ derived from the atmosphere and from silicate, carbonate and evaporite/anthropogeniccontributions in the Rhone River.

Atmosphere (%) Carbonates (%) Silicates (%) Eva/Ant. (%)

Ca2+ 3–7.8 72–89 7–18 0–4Mg2+ 3.7–7.9 14–35 58–78 0–3.9Na+ 12–33 1.3–5.2 37–79 0–42K+ 14–45 45–95

Physical and chemical erosion in the Rhone basin 917

Xa ¼ ½Cl�cyclic �X

Cl

� �rw

ð2Þ

The atmospheric corrections range from 14% to 58% forchloride (with a mean input of 32%), from 12% to 33%(22%) for sodium, and from 14% to 45% (27%) for potas-sium. By contrast, the corrections are minor for Ca andMg, ranging from 3.0% to 7.8% (4.7%) and from 3.7% to7.9% (5%), respectively (Table 3).

4.2.3. Particulate composition

Calcite is the most abundant of the coarse minerals inthe SPM of the Rhone River, accounting for 28% up to60% of non clay minerals. The other coarse minerals areby abundance: Quartz (27–48%), Micas (3–16%), Na- andK-Feldspar (2–7% and 1–8%, respectively) and Dolomite(1–3%). Illite is the dominant clay mineral and represents38–84% of the clay mineral fraction. The other clay miner-als are: Smectite (3–43%), Kaolinite (3–14%), Chlorite (4–11%) and mixed Illite/Chlorite (0–9%). The proportion ofprimary minerals and the dominant type of clay are highlydependant on the origin of flood.

The SPM chemical composition is characterized by highCa2+ concentrations, representing more than 20% of the to-tal cations. The major cations ratios have been plotted inFig. 4 for comparison with the dissolved phase. From therelationship between the molar ratios Na/Ca and Mg/Cain the particulate phase, we calculate that 12–40% ofSPM is composed by calcite (with an average calcite con-tent of about 29%). The SPM data in Fig. 4 are clearly dif-ferent from those in solution, with a broad linearcorrelation between Na/Ca and Mg/Ca, lower Na/Ca inthe SPM, and also partially lower Mg/Ca ratios. The rangeof variation is also larger in the particles than in solution.The Rhone data is spread between the values obtained inthe Ouveze and Durance rivers on the high Ca side, andthose from the Gard and Ardeche at the other end. How-ever, a significant scatter is observed around the averageline. In general, major and trace elements particulate con-centrations tend to decrease with increased discharge exceptfor Ca, Sr and Ba concentrations which increase slightly. Asa consequence, the high-calcium samples (low Na/Ca andMg/Ca ratios) are those collected during the floods, as al-ready noted for the dissolved composition.

5. DISCUSSION

In the following discussion, we show that the relativecontribution from carbonate and silicate bedrocks can bediscriminated from each other in the dissolved and particu-

late load transported by the Rhone River. We estimate thechemical fluxes issued from these two sources and theirrespective contribution to the physical and chemical erosionrates, as well as the seasonal variation of their relationshipduring the hydrological cycle. These results are then dis-cussed by comparison with global estimates compiled fromthe literature on the large world rivers. Finally, we applythe steady-state erosion budget model of Gaillardet et al.(1997) to our data and discuss the results of this model inthe light of the recent evolution of this river over the lastcentury.

5.1. Physical erosion

The annual solid discharges were calculated for the 3years of survey by integrating the measured SPM fluxes,using the hourly flow rate in Arles given by the CNR data.We obtained SPM fluxes of 5.1 ± 0.4 � 106 t in 2001,10.9 ± 0.9 � 106 t in 2002 and 4.8 ± 0.4 � 106 t in 2003.Uncertainties on these numbers (8.6%) correspond to twosigma standard deviation of the SPM measurements ob-tained on depth profiles in the middle of the river in frontof the sampling site. This method is the most appropriateto take into account the high variability of the SPM versusdischarge relationships both during the flood events, andbetween different floods. However, the results are very sen-sitive to the curvature of the SPM versus discharge relation-ship in the early part of the flood events. We checkedcarefully that the hourly resolution of our sample spacingduring the floods was sufficient to monitor these variations,and that the linear interpolation between samples did notinduce uncertainties larger than those quoted above.

Typical of Mediterranean rivers, the strong inter-annualvariability is directly related to the number and intensity ofthe flood events. Over the 3 years, 82%, 88% and 78% of theSPM annual fluxes were discharged during floods (i.e. 12%of the time period), compared to only 38%, 31% and 11% ofthe annual water flux (Fig. 2). Considered alone, the floodof November 2002 contributed 38% of the total SPM fluxover all the study period (1128 days) in less than 4% ofthe time (36 days).

It is important to emphasize that, due to the high vari-ability of the SPM relationship with flow rate from oneflood to another, the solid discharge is not proportionalto the peak discharge, nor to the integrated water dischargeof the flood events. In Table 4, we compare our results withthose obtained with the traditional method of using log–lograting curves such as Eq. (1) above (Walling, 1974, 1978).While the agreement between the two methods is wellwithin error for 2002 and 2003, the log–log estimate would

Table 4Water and solid fluxes (flood events and annual discharge) of the Rhone River in 2001, 2002 and 2003 using integration method and log–lograting curves from this study (see text) and log–log rating curves from Pont et al. (2002) and Sempere et al. (2000). Results take into accountthe contribution of the “petit Rhone” which carries about 15% of the total water liquid flux. (�): Data are corrected by using the correctionfactor of Ferguson (1986, 1987) (CF = exp(2.65r2) = 1.289 where r2 is

Pnðlog Cobs � log CestÞ2ðn� 2Þ and Cobs is the actual sedimentconcentration, Cest the estimate of actual concentration for the same observation and n the number of observations).

Floods Total annual

Mar.–Apr. 2001 Sept. 2002 Nov. 2002 Dec. 2003 2001 2002 2003

Peak discharge (m3 s�1) 5868 9034 9250 10600Integrated water flux (109 m3) 25.2 1.62 16.5 4.8 66.9 57.7 44.6

Solid discharge (106 t)

Point-to-point integration (this work) 4.1 ± 0.4 1.4 ± 0.1 8.2 ± 0.7 3.8 ± 0.3 5.1 ± 0.4 10.9 ± 0.9 4.8 ± 0.4log–log rating curve (this work) 5.4* 0.55* 8.7* 3.7* 7.5* 11.0* 5.1*

log–log (Pont et al., 2002) 10.2* 1.3* 20.7* 9.5* 13.8* 24.7* 11.6*

log–log (Sempere et al., 2000) 9.2* 1.0* 17.5* 8.0* 12.3* 21.0* 9.9*

918 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

overestimate the solid discharge by 49% for 2001 (due to therelatively diluted SPM concentration during the 2001 floodevent). In the same way, if we had applied the log–log rela-tionships reported by Pont et al. (2002) or Sempere et al.(2000), based on measurements from 1992 to 1995, wewould have also overestimated the solid discharges duringour survey by 99–131%, well outside the range ofuncertainty.

For each year, the average SPM concentrations were75.8, 189 and 108.5 mg l�1, with an average of 122.4 mg l�1

in good agreement with that of Pont (1997) (123 mg l�1 forthe period 1994–1995 also including important floods).These values are higher by a factor 2–6 compared to theSeine and Loire rivers (1959–1994 long term averages of44 and 30 mg l�1, respectively; Meybeck and Ragu, 1996).Nevertheless, at the global scale, the Rhone River remainsamong rivers that have low SPM concentrations, comparedwith the world river average of 535 mg l�1 (Berner and Ber-ner, 1996).

Table 5Water and solid fluxes of the Rhone River. Results take into account thetotal water liquid flux. $ in 106 m3 yr�1 of solid discharge.

References Method

This work See textPont et al. (2002) measured, log/log

Pont et al. (2002) extrapolated from log/logSempere et al (2000) measured, log/logSempere et al (2000) extrapolated from log/logPont (1997) measured, log/logPont (1997) extrapolated from log/logRoditis and Pont (1993) extrapolated from log/logCauwet et al. (1990)El-Habr and Golterman(1987)Milliman and Meade (1983)

Parde (1942) calculated from Rhone tributariesSurell (1847) calculated from sediment deposition on th

delta

The annual average SPM fluxes reported here for theperiod 2001–2003 are consistent with previous works forthe period 1967–1996 (El-Habr and Golterman, 1987; Cau-wet et al., 1990; Roditis and Pont, 1993; Pont, 1997; Sem-pere et al., 2000; Pont et al., 2002) (Table 5 and Fig. 7).By contrast, a strong decrease appears clearly, when com-pared to the estimates reported for the 19th and the begin-ning of the 20th century. Indeed, Surell (1847), Parde (1925,1942) and Milliman and Meade (1983) showed that the pre-damming total amount of suspended matter entering theGulf of Lions was as high as 21 � 106 t yr�1, 31 � 106 t yr�1

and 45 � 106 t yr�1.

5.2. Chemical erosion: major sources of the dissolved load

Previous studies have shown that the major elementcomposition of the world’s main rivers can be interpretedin terms of mixing between three main end-members corre-sponding to the weathering products of limestone, silicates

contribution of the “petit Rhone” which carries about 15% of the

Period SPM flux(106 t yr�1)

Water flux(km3 yr�1)

Range Average Range Average

2001–2003 5.1–10.9 7.3 44–67 56.210/92–05/95 10.6–

19.715.2 53–69 62.6

1967–1996 1.2–19.7 7.4 33–75 53.606/94–05/95 11.7 661987–1996 2.3–22.7 9.9 ± 6.4 33–69 54 ± 1006/94–05/95 13.9 69.81961–1997 1.4–16.4 6.2 33–75 541960–1991 1–8 3.4 ± 2.41986–1987 2.8 531984–1985 2.6 47–53 50

Pre-damming

31

1850–1940 25–45 30e Rhone 1840–1845 21$

20001990198019701800 1900

annu

al S

PM f

luxe

s (1

06 ton

s)

40

35

30

25

20

15

10

5

0

Pre-dammingperiod

This study

El-Habr & Golterman (1987)

Cauwet et al. (1990)

Sempéré et al. (2000) extrapolated

Pont et al. (2002)

Pont (1997) Pont et al. (2002) extrapolated

Sempéré et al. (2000)

Fig. 7. Time evolution of annual Suspended Particulate Matter (SPM) fluxes in the Rhone River. Hachured area corresponding to the annualSPM fluxes estimated by Surell (1847), Parde (1925, 1942) and Milliman and Meade (1983) during the 19th and the beginning of the 20thcentury (pre-damming period). Mean value (—) and standard deviation (- -) of SPM flux for period 1967–2003 are also indicated.

Physical and chemical erosion in the Rhone basin 919

and evaporites (Negrel et al., 1993). The same approach canbe followed for our data on the Rhone, as illustrated in theNa/Ca, Mg/Ca and Cl/Ca plots shown in Figs. 4–6. Inthese plots, the contribution from limestone weatheringcorresponds to the trend towards the high-calciumend-member close to the origin. The chemical compositionof this end-member (Na/Ca = 0.007–0.008 and Mg/Ca =0.04) is given by the data from the particles of the Ouvezeand Durance rivers, left bank contributors of the Rhone is-sued from calcareous formations in the western Pre-Alps.This composition is also very close to that obtained byMeybeck (1986) for waters draining limestones in theDoubs river (Saone watershed, Na/Ca and Mg/Ca ratiosof 0.008 and 0.053), and also in agreement with the rangeof composition given by Han and Liu (2004) for the Wuji-ang and Yuanjiang rivers in calcareous basins from centralChina (Guizhou Province) (Na/Ca � 0.005 and Mg/Ca � 0.03). These values, however, are significantly differ-ent from those selected by Negrel et al. (1993), or Gaillardetet al. (1997) for the calcareous end-member in the Congoand Amazon basins (0.017 and 0.022 for Na/Ca; 0.5 and0.33 for Mg/Ca, respectively).

As mentioned above, high concentrations of Na+ andCl� originate from the Saone watershed. Na/Ca and Cl/Ca ratios vary from 0.090 to 0.345 and 0.051 to 0.262,respectively, for the Rhone River, whereas they reach 1.8and 2 for the Saone at Lyon. These high concentrationsmay be explained by the presence of Triassic evaporite for-mations of halite and gypsum in the Saone watershed.However, another possibility could be the intense humanactivities within this basin, which includes the most indus-trialized part of Lyon, 2nd largest town of France. Indeed,sodium, chloride and sulfate concentrations can be heavilyimpacted by the anthropogenic contamination from indus-trial and urban waste waters (Meybeck, 2003). Neverthe-less, the RMC data show that the high Cl and Naconcentrations are observed in all stations along the SaoneRiver, including upstream regions with low densities ofpopulation. It appears also that SO4 and Cl are broadlycorrelated with each other in the Rhone River suggesting

that CaSO4 and NaCl both contribute to the chemical com-position of this end-member. Therefore, we favour thehypothesis that the Saone end-member is indeed dominatedby the evaporite-type composition. As seen in Fig. 5, Mg/Ca ratios are low in the Saone River, and show a slight in-crease up to 0.120 with increasing Cl/Ca ratios. Consideringthese results, as well as literature data based on simple ormono-lithologic terrains (Meybeck, 1986; Gaillardet et al.,1997), the end-member corresponding to the Saone watersis chosen with Na/Ca = 3 ± 1 and Mg/Ca = 0.12 ± 0.01.

Finally, the influence of the third end-member, issuedfrom silicate weathering, is reflected in the highest Mg/Caand Na/Ca ratios of the Rhone River samples (from0.099 to 0.195 and 0.090 to 0.345, respectively, Figs. 5and 6), as well as by the data from the Gard and Ardecherivers, the headwaters of which drain siliceous metamorphicterrains in the Massif Central. Also plotted for comparisonin Fig. 4 are the data obtained by Grosbois et al. (2000) inupstream waters of the Loire River, that flow also from thesame crystalline formations. We selected a chemical compo-sition of Na/Ca = 1.3 ± 0.1 and Mg/Ca = 0.9 ± 0.2 for thedissolved silicate end-member (Fig. 4). This value was cho-sen such that (1) the evaporate–carbonate–silicate triangleincluded all our data, (2) the silicate end-member lies onthe extrapolation of the trend defined by the Loire data,(3) the dissolved silicate has a higher Na/Ca ratio thanthe bedrock (see UCC-NASC line in Fig. 4). The uncertain-ties in the end-member composition (Na/Ca = 1.3 ± 0.1;Mg/Ca = 0.9 ± 0.2) reflect a plausible range of variationwithin these assumptions. Similar to the evaporiteend-member, some ambiguity persists for the silicate end-member, regarding the possible contribution from anthro-pogenic sources. Grosbois et al.(2000) proposed that thehigh Ca, Mg and Na concentrations observed in the Loiresamples could be related to agricultural effluents that mightprovide as much solutes as silicate weathering in this wa-tershed (i.e. 34%, 20% and 17% of the dissolved load inthe Loire river). Since the major element composition ofthis input is close to that selected for the silicate contribu-tion, it is not possible to distinguish between them without

Table 6Chemical molar ratios of carbonates, silicates and evaporates end-members.

End-members Carbonates Silicates Evaporites

Na/Ca 0.0076 ± 0.001 1.3 ± 0.1 3 ± 1Mg/Ca 0.041 ± 0.005 0.9 ± 0.2 0.12 ± 0.01

95

90

85

80

75

70

35

30

25

20

15

5

4

3

2

1

00 2000 4000 6000 8000 10000

Ca

carb

(%

)

water discharge (Q, m3.s-1)

Mg

carb

(%

)N

a ca

rb (

%)

Fig. 8. Variation of the percentage of dissolved Ca2+ (Cacarb),Mg2+ (Mgcarb) and Na+ (Nacarb) concentrations issued fromcarbonate weathering with increasing water discharge.

920 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

further investigation. Therefore, the silicate weatheringfluxes that we calculate below must be taken as upperlimits.

Once the compositions of the three end-members havebeen defined (see Table 6 for summary), mixing equationscan be solved for the respective contribution of these end-members to the river chemical composition:

XCa

� �riv

¼X

i

XCa

� �i

aCað Þi ð3Þ

X/Ca is the elemental molar ratio measured in the river(riv), or assumed in each end-member (i) (carbonates, sili-cates or evaporites). aCað Þi is the mixing proportion ofCa2+, from which the mixing proportions of the other ele-ments are easily deduced. The full equations system is listedin Appendix A.

The proportions of Ca2+, Mg2+ and Na+ originatingfrom the different reservoirs are given in Table 3. For dis-solved Ca2+, 72–89% is derived from carbonate weathering.In general, dissolved Mg2+ is dominantly from silicates (58–78%), although in some samples, up to 35% Mg2+ is fromcarbonates. The proportions of Ca2+ and Mg2+ originatingfrom the dissolution of evaporites are less than 4%. By con-trast, evaporites contribute up to 42% of Na+ (with a meanvalue of 9.5%).

An interesting result of this calculation is that the pro-portion of Ca2+, Mg2+ and Na+ concentrations suppliedfrom carbonate weathering increases with increasing waterdischarge (Fig. 8). This is consistent with the major iondata, corrected for rainwater contribution, which show aconcomitant increase in Ca/Na, Mg/Na, HCO3/Na ratios(data not shown). This could be due either to differentsources in the watershed, or to variation in the intensityof weathering with flow rate, as discussed further below.

The potassium composition of the carbonate and evap-orite end-members is poorly documented and may be highlyvariable. Moreover, it is generally accepted that K+ concen-tration is heavily influenced by recycling of the vegetationand by salt and fertilizers. We show that, based on the so-dium budget and using a typical ratio K/Na = 0.1 for waterdraining silicates (Meybeck, 1986; Edmond et al., 1995;Gaillardet et al., 1999a), from 45% to 95% of K can be re-leased by silicate weathering (after atmospheric correction).This range of variation, and the unexplained K, may resulteither from the uncertainty on the K/Na ratio, or fromadditional sources of K. The uncertainty related to thisassumption is expected to be small because of the lowpotassium concentrations measured in the Rhone River.Assuming that K comes only from silicates, Ksil correspondto 3.8–8.5% of the total cationic weathering rates of silicates(TDSsil = [Ca]sil + [Mg]sil + [Na]sil + [K]sil). Excluding K or

assuming that all K comes only from silicate affects CO2

consumption from silicate weathering by less than 5%.In the same way, the (HCO3

�)sil concentrations originat-ing from silicate weathering can be calculated from thecharge balance of major cation. (HCO3

�)sil is then1.07 ± 0.20 mmol l�1 on average, i.e. about 36% of theHCO3

� concentration measured in the Rhone River.

5.3. Carbonate versus silicate denudation rates and CO2

consumption

Using the proportions calculated above, we can estimatethe chemical erosion rate of carbonates (TDScarb) and sili-cates (TDSsil) in the Rhone catchment, as well as the relatedrates of consumption of CO2 (Table 7).

The carbonates chemical erosion rate (TDScarb) is calcu-lated as:

TDScarb ¼ ½Ca�carb þ ½Mg�carb þ 0:5½HCO3�carb ð4Þ

where [HCO3�]carb is calculated by difference between total

[HCO3�] and [HCO3

�]sil, assuming that HCO3� concentra-

tions are not influenced by anthropogenic pollution (Mey-beck, 2003). The values of TDScarb range from 45 to

Annual average

0.1

1000

Carbonate

Silicate

Silicate (world rivers)

(Rhône, this study)

(Rhône, this study)

physical erosion rate (t/km2/yr)1 100010010 10000

0.1

100

10

chem

ical

wea

ther

ing

rate

(t/k

m2 /

yr)

100000

1:1

1

0.1 1 100010010 10000 100000

(Rhône, this study)

Fig. 9. Relationship between chemical weathering and physicalerosion for carbonate ( ) and silicate (s, ) rocks of the Rhonewatershed. Physical erosion rates for silicates were corrected forcarbonate contribution from the relationships between molar ratiosNa/Ca and Mg/Ca in the particulate phase. Silicate weathering anderosion rates for other world rivers ( ) from Millot et al. (2002)and Zakharova et al. (2005) are also indicated.

Table 7Rock weathering rates and CO2 consumption for the Rhone River at Arles. TDSsil and TDScarb are the sum of dissolved species originatingfrom silicate and from carbonate, respectively (see text for details).

Rock weathering TDSsil TDScarb CO2 consumption CO2sil CO2carb

mg l�1 7–35 45–154 lmol l�1 380–1830 110–1460106 t yr�1 1.0–1.9 5.7–11.3 109 mol yr�1 55–102 39–98t km�2 yr�1 10–19 57–114 103 mol km�2 yr�1 560–1030 400–990

Physical and chemical erosion in the Rhone basin 921

154 mg l�1, with the lowest values measured during lowflow and during the flood of September 2002. The chemicalweathering rate of carbonates has a mean value of89 ± 5 t km�2 yr�1 (from 62 ± 4 to 109 ± 5 t km�2 yr�1),twice that obtained by Roy et al. (1999) for the Seine River(48 t km�2 yr�1), and in good agreement with the averagevalue obtained by Gaillardet et al. (1999a). The correspond-ing consumption of CO2 by carbonate weathering is72 ± 6 � 109 mol yr�1. The Rhone River belongs to the riv-ers that have high consumption of CO2 by carbonatesweathering (rank 17th of largest world rivers; Gaillardetet al., 1999a).

Using the solid discharge values calculated in Section5.1, and taking into account that 25–30% of the mass ofsuspended particulate matter transported by the RhoneRiver is composed of calcite, we obtain a physical erosionrate of carbonates of 12–33 t km�2 yr�1 (mean value of19 ± 3 t km�2 yr�1). The total erosion rate of carbonatesis thus 108 ± 7 t km�2 yr�1. Using a mean density of2.7 g/cm3, we calculate that 40 mm/1000 years of carbon-ates are being eroded in the Rhone basin. The mechanicalto chemical weathering ratio is about 0.12–0.36 for the car-bonate bedrocks of the Rhone River.

The cationic weathering rate of silicates (TDSsil) can becalculated from the dissolved concentrations of Ca2+,Mg2+, Na+ and K+ resulting from silicate weathering, as inGaillardet et al. (1999a). Silica concentration is not includedin the weathering budget. The mineral phases controlling sil-ica in river waters are poorly known (Millot et al., 2002):

TDSsil ¼ ½Ca�sil þ ½Mg�sil þ ½Na�sil þ ½K�sil ð5Þ

The mean value of the TDSsil is 1.42 ± 0.3 � 106 t yr�1, i.e.about seven times lower than for carbonates. However, thisvalue is a lower limit of silicate weathering. Indeed, it hasbeen showed that the weathering of silicates in the Seine ba-sin is characterized by ratios of dissolved SiO2 toNa + K + Ca + Mg that can be as high as 2 (Roy et al.,1999). Here, dissolved silica is not included in the silicateweathering calculation. Considering the total surface areaof the Rhone basin, this corresponds to 14.4 ± 3t km�2 yr�1, higher than that estimated for the silicates ofthe Seine River (1.2–2.6 t km�2 yr�1; Roy et al., 1999).The consumption of CO2 by silicate weathering is 78 ±14 � 109 mol yr�1, i.e. similar to the consumption of CO2

by carbonates.Considering again the 25–30% of calcite in the

suspended particulate matter, we estimate a mean physicalerosion rate for the silicates of 5 � 106 t yr�1 (3.4–8.2 �106 t yr�1). The total silicate denudation rate (mechanicaland chemical) is thus 66 ± 5 � 106 t km�2 yr�1 or 24 mm/1000 years, using a mean rock density of 2.7. The ratio

between mechanical erosion and chemical weathering isthus about 2 - 7 for the silicates of the Rhone River. Thisratio is an upper limit, as dissolved silica is not includedin the silicate weathering calculation.

5.4. Physical versus chemical erosion in the Rhone River

Several recent studies have emphasized that chemicalweathering rates and physical erosion rates are highly cor-related for the silicate continental crust at the global scale,under various climatic and geological conditions (Bluth andKump, 1994; Gaillardet et al., 1997; Millot et al., 2002; Za-kharova et al., 2005). This reflects that the main factor con-trolling chemical weathering is the supply rate of freshunaltered minerals through mechanical erosion, which isgenerally dominating other factors as temperature, runoffor vegetation cover except in very high erosion environ-ments. In Fig. 9, we have plotted the flux of chemical

922 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

weathering calculated above versus the physical erosionrate for our samples from the Rhone River, and these dataare compared with a compilation of the silicate componentof erosion in the world’s main rivers (McDowell andAsbury, 1994; Edmond et al., 1995; Louvat and Allegre,1997, 1998; Viers et al., 2000; Millot et al., 2002; Zakharovaet al., 2005). We have plotted both the chemical weatheringand physical erosion rates of the carbonate and silicatecomponents, as calculated in the previous section. Thiscomparison illustrates directly that the chemical weatheringis largely dominated by the weathering of carbonates, as al-ready stressed above. Our data confirm, within one river,the strong positive correlation between chemical weatheringand physical erosion, both for carbonate and silicate fluxes(r2 = 0.870 and 0.848, respectively). We also observe thatthe slope of the correlation is significantly steeper for thecarbonates than for silicates (0.289 ± 0.029 compared to0.231 ± 0.022). This illustrates graphically that the carbon-ate weathering rate increases with flow rate. Indeed, wehave shown above that the carbonate/silicate fraction in-creases during flood events both in the dissolved and theparticulate phases, which could be due to changes in theprovenance of solutes and particles from different parts ofthe watershed during floods. However, the difference inslope in the chemical weathering versus physical erosiondiagram demonstrates that this increase in the carbonatefraction is enhanced in the dissolved phase compared tothe particulate phase, and thus must be related to increasedweathering efficiency. Tipper et al. (2006) and Rai andSingh (2007) have discussed in detail of a linear correlationbetween chemical weathering flux and runoff, and haveunderlined the contrasting response of carbonate and sili-cate lithologies to climate forcing, in the headwater tribu-taries of the Marsyandi River and in the BrahmaputraRiver. They showed a relatively lower contribution from sil-icate weathering to the major ion budget during monsoonand a concomitant increase in carbonate weathering contri-bution. The faster dissolution kinetics of carbonate can ex-plain why carbonate weathering exhibits a greatersensitivity to runoff than silicate lithologies. At high runoff,the time of contact between the mineral phases and riverwater is short, causing an increase in the proportion of car-bonate to silicate derived ions.

In addition, two other interesting features appear fromthe comparison of the silicate fluxes between the Rhoneand the other world rivers: (i) the slope is shallower forthe Rhone than for the global trend and (ii) the average val-ues are significantly offset for the Rhone compared to theothers, with lower physical erosion flux for a given chemicalweathering flux. West et al. (2005) have discussed in detailthe significance of the correlation slope in the chemicalweathering versus physical erosion diagram. They haveshown that the slope may vary from an upper limit ofone (transport limitation) to a lower theoretical limit ofzero (kinetic limitation). Except in low erosion environ-ments like cratonic basements, where the data fit the trans-port-limited slope, most world rivers are scattered aroundan intermediate average slope of 0.37, the scatter reflectingcombined control of the chemical weathering rate by phys-ical erosion, temperature, and runoff.

Within one single river like for our data from the Rhone,the observed correlation line is in fact mainly due to thelog–log correlation between SPM concentration and flowrate.

Relation (1) above (Section 4.1) can be written again as:

F p ¼ a� Fd

TDS

� �ðbþ1Þ

ð6Þ

where Fp and Fd are the particulate and dissolved fluxes, a

and b are the ordinate and slope of the log–log correlationbetween SPM concentration and water flux (relation (1)above), and [TDS] is the total dissolved load. Even though[TDS] also depends on flow rate, the observed range of var-iation is much lower than for the SPM concentration, andthe overall slope in the chemical weathering versus physicalerosion diagram is thus close to (b + 1).

Finally, we interpret the apparent shift of the Rhonedata towards higher chemical weathering, or lower physicalerosion rates compared to other large world rivers, as directevidence of the artificial reduction of the suspended loaddue the extensive damming of this river. A 3–6 factor ofreduction of the natural SPM flux is suggested by the com-parison with the global trend for comparable weatheringfluxes. We will corroborate this conclusion in the followingsection, from an independent perspective.

5.5. Steady-state prediction of the suspended sediment

discharge

Gaillardet et al. (1995) have shown that the suspendedload transported by a river can be determined when the ero-sion regime is supposed to be at steady-state, by solvingmass budget equations between the chemical compositionof the dissolved and suspended loads of the river, and theaverage composition of the material undergoing erosionin the watershed. Our data allow us to check for consistencyon a suite of samples collected over the hydrological cycleof one river.

Considering only the silicate component in the budgetequations, and neglecting the bottom transport by sand,the suspended sediment concentration predicted at steady-state, P (mg/l), can be calculated as:

P ¼ Cw

ðC�p � CpÞð7Þ

Cw and Cp are the concentrations of a soluble element mea-sured in the dissolved and particulate phases, after correc-tion for carbonate and evaporite contributions. C�p is the“pristine” concentration of this element in the particles,prior to the fractionation resulting from weathering. C�pdiffers from Cc due to dilution by organic matter, quartzor any other diluting phases. C�p can thus be calculatedfrom:

C�pðSÞ ¼CpðIÞCcðIÞ

� �� CcðSÞ ð8Þ

(S being the soluble element and I an insoluble element cho-sen as reference. “p” and “c” refer to the suspended parti-cles and the average parent rock composition chosen fornormalization). Practically, each soluble element is referred

Physical and chemical erosion in the Rhone basin 923

to the closest insoluble element in the classical pattern ofcrustal enrichment plots (i.e. (Mg, Al), (Ca, Ti), (Na, Nd),(Na, Sm), (Na, Th), (K, Th), (Sr, Nd) and (Ba, Th)).

Since the budget equations apply to the silicate compo-nent only, the particulate composition Cp must be correctedfor the presence of limestone in the particles. This is animportant difference with previous studies on large rivers(Congo, Amazon, Niger), where the pH is lower and thusthe particles can be assumed as residual silicates only

20

1000 x C

Mg

Ca

0

0.1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

1

Na

Cp

/ Cp*

0 200 400

Mg

Ca

N

P measured(2002) Pth (mg

Ba

1000 x C

Mg

Ca

0

0.1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

1

Na

Cp

/ Cp*

0 200 400 600

Mg

Ca

Na

P measured(2001) Pth (mg/l)

Ba

20

Pth.fl.

Fig. 10. Illustration of the steady-state erosion model for the Rhone Rive2001 (top) as an example when model is applied to the data measured duParticulate Matter (SPM) and dissolved load, after correction for atmosphconcentration in the parent rock chosen for reference. Two extreme caseMcLennan, 1985) crustal compositions. The theoretical value of the riversteady-state line with the X-axis (Gaillardet et al., 1999b). The dotted linethe theoretical value of the river SPM concentration, respectively, whenApril 2001. The hatched band illustrates the range of SPM concentrationof the 20th century. The measured average suspended load correctedcomparison.

(Gaillardet et al., 1995, 1997, 1999b; Picouet et al., 2002).As discussed above, both the mineralogical observationand the chemical data of the suspended sediments of theRhone indicate that 12–40% of the mass of the suspendedparticulate matter is composed of calcite and about 1–3%of dolomite. As a result, Ca2+ and Mg2+ concentrations is-sued from silicates represent thus only 0.24 mmol g�1 (from0.11 to 0.55 mmol g�1) and 0.25 mmol g�1 (from 0.12 to0.56 mmol g�1) in the suspended sediment, respectively.

UCCNASC02

w / Cp*600 800 1000

a

Pth (mg/l)/l)

w / Cp*800 1000

UCCNASC

Pth (mg/l)

1200

01

Pth.fl.

r in 2001 (top) and 2002 (bottom), and for the flood of March–Aprilring floods. Cp and Cw denote the concentrations in the Suspendederic, carbonate and evaporate contributions except for Ba. C�p is thes are shown, based on NASC (Condie, 1993) or UCC (Taylor andSPM concentration (Pth in mg/l) is given by the intersection of theand “Pth.fl” (on the top) correspond to the steady-state line and to

the model is applied to the data measured during flood of March–(corrected for carbonates) estimated for the 19th and the beginningfor carbonates (this study) in 2001 and 2002 is also shown for

924 P. Ollivier et al. / Geochimica et Cosmochimica Acta 74 (2010) 907–927

Given those parameters, a model value of P can be cal-culated from Eq. (7) for each chemical element. The resultsobtained for Na, Ca, Mg and Ba are illustrated in the Cp/C�pvs. Cw/C�p plot of Gaillardet et al. (1999b) (Fig. 10). At stea-dy-state erosion, all elements should plot on a straight line(the steady-state line) whose intercept with the X-axis isPmodel (g l�1) and intercept with the Y-axis is unity.

The steady-state model of erosion was applied both tothe annual average values and to the specific flood periodsof 2001 and 2002. The average concentrations of dissolvedand particulate elements used in the model are calculatedfrom element fluxes which are obtained as for SPM fluxes(cf Section 5.1). In 2003, we analyzed only 11 SPM, includ-ing nine samples during flood of December. The steady-state model was applied only to that flood and not to theannual average.

As shown by Gaillardet et al. (1999b), the results ofthese equations depend largely on the choice of the chemi-cal composition for the bedrock. We show in Fig. 10 the re-sults obtained separately with UCC and NASC crustalcomposition taken as reference. In the Rhone basin, theoutcrops of siliceous crystalline and metamorphic rocksare small and restricted principally within the Massif Cen-tral, the Cevennes and the Alps mountains. Alumino-sili-cates are mostly clays and silts in sedimentary formations.As a whole, the average composition of the parent rockover this heterogeneous watershed must be intermediate be-tween the crystalline silicate composition and more de-pleted compositions such as observed in shale formations.The model P values range from 470 to 960 mg l�1 and from425 to 800 mg l�1 for the average annual suspended sedi-ment concentrations in 2001 and 2002, respectively(Fig. 10), while the measured values corrected for carbon-ates are only 50 mg l�1 in 2001 and 130 mg l�1 in 2002.We show also that, in 3 cases out of 4, the results obtainedwhen the model is applied to the data measured duringfloods only are in good agreement with the theoretical an-nual average. The flashflood of September 2002 stands asan exception (P values between 230 and 370 mg/l), due tohigh dilution of the dissolved concentrations (low Cw).Flood of April–March 2001 is showed as an example inFig. 10. There is thus a clear discrepancy between the valueof P derived from the mass budget model and the sus-pended load measured in the field. Today, the Rhone Riverseems to export less material than produced by steady-statedenudation in its watershed. The same conclusion was alsoreached in the previous section from the comparison be-tween the Rhone and other large rivers of the world. More-over, the sediment flux inferred from the steady-statecalculation (21–56 � 106 t yr�1) is on the same order asthose estimated by Surell (1847), Parde (1925, 1942) andMilliman and Meade (1983) for the 19th and the beginningof the 20th century (21–45 � 106 t yr�1, i.e. about 15–35 � 106 t yr�1 corrected for carbonates).

These results suggest thus a significant reduction of thesediment transport in the Rhone basin since the beginningof the 20th century. This reduction can be explained bythe trapping of alluvion by the numerous dams that havebeen installed since 1952 on the river and its tributaries.This effect has been well documented on other Mediterra-

nean rivers such as the Ebro (Palanques et al., 1990; Guillenand Palanques, 1992), or the Nile (Kempe, 1993; Abdel-Moati, 1999), whose sediment delivery have been reducedby 95% and by a factor 400, respectively. The steady-statemodel can thus be used as a geochemical case study to testthis hypothesis. Our data provide us with an upper estimateof 75 � 106 t yr�1 (i.e. 80 � 106 t yr�1 (theoretical value:56 � 106 t yr�1 for silicates +24 � 106 t yr�1 for carbon-ates) � 5 � 106 t yr�1 (measured value) in 2001) for the rateof sediment deposition in the Rhone river bed. It is interest-ing to note that the good agreement between the pre-damvalue of P and the theoretical steady-state value also sug-gests that human activities related to the agriculture havenot led to significant loss of soils in the Rhone basin, asin other part of the world.

However, a second factor may also contribute to weak-ening the extraction and/or the transportation of particlesout of soil profiles. Indeed, the floods frequency has de-creased significantly and regularly since the beginning ofthe 19th century, as shown by hydrological measurementsmade at Arles since 1800 (Folland et al., 1993; Pichard,1995; Pont et al., 2002). This may also result in a deficitof sediment transport with respect to soil formation anddeepening. In this case, the imbalance between the valuesof P derived from the mass budget model and the sus-pended load measured in the field is not only apparentand due to an anthropogenic bias as described above. TheRhone would be actually under a transient erosion regimefollowing climate change. The characteristic response timeto such a perturbation remains unknown.

6. CONCLUSIONS

1. We have obtained a more precise estimate value ofthe Rhone river solid discharge over a three-year per-iod of study characterized by massive floods(7.3 ± 0.6 106 t yr�1). The 2001–2003 solid dischargeof the Rhone River is in the same range of variationas previous determinations during the 1967–1996 per-iod. These values are at least a factor 3 lower thanhistorical estimates reported in the literature for the19th and the beginning of the 20th century.

2. Based on our high temporal resolution sampling, wehave shown that the precision on the SPM determi-nation is highly dependent on careful monitoring ofthe increase in suspended load during the early partof flood events. Uncertainties higher than 50% mayoccur from using the log–log correlation betweenSPM and flow rate for the determination of theannual solid discharge.

3. During our study, more than 80% of the SPM fluxwas discharged during the flood events, i.e. in only12% of the time period. The strong inter-annual var-iability of yearly SPM fluxes (4.8 � 106–10.9 � 106 t)is directly related to the frequency and intensity ofthe flood events.

4. Our data on the chemical composition in the dis-solved phase show that the Rhone River has a majorelement signature distinctly different from the other

Physical and chemical erosion in the Rhone basin 925

large rivers in France (Seine, Loire and Saone). Wealso show that the contribution from silicates andevaporites can be identified and quantified in spiteof the dominance of contribution from limestone dis-solution. However, the results of this apportionmentare sensitive both to the choice of composition of theend-members, and to the correction of the atmo-spheric deposition. In our case, the silicate end-mem-ber is best represented by the extreme compositionfound in the Loire River, and the evaporite end-member by the Saone samples. The role of anthropo-genic input from sewage plants or agriculture efflu-ents remains to be fully evaluated.

5. The chemical and the mineralogical composition ofthe particulate matter both indicate the presence ofcarbonates in the solid phase, as high as 29% of theSPM mass on average. The proportion of carbonatesshows a clear increase during the flood events, bothin the solid and dissolved phases.

6. We have calculated the chemical and physical denu-dation rates of the carbonates (8.8 ± 0.6 � 106 and1.9 � 106 t yr�1, respectively), and of the silicates(1.42 ± 0.3 � 106 and 5 � 106 t yr�1), and the relatedCO2 consumption rates in the Rhone watershed.

7. Our data show, within the hydrological seasonalcycle of the Rhone River, a strong positive correla-tion between chemical weathering rate and physicalerosion rate. A similar correlation has been discussedrecently in the literature, based on compilations ofworld river annual average compositions (Millotet al., 2002), or from case studies of soil profiles basedon cosmogenic isotopes and immobile elementschemical budgets (Riebe et al., 2003). Within one riv-er, this correlation is essentially determined by thepower-law relationship between SPM concentrationand flow rate.

8. The slope of the correlation between chemical weath-ering and mechanical erosion is steeper for the car-bonates than for the silicates. This indicates higherweathering efficiency for the carbonates at high flowrate.

9. The comparison between the physical erosion and thechemical weathering annual fluxes of the Rhone Riv-er and the world rivers global correlation indicates adeficit in the suspended load of this river, most prob-ably linked principally to sediment trapping by thenumerous dams along its course.

10. Based on the hypothesis of a steady-state erosionregime, where the total denudation of the crust (alter-ation plus erosion) is at equilibrium with the solidand dissolved transport by the river, and using thebudget equations of Gaillardet et al. (1999b), we findthat the Rhone is currently out of equilibrium. Thisreflects again in the reduction of sediment transportdue to the influence of the dams. An upper estimateof 75 � 106 t yr�1 of sediment trapping by the damsis inferred from this analysis. However, the potentialadditional effect of reduced frequency of the floodevents related to recent climate change could alsocontribute to this disequilibrium, and should deserve

further investigation in the future, through detailedmonitoring of the sediment discharge during floods,as well as of inventories of recent sediment accumula-tion behind the dams.

ACKNOWLEDGMENTS

This work benefited from P.O. Ph.D. scholarship from FrenchMinistry of Research and Education, and from support by INSU(Institut Nationale des Sciences de l’Univers) through the researchobservatories RESYST and ORME. We thank M. Meybeck and T.Church for constructive comments improving the manuscript, andJ. Gaillardet, S.K. Singh, an anonymous reviewer and associateeditor S. Krishnaswami for their detailed and thorough reviews.

APPENDIX A

List of the three equations used to estimate the inputs ofthe different end-members to the dissolved load.

NaCa

� �riv

¼ NaCa

� �carb

� aCað Þcarb þNaCa

� �sil

� aCað Þsil

þ NaCa

� �eva

� aCað Þeva ðA1Þ

MgCa

� �riv

¼ MgCa

� �carb

� aCað Þcarb þMgCa

� �sil

� aCað Þsil þMgCa

� �eva

� aCað Þeva ðA2Þ

aCað Þcarb þ aCað Þsil þ aCað Þeva ¼ 1: ðA3Þ

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Associate editor: S. Krishnaswami