Chemical denudation rates of the western Canadian orogenic belt: the Stikine terrane

Chemical Geology 201 (2003) 257 – 279 www.elsevier.com/locate/chemgeo

Chemical denudation rates of the western Canadian orogenic belt: the Stikine terrane Je´roˆme Gaillardet a,*, Romain Millot a, Bernard Dupre´ b a

Laboratoire de Ge´ochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universite´ Paris VII, UMR 7579, Tour 14/24 3e`me e´tage 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire de Ge´ochimie, CNRS-OMP UMR, 5563, Universite´ Paul Sabatier, 38, rue des 36 Ponts 31400, Toulouse, France Received 24 September 2002; accepted 11 July 2003

Abstract We report in this paper major element and Sr isotopic ratio analyses for 19 rivers draining the Stikine Province in the Western Canadian Cordillera. Dissolved solutes, suspended sediments and river sands were analyzed. The aim of the paper is to calculate chemical denudation rates for this uplifted accretionary prism, mainly dominated by volcanics and volcanic-derived sedimentary rocks. The major chemical features of the Stikine rivers are their dilute character and the excess of Ca, Mg and Sr in the dissolved load with regards to Na. Molar Ca/Na ratios from 2 to 14, mean value of 5.4, are observed although the region is dominated by silicate rocks. Suspended sediments show Ca/Na ratios close to 0.4. Isotopically, most of the rivers investigated display lower Sr isotopic ratios in the dissolved load compared to the suspended load, which is very unusual. To account for the excess of cations, three different hypotheses are tested by an inversion method allowing us to determine the origin of each major solute. The first scenario assumes that the excess of Ca and Mg is due to the dissolution of unradiogenic carbonates. The second scenario supposes that the excess of base cations is derived from the preferential dissolution of Ca- and Mg-rich minerals, such as amphiboles. In the third scenario, all solutes are assumed to derived from silicate weathering. The cationic weathering rates of silicates, although imprecise, are in the order of 2 – 5 tons/km2/year for the two first hypotheses. If all solutes are taken into account, denudation rates are between 9 and 15 tons/km2/year. On a global scale, these numbers are among the lowest found for rivers draining basaltic rocks. They are however found to be consistent with the global law for chemical weathering rates established for basaltic rocks showing that chemical denudation rate are linearly correlated to runoff and depend on temperature according to an Arrhenius law. The rivers draining the complex assemblage of volcanic and volcanic-derived rocks in the subduction/collision geological context of the Stikine Province thus respond similarly to the other volcanic provinces of the world. This does not seem to be true for associated CO2 consumption rates. The abundance of sulfate ions in the dissolved load, possibly derived from oxidative weathering of sulfides, may indeed provide a non-negligible part of the protons necessary for chemical weathering. Within Canada, the Stikine geologic province is one whose silicates weather at a high rate. The order of chemical weatherabilty of Canadian geological province from the lowest to the highest is established as follows: granites < shales of the rockies < volcanics of the Western Cordillera < shales of the interior platform < carbonates. D 2003 Elsevier B.V. All rights reserved. Keywords: Silicate weathering; Canadian Cordillera; Rivers;

87

Sr/86Sr; Ca/Na ratio

* Corresponding author. Tel.: +33-1-44-27-49-15; fax: +33-1-44-27-37-52. E-mail address: [email protected] (J. Gaillardet). 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.07.001

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J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

1. Introduction At the surface of the continents and over geologic time, chemical weathering of silicate rocks is a major process in the global climate of the Earth by consuming atmospheric CO2 (e.g. Berner and Kothavala, 2001). In the last decade, many studies have focused on river geochemistry in order to estimate silicate weathering rates as well as CO2 consumption rates associated and to determine and constrain parameters controlling these silicate erosion fluxes (Ne´grel et al., 1993; Gaillardet et al., 1995, 1999; Edmond and Huh, 1997; Millot et al., 2002, 2003). The geochemistry of rivers draining basaltic rocks have risen a special interest and numerous recent papers have focused on the geochemical investigation of rivers draining oceanic islands such as Iceland (Gislason et al., 1996), La Re´union (Louvat and Alle`gre, 1997), Azores (Louvat and Alle`gre, 1998) or provinces of flood basalts (Taylor and Lasaga, 1999; Dessert et al., 2001). Basically, two main geochemical issues are attached to the study of basaltic rivers: – As recently demonstrated by the surveys of Louvat and Dessert, basaltic rocks weather at rates which are 10– 100 times higher than those of the granitic or sedimentary rocks constituting the continents. This is especially true in terms of atmospheric CO2 consumption during the interaction of rain + soil waters with volcanic rocks. Although outcropping 5% of the surface continental area of the Earth, basalts are responsible for a global CO2 consumption flux up to f 30% (Dessert et al., 2003). The reason why basalt weathering has such an intense impact on the long-term CO2 budget of the Earth, is due to the fact that basalts are rich in felsic minerals and glass, both phases being easily weatherable chemically. The study of Dessert et al. (2001) showed that basalt weathering fluxes are controlled, at a global scale, by temperature according to an Arrhenius type law whose preexponential factor depends linearly on runoff. As a consequence, basalt weathering may serve on Earth, as a natural thermostat, regulating the concentration of CO2 in the atmosphere. Such a feature is illustrated by Dessert et al. (2001) who showed that the emplacement of the Deccan Traps and associated CO2 degassing, increased chemical

weathering of rocks at the surface of the Earth and led (in only 2 –3 Ma) to a complete removal of the CO2 injected by the volcanic eruption in the atmosphere. With no doubt, continental basalt weathering is one of the major mechanisms of the Earth’s surface climatic evolution. The weathering of the basalts of the ocean floor has also been demonstrated to potentially affect long-term atmospheric CO2 concentrations by Brady and Gislason (1997). – Most of the rocks exposed to chemical weathering on the continents have undergone several cycles of erosion/deposition during their history (Gaillardet et al., 1999). Basalt weathering studies offer the possibility of understanding the chemical and isotopic changes associated with first cycle weathering and are probably a key to understand how felsic rocks are able to evolve toward more acidic compositions due to chemical alteration. So far, it is striking to note that the investigation of rivers draining volcanics are only addressed oceanic and flood province basalts. The study of weathering of basaltic rocks and associated lithologies on active continental margins and volcanic arcs has received little attention (Dethier, 1986; Cameron et al., 1995; Anderson et al., 1999). This paper is an attempt to fill the gap and is part of a systematic investigation of the geochemistry of northwestern Canadian rivers. After studying the rivers of the granitic Slave Province (Millot et al., 2002) and those draining the North American stable margin (Millot et al., 2003), here we focus on the geochemistry of the rivers of the Stikine Province, one of the main allochtonous terranes accreted to the North American Margin during the Mesozoic. Relatively high precipitation rates, the presence of glaciers and the predominantly volcanic and volcaniclastic nature of exposed rocks are factors that should favor erosion processes. Our investigation is based on the analysis of both the dissolved load and suspended load of the rivers. A clear contrast is observed between the chemistry and the weathering fluxes of the Stikine region with those found in the adjacent Rocky and Mackenzie Mountains (Millot et al., 2003). Weathering fluxes of the Stikine Province are one order of magnitude higher that those of the Rocky mountains, but relatively low compared to the rivers draining volcanics at the global

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

scale. These relatively low rates are essentially due to the low temperatures.

2. Setting of the Western Canadian Cordillera river basins The Canadian Cordillera (Fig. 1) is an orogenic belt inherited from a complex history, dominated by the accretion of exotic terranes to the North American margin. The largest allochtonous terrane of this mosaic assemblage is the Stikine terrane, extending from the Canada/US border to the Yukon/Alaska border. The three main river basins investigated in this study (Skeena, Nass and Stikine) are limited to the South by the Fraser River Basin and to the North by the Liard River Basin and are therefore mainly draining the Stikine terrane, most of it being constituted by the sedimentary Bowser Basin. It is thought that the Stikine block was accreted to the North American margin during the Middle Jurassic (Samson et al., 1989). The rocks outcropping into the Stikine terrane reflect the migration of the terrane and its accretion to North America. The volcanic arc-type rocks associating basalts, andesites or granodiorites, have been formed during the subduction processes. Post-accretion volcaniclastic sediments (greywackes and shales), constituting most of the Bowser sedimentary basin, are derived from the erosion of the relief formed during the collision (Monger, 1977; Coney et al., 1980). The isotopic study of Samson et al. (1989) showed that the Stikine terrane is almost completely composed of juvenile, mantle-derived rocks, in contrast to other Phanerozoic orogenic belts, such as for example the Mackenzie and Rocky Mountains (Mackenzie River Basin). The initial eNd and 87Sr/86Sr systematics of various rocks of the Stikine region clearly showed such a feature: igneous rocks as well as sediments plot into the field of MORB or island-arc volcanics non-contaminated by continental sediments (such as the Aleutians and Marianas). Only the sediments of the Bowser sedimentary basin show a decreasing eNd value with decreasing age, indicating a contribution from the evolved North American margin. Mass budget calculations indicate that about 9
106 km3 of new continental crust were generated during the period of magmatic activity of the Stikine terrane (from Permian to Jurassic, Samson et al.,

259

1989). The region under consideration is therefore a preferential locus of Mesozoic continental crust. The lithological nature of the Skeena, Nass and Stikine basins is complex and not well characterized. Bowser Basin contains over 5000 m of deltaic to submarine fan assemblages of Middle to Late Jurassic age, succeeded by Early Cretaceous fluvial and alluvial fan deposits, in the north, and deltaic sedimentary rocks in the south. The Late Cretaceous Sustut – Skeena Basin represents 2000 m of non-marine foreland deposition derived in part from Bowser strata in the Skeena Fold Belt. The rocks of the Bowser basin are mostly black shsales, coal, sandstone, greywackes and conglomerates. Basaltic intrusions are frequent in the southern part of the Bowser Basin and in the Skeena Arch (Basset and Kleinspehn, 1996). The rivers under study belong to three main river systems: the Skeena (42.2
103 km2), the Nass (19.2
10 3 km 2 ) and the Stikine River Basin (29.3
103 km2). Precipitation decreases strongly from West to East and the mean value ranges from 1500 to 1750 mm/year. Mean runoff values for the main rivers sampled in this paper range between 700 and 1300 mm/year. In addition, mean annual air temperature range from 0 to 5 jC. The regions under consideration are characterized by moderate relief. Altitudes range from 1500 to 2500 m in the upper part of the river basins. Glaciers are present on the tops. The landscape is typical postglacial with abundant till deposits in the lower part of the valleys. Vegetation is dominated by conifers and population is sparse, making this region remarkably pristine. Suspended sediments concentrations measured during the sampling cruise are respectively 50, 294 and 450 mg/l. Physical denudation is relatively high as indicated as reported by Milliman and Syvitski (1992), sediment yields are 260 and 1100 tons/km2/ year, respectively, for the Skeena and the Stikine River, and these physical denudation rates are higher than global average. Theses numbers do not take bedload into account.

3. Sampling and analytical methods We have sampled main river basins of the Western Cordillera along the Cassiar Highway in Canada, as

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J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

Fig. 1. Map of the Western Cordillera showing sampling sites for the three major river basins: Stikine, Nass and Skeena, as well as the tectonostratigraphic terrane map of the Cordillera (adapted from Samson et al., 1989). Terranes are labeled as follows: Ax: Alexander, Ch: Cache Creek, E: Eastern assemblage, St: Stikine terrane, W: Wrangellia and Mc: Mackenzie lowlands. Numbers refer to sample numbers in Table 1. The contours of the sedimentary Bowser Basin are figured by the dotted line. Skeena and Stikine archs limit the Bowser Basin to the South and the North, respectively.

well as some small stream waters during high flow in June 1999. River waters were sampled in the middle of the channel, 1 m below the surface. Suspended sedi-

ments were also collected after shaking off the 0.2 Am filters. Bed sands deposited by the Nass and Stikine were also collected for major elements analysis.

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

In the field, 10– 15 l of river water was collected into acid-washed containers. Samples were filtered a few hours later, using a Sartorius 0.2 Am cellulose acetate filter (142 mm diameter). Filtered samples were stored in acid-washed polypropylene bottles and separated from the same batch into several bottles. Samples for cations, trace elements and Sr isotope analysis were acidified in the field to pH = 2 with ultrapure HNO3. The bottles for anion analysis were kept non-acidified. pH and alkalinity (Gran method titration) were determined in the field. Filters were placed in 125 ml of filtered water and suspended sediments energetically shaken off the filters. The sediments were then centrifuged in the lab a few weeks after, the supernatant eliminated and the suspended sediments dried at 60 jC. No chemical variation between the supernatant and the river dissolved phase was observed after 3 weeks, demonstrating that no chemical evolution of the above concentrated suspended sediments occurred during the storage. In the laboratory, major element concentrations (Na, K, Mg, Ca, Cl, NO3 and SO4) were measured by Ion Chromatography (Dionex 300) with a precision of F 1– 3% for river waters, and were measured by ICP-AES for suspended sediments with a precision of F 2 –5%. Dissolved silica concentrations in river waters (H4 SiO4) were measured by spectrophotometry (molybdenum complex) at Toulouse University. Trace element concentrations (for river waters and suspended sediment samples) were determined by ICP-MS in Toulouse (VG PlasmaQuad II+) with indium as an internal standard, and a precision better than F 3%. For both dissolved and suspended load, strontium concentrations and isotopic ratios were measured in Paris by thermal ionization mass spectrometry (TIMS) using a VG 354 mass spectrometer, after chemical separation on a Sr Spec column and dissolution of suspended sediments by acid attack (HF, HNO3 and HClO4). Strontium concentrations were determined by isotopic dilution with a precision better than F 1%. Blanks for the complete analytical method were less than 80 pg of Sr for dissolved strontium extraction, and around 400 pg for acid dissolution of suspended sediment. In both cases, blank levels were negligible compared to the Sr content of the samples. Accuracy of the measurements was checked by running the NBS 987 standard (n = 80), which yielded 87 Sr/86Sr = 0.710250 F 0.000018 (2j error).

261

In the present work, suspended sediments were used in order to constrain the 87Sr/86Sr ratio of the silicate phase. We have performed leaching of both carbonate and dolomite phases in order to determine the Sr isotopic signature of the silicate phase in the river suspended sediments. About 150 mg of suspended sediment was crushed and leached with 4 ml of 4 N acetic acid for 2 h at 80 jC in order to dissolve the calcitic phase. The acetic acid was then removed and suspended sediment was dried and crushed before the second leaching. The second leaching was carried out with 8 ml of 0.1 M HCl during 2 h at 80 jC in order to remove the dolomitic phase of the suspended sediment. Finally, the HCl was eliminated and the residue was dried before complete acid digestion in HF, HNO3 and HClO4.

4. Results and comments 4.1. Dissolved load Data for major and trace elements in the dissolved load are compiled in Table 1. The 19 rivers of this study are quite homogenous in terms of major element concentrations. Total dissolved solids (TDS) load range from 44 to 152 mg/l, with a mean value around 74 mg/l. The highest concentrations are found in the northern part of the Stikine Province. S+ and S (which represent the sum of the cations and the sum of anions) range, respectively, from 535 to 2025 and 525 to 2140 Aeq/l. More precisely, sodium concentrations are between 39.1 and 127.0 Amol/l. Potassium concentrations range from 4.9 to 18.9 Amol/ l and Mg and Ca concentrations are between 39.9 and 337, and 185.3 and 621.4 Amol/l, respectively. An important observation that is apparent from Table 1 and Fig. 2 is that the rivers of the Stikine Province have relatively high Ca/Na molar ratios (from 2 to 14) for what would be expected for rivers draining silicates. In addition, Mg/Na and Sr/Na ratios obey the same rule. Similar enrichments in Ca relative to Na have also been reported by Dethier (1986) in catchments in the Pacific Northwest, USA (Ca/Na ratios from 1.5 to 3.5 from basaltic and intermediate volcanic rocks). Dissolved silica concentrations in the river basins are between 26.9 and 76.8 Amol/l with a mean value

262

Table 1 Major, trace element concentrations and Sr isotopic composition for the dissolved phase of the Western Cordillera rivers Sample number Skeena CAN99-23 CAN99-24

CAN99-27

Sr/86Sr

pH

TDS (mg/l)

0.705034 0.704856

7.1 7.3

9/06/99 9/06/99

0.704618 0.704738

10/06/99

Sampling date

TSM (mg/l)

Buck Bulkley at Houston Telkwa Bulkley at Telkwa Babine at Hazelton Skeena at Kitwanga

9/06/99 9/06/99

14

S+ (Aeq/l)

Si (Amol/l)

Cl (Amol/l)

860 915

n.d. 76.8

10.7 20.0

0.3

453 659

150.7 70.9

0.49 0.45

587 558

50.4 48.2

3.9 7.3

3.7 1.3

504 448

224.2

0.87

633

31.5

3.1

4.0

Na (Amol/l)

K (Amol/l)

Mg (Amol/l)

Ca (Amol/l)

Sr (Amol/l)

58.2 69.2

113.9 109.6

18.9 15.9

137.0 129.6

226.7 265.1

0.86 0.79

7.1 7.0

47.6 44.5

53.0 55.2

4.9 7.9

56.8 52.3

208.0 195.3

0.704766

7.2

49.8

53.5

4.9

63.0

NO3 (Amol/l)

HCO3 (Amol/l)

SO4 (Amol/l)

S (Aeq/l)

NICB (%)

DOC (mg/l)

Calcite Saturation Index

765 821

11 10

– 11.43

 1.97  1.54

24.3 34.1

560 525

5 6

– –

 1.95  2.15

501

53.0

614

3

–

 1.82

10/06/99

53

0.704741

7.5

53.1

56.5

5.6

63.4

227.4

0.85

644

35.1

4.2

3.5

551

51.1

661

3

4.48

 1.48

Cranberry Nass Bell Irving Taft Creek Snowbank Snowbank

10/06/99 10/06/99 10/06/99 10/06/99 10/06/99 10/06/99

164 294 160 28 234

0.705590 0.705250 0.705514 0.705392 0.705368 0.705417

7.7 7.6 7.8 7.5 7.5 7.8

73.1 54.3 63.3 53.0 60.3 88.0

76.1 43.5 42.2 40.9 47.8 39.1

7.9 5.6 6.6 6.4 6.1 5.1

161.3 79.8 101.2 100.8 127.2 135.8

273.8 230.4 281.8 185.3 361.8 408.2

2.17 0.97 1.10 0.93 1.36 1.34

954 670 815 619 1032 1132

n.d. 26.9 27.0 n.d. n.d. n.d.

11.3 3.7 3.1 3.1 2.5 2.8

4.8 11.1 11.5 11.8 12.9 11.9

855 536 565 485 593 637

34.1 70.1 119.8 120.1 45.1 287.9

940 691 819 740 698 1227

2 3 0  19 32 8

– 2.65 – – – –

 1.07  1.41  1.15  1.68  1.25  0.96

Ningunsaw Burrage Willow Creek Todagin Zetu Stikine Tanzilla

10/06/99 11/06/99 11/06/99

322

0.705258 0.705536 0.705268

7.7 7.6 8.1

81.3 87.3 122.4

49.6 63.9 103.5

5.9 9.2 10.7

90.9 227.6 306.2

395.3 311.0 429.4

1.70 1.46 2.03

1028 1150 1585

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

4.2 2.8 3.4

13.7 1.1 2.1

569 806 1099

272.3 189.2 288.9

1132 1188 1682

 10 3 6

– – –

 1.11  1.17  0.40

0.705624 0.705158 0.705509 0.704304

8.0 8.0 8.0 7.5

151.7 128.5 66.0 46.1

127.0 43.9 56.5 39.6

11.8 7.4 8.7 9.0

337.0 176.1 113.2 39.9

605.5 621.4 249.1 204.0

2.54 1.28 0.90 0.69

2024 1646 790 536

n.d. n.d. 36.2 41.9

3.7 3.7 3.1 3.9

3.7

1013 1293 663 403

559.9 198.3 92.1 87.9

2141 1693 851 584

6 3 8 9

– – 3.89 –

 0.43  0.29  0.86  1.68

Sample number

River name

La (ppb)

Ce (ppb)

Pr (ppb)

Nd (ppb)

Sm (ppb)

Yb (ppb)

Lu (ppb)

CAN99-24

Bulkley at Houston Babine at Hazelton Skeena at Kitwanga Nass Bell Irving Stikine

0.1438

0.1448

0.0430

0.2146

0.0531

0.0299

0.0049

0.0491

0.0635

0.0153

0.0819

0.0210

0.0096

0.0012

0.0509

0.0637

0.0151

0.0810

0.0230

0.0100

0.0017

0.0197 0.0117 0.0477

0.0301 0.0148 0.0712

0.0066 0.0034 0.0141

0.0373 0.0198 0.0683

0.0131 0.0046 0.0193

0.0050 0.0025 0.0097

0.0009 0.0005 0.0016

CAN99-28

Nass CAN99-29 CAN99-30 CAN99-31 CAN99-32 CAN99-33/a CAN99-33/b Stikine CAN99-34 CAN99-35 CAN99-36 CAN99-37 CAN99-38 CAN99-39 CAN99-40

CAN99-27 CAN99-28 CAN99-30 CAN99-31 CAN99-39

11/06/99 11/06/99 11/06/99 11/06/99

390 450

0.2 1.5

Concentrations for major elements are in Amol/l except for the total suspended matter (TSM), total dissolved load (TDS) and DOC concentrations in mg/l. Trace element concentrations (La, Ca, Pr, Nd, Sm, Yb and Lu) are in ppb.

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

CAN99-25 CAN99-26

87

River name

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

Fig. 2. Relationships for major element ratios in the dissolved load (where * means that the concentration is corrected from atmospheric input, using the chloride concentration in rainwater and the CL normalized ratio of seawater). Note that atmospheric correction remains low. (a) Histogram of molar Ca/Na ratio for basaltic rivers (n = 117) world-wide compiled by Dessert et al. (2003); (b and c) two groups of rivers have been distinguished. Black circles (Bulkley, Babine and Skeena Rivers) represent Southern tributaries and Northern tributaries by black squares (Nass, Bell Irving and Stikine Rivers). Bigger symbols represent the largest river basins.

around 41.5 Amol/l. This value is similar to the mean Si concentration found in the eastern part of the Rockies and in the Mackenzie Mountains (Millot et al., 2003). On a worldwide basis, Si concentrations found in the Canadian Cordillera are quite low com-

263

pared to the mean average of 130 Amol/l based on the 60 largest rivers of the world (Gaillardet et al., 1999). Chloride concentrations are also very low (mean value of 5.3 Amol/l), this denotes the low atmospheric input, except for the Bulkley River at Houston which has the highest value of chloride with 20.0 Amol/ l (however this values is still low relative the global mean). For all the river waters, nitrate concentrations are low (mean value of 5.8 Amol/l). By contrast, bicarbonate concentrations (HCO3) are rather high, ranging from 400 to 1290 Amol/l, with a mean value of 665 Amol/l. For the large majority of rivers, SO4 concentrations are less than 100 Amol/l but a small number of them have sulfate concentrations greater than 200 Amol/l. All these rivers are located in the Bowser basin. The relatively low SO4 concentrations found here contrast with the adjacent Mackenzie River Basin where high levels of SO4 were measured in the river waters (Millot et al., 2003). In addition, all the rivers of the Stikine Province are undersaturated with respect to calcite, having a mean saturation index of calcite of  1.3 (from  2.1 for the Bulkley River to  0.3 for the Zetu River). The Calcite Saturation Index (CSI) is defined as CSI = log X, where X={Ca2 +}
{CO32 }/KScalcite and {} denotes activities. Strontium isotopic ratios are low, reflecting the weathering contribution of the volcanic rocks within the Cordillera. 87Sr/86Sr ratios are indeed ranging from 0.7043 to 0.7056 (respectively for the Tanzilla and the Todagin River) with a mean value of 87 Sr/86Sr = 0.7052. Sr isotopic ratios for the Skeena, the Nass and the Stikine River are in a good agreement with previous values given by Wadleigh et al. (1985) and Cameron et al. (1995) for the volcanic rivers of the Cascade and intermontane Belt (Fraser Basin). The Western Canadian Cordillera is separated from the Rocky Mountains in the East by the Tintina fault. In the Eastern part of the Cordillera, rivers draining the Rocky Mountains belong to the Mackenzie River Basin and have been documented in details in a previous paper (Millot et al., 2003). It is clear that the rivers of the Western Cordillera are very different from those of the Rocky Mountains in terms of chemical composition. For example, mean value for Ca/Na, Mg/Na and 1000
Sr/Na ratios are 5, 2 and 20, respectively, in the river waters of the Cordillera,

Sr isotopic composition for bulk suspended sediment and values for leached suspended sediment are also given. Suspended sediment samples have been leached in order to dissolve the dolomitic and the carbonate constituent of the suspended phase. Details are given in the analytical section with the technique of leaching with acetic and chlorhydric acids.

961 4.93 18.97 38.25 4.87 19.56 4.28 1.12 4.10 0.65 4.10 0.82 2.46 0.36 2.30 0.37 654 852 14 652 10 650 15 358 15 607 4196 387 10 733 10 936 17 509 14 693 2757 0.706146 0.706563 56.2 216.5 292 740 75 071 45 241 341 880 59 500 30 906

5933 14 097 14 777 4196 1135 3.62 15.12 31.73 4.06 17.30 3.98 1.07 3.82 0.60 3.76 0.76 2.23 0.32 2.18 0.32 852 17 908 0.707207 0.707643 50.2 148.5 295 820 72 265 47 829

6076 14 690 14 777 4017 1004 3.90 16.55 33.79 4.35 18.67 4.31 1.08 4.12 0.70 4.25 0.89 2.61 0.37 2.49 0.38 9149 17 954 16 519 3537 1004 774 16 522 697 15 255 0.706711 0.706914 56.4 159.3 298 247 73 376 46 500 311 780 72 264 42 024

0.704820 0.704840 43.1 282.3 288 167 83 647 48 598 1007 11 698 11 294 19 216 13 033 4676 1092 3.84 17.01 34.55 4.56 19.11 4.44 1.21 4.30 0.64 3.98 0.81 2.44 0.35 2.26 0.35

929 11 879 10 365 17 955 14 279 4436 1092 3.87 16.09 33.14 4.33 18.28 4.22 1.19 4.16 0.63 3.94 0.80 2.41 0.34 2.21 0.35 46.3 256.3 280 513 86 559 49 507 –

– – – – – – – – – – 382.0 – 0.704881 0.704939

CAN99-24 Bulkley at Houston CAN99-27 Babine at Hazelton CAN99-28 Skeena at Kitwanga CAN99-30 Nass CAN99-30 Nass sands CAN99-31 Bell Irving CAN99-39 Stikine CAN99-39 Stikine sands

0.704855

Tm Er Ho Dy Tb Gd Eu Sm Nd Pr Ce La Th P Ti K Na Ca Mg Mn Fe Al Si Sr Rb 87 Sr/86Sr leached 87 Sr/86Sr bulk

River name

River suspended sediments of the Canadian Cordillera have been analyzed for major, trace elements and Sr isotopic compositions (Table 2). The bed sands of the Nass and Stikine were also analyzed for major elements and do not show any striking difference with the suspended material. We have represented in Fig. 3a, REE concentrations in the suspended sediment (CP) normalized to the concentrations in the Primitive Mantle (CPM). In contrast to the suspended sediments of the world’s largest rivers, the REE patterns of the suspended sediments of the Stikine Province show considerable LREE depletion compared to the upper continental crust (UCC) pattern. As REE are insoluble elements, they are mainly transported as sediments and the REE patterns of the suspended sediments reflect the composition of the source rocks. In Fig. 3b, we have represented the REE concentrations in the

Table 2 Major and trace elements concentration for suspended sediments (ppm) and riverbed sands

4.2. Suspended and bottom sediments

Yb

whereas they are higher with values respectively around 20, 10 and 38 in the river waters of the Rocky Mountains of the Mackenzie Basin. We also notice a large contrast in Sr isotopic compositions. Whereas 87 Sr/86Sr ratios are ranging from 0.7043 to 0.7056 in the Cordillera, isotopic Sr ratios are much higher in the Rocky Mountains with isotopic ratios comprised between 0.7066 and 0.7580 (mean value around 0.7195; Millot et al., 2003). This contrast between the Cordillera and the Rocky and Mackenzie Mountains river basins is the result of a strong difference in terms of lithology. Whereas volcanics and volcaniclastics dominate the river basins of the Cordillera, limestones and dolostones are rather abundant in the river basins of the Rocky Mountains with a Sr radiogenic signature (see Millot et al., 2003 for discussion). Based on Sr isotopes and Ca/Na systematics, the rivers sampled in this study can be separated into two groups. The first group is composed by rivers such as the Bulkley, the Babine and the Skeena River, located in the southern part of the Canadian Cordillera. They exhibit the lowest Sr isotopic ratios and base cation normalized ratios. The second group of rivers, composed by other river basins such as the Nass, the Bell Irving and the Stikine River, is located in the northern part of the Canadian Cordillera. These rivers show relatively high Ca/Na ratios and have the highest Sr isotopic ratios.

3.77 20.53 42.14 5.42 21.86 4.65 1.32 4.67 0.68 4.13 0.81 2.46 0.35 2.27 0.35

Lu

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Sample number

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265

Fig. 3. Trace element concentrations in the suspended sediment phase (CP) normalized to the Primitive Mantle (CPM) and to the Andesite model composition for the bulk crust (Taylor and MacLennan, 1985).

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river suspended material normalized to REE concentrations in the Andesite model composition for the bulk crust (Taylor and MacLennan, 1985). The flatness of the patterns obtained indicates that the rocks exposed to weathering in the Stikine Province are representative of juvenile continental crust. No distinction between the rivers of the North (Stikine, Nass and Bell Irving) and rivers form the South (Bulkley, Babine and Skeena) is observable. This observation is in contrast with Sr isotopic ratios. The river suspended sediments have Sr isotopic compositions ranging from 0.7048 to 0.7072, with a mean value of 0.7058, slightly higher in the suspended sediments compared to the dissolved phase. The river suspended sediments from the North (Stikine, Nass and Bell Irving) are more radiogenic (about 0.707) than those from the South (Bulkley, Babine or Skeena River with Sr isotopic ratios around 0.7049) and a relatively good correlation is observed between Sr isotopic ratios and 87Rb/86Sr ratios (Fig. 4). There are at least two possible interpretations for such a correlation. The first is to interpret it as an isochron reflecting the age of the source rocks. This implies that Rb and Sr were not fractionated by weathering processes and that the eroded rocks were formed by a single event at 280 Ma (Early Permian). This result

Fig. 4. Sr isotopic ratio of suspended sediments from the Stikine rivers as a function of their Rb/Sr chemical ratio. Interpreted as an isochron, this correlation would lead to an age of 280 Ma. We favor the mixing interpretation between a volcanic endmember of low Sr isotopic composition and a sedimentary component of higher Sr isotopic ratio.

seems unlikely given that volcanism in the Stikine region is known to span from the Devonian to Jurassic. Our favored interpretation is that the correlation in Fig. 4 represents a mixing between the volcanic dominated products of weathering and those derived from the Bowser Basin having a volcaniclastic sedimentary origin. This idea is consistent with the database published by Samson et al. (1989) concerning Rb – Sr data for more than 20 rocks from the Stikine Terrane. The volcanic rocks (especially outcropping in the South of the studied area) have relatively low Sr isotopic ratios and associated Rb/Sr (lower than 0.5), while the volcanic derived-sedimentary rocks of the Bowser Basin have higher Sr isotopic ratios (up to 0.709) and higher associated Rb/Sr ratio (2.5 – 3). The Rb enrichment of the sediments of the Bowser Basin is inherited from the pre-depositional weathering processes that preferentially removed Sr from the exposed bedrock. With time, the Rb enrichment of the Bowser Basin sedimentary rocks led to relatively high Sr isotopic ratios. The scenario is consistent with the correlation in Fig. 4 and shows that the source rocks of the Stikine, Bell Irving and Nass rivers are more influenced by sedimentary rocks (volcaniclastics) than those of the Skeena and Babine River. The inspection of Ca concentrations in the suspended sediments shows the greater depletion of the Nass suspended sediments compared to the Stikine and Skeena rivers. Acid leaching of the suspended sediment was carried out in order to remove both the potential carbonate and dolomitic phases (see analytical section for details). It clearly appears (Fig. 5) that no difference of isotopic ratios exists between the unleached and leached suspended sediments of the Skeena and Bulkley rivers, while a component having a lower isotopic ratio is removed from the sediments by acid leaching from the rivers of the North (Nass, Stikine and Bell Irving). As a conclusion, the geochemistry of suspended sediments remarkably matches the major geological features of the Stikine Province. The rocks submitted to chemical weathering in the river basins of the Skeena, Nass and Stikine are predominantly mantle derived juvenile rocks, mostly igneous for the Skeena and sedimentary rocks derived from a first weathering cycle of older mantle derived-igneous rocks for the northern rivers (Bowser Basin). In both cases, the pattern of insoluble elements such as REE is remark-

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

Fig. 5. Sr isotopic composition of bulk suspended sediments plotted as a function of the Sr isotopic ratio of acid-leached suspended sediments. Suspended load samples have been leached in order to remove dolomitic and carbonate phases, with hydrochloric and acetic acids (see analytical section for details). River basins from the South (circles) have been distinguished from those of the North (squares). Dotted line is the 1/1 line.

ably similar to that of the andesite model for newly formed continental crust.

5. Identification and contribution of each rock reservoir We have represented in Fig. 6 the Sr isotopic composition of the dissolved load as a function of Ca*/Na* and 1000
Sr/Na* (where * means that the concentration is corrected from marine input). The areas corresponding to the two silicate endmembers deduced from the composition of the suspended sediments after leaching with acids are also plotted. According to Section 4, the ‘‘high’’ Sr isotopic ratio represents predominantly a sedimentary silicate endmember while the ‘‘low’’ Sr isotopic ratio represents an igneous endmember. Under the working hypothesis (that will be discussed in Section 7) that river suspended sediments are the residue of silicate chemical weathering, these fields constrain the composition of the river water having interacted with silicate rocks. The Sr isotopic signature of the river waters will be that of sediments assuming that no major fractionation of Sr isotopes occurs during chemical weathering. In addition, under the classical scheme that Ca, Sr and Mg are less soluble than Na, the Ca/Na, Mg/Na and

267

Sr/Na ratios of suspended sediments are the maximum possible values for the endmember of waters draining silicates. These ratios are about 0.4, 1 and 2
10 3, respectively. According to this model, a third endmember is needed in order to explain the whole range of isotopic and chemical variations for the river waters of the Skeena, Nass and Stikine basins. As shown in Fig. 6, this third endmember should have both high Ca/Na, Sr/Na as well as Mg/Na ratios and should also have a Sr isotopic composition lower than that of the silicate component. The most simple hypothesis is to suppose that this component corresponds to the weathering of carbonate. There are also two good arguments in favor of carbonate weathering. Firstly, carbonates are found in the Stikine sedimentary formations either in the form of post-magmatic calcite in igneous rocks or as sedimentary carbonates (Basset and Kleinspehn, 1996). Secondly, during the leaching experiments with the river suspended sediments, a component of Sr having a lower isotopic ratios than the bulk suspended sediment is released with acetic acid treatment. The carbonate endmember characterized by high Ca/Na, 1000
Sr/Na and Mg/Na ratios of 50 F 20, 75 F 25 and 20 F 8 (see Gaillardet et al., 1999 for discussion) has been represented on Fig. 6a and b. Theoretical mixing hyperbolas have also been calculated and plotted in Fig. 6. Two trends are observed. The mixing of sedimentary silicates and carbonates explains relatively well the composition of the dissolved load of the rivers from the North. The rivers of the South appear to be the result of a mixture between the dissolution products of igneous silicates and carbonates. These two trends are consistent with the geology of the Bowser Basin, dominated by sedimentary rocks and of the Skeena Arch, dominated by igneous rocks (Basset and Kleinspehn, 1996). It appears that the rivers of the North are more influenced by carbonate dissolution than those of the South. Both trends seem to converge toward the same endmember having a Sr isotopic composition of about 0.703 – 0.704. This Sr isotopic signature inferred for the carbonate endmember is low compared to the Sr isotopic composition of marine carbonates over Phanerozoic time (from 0.707 to 0.709), implying that the contributing carbonates are not of marine origin. There are different possible origins for these carbonates. Carbonates could have been formed in a confined basin (continental or isolated marine) in which

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Fig. 6. Sr isotopic composition of the dissolved load plotted as a function of Ca*/Na* and 1000
Sr/Na*. Sr isotopic signature for igneous and sedimentary silicate rocks are from Samson et al. (1989) and remarkably compatible with those measured in the silicate residue of the suspended sediments. Bigger symbols represent the largest river basins. Calculated hyperbolas for mixing between the endmembers have been represented on these figures. Squares are for the Northern rivers, circles for the Southern rivers.

the local Sr isotopic signature was coprecipitated. Non-marine sediments are described in the Bower Basin and Skeena Arch (Basset and Kleinspehn, 1996) but it is not within the scope of this paper to relate the presence of non-marine carbonates to Sr isotopic composition of rivers. The systematics of Rb –Sr data for rocks of the Stikine terrane by Samson et al. (1989) showed that the 87Sr/86Sr initial isotopic value for the igneous rocks of the Stikine terrane is about 0.704. This value corresponds to the mean value

needed here for the carbonate endmember (Fig. 6). Therefore, another possibility is that the high Ca, Mg and Sr endmember described above represents the dissolution of calcite of hydrothermal origin, quite widespread in the rocks of the Stikine Province like in any setting of andesite and arc-basalt formation. The possibility of carbonate cements or veins is also invoked by Dethier (1986) in the Pacific Northwest, USA as a source of calcium in weathering solutions. As shown above, the Sr isotopic composition of Sr

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

acid-leached from the suspended sediments of the rivers of the northern part of the Stikine is higher than 0.704. We do not consider that this observation contradicts with the occurrence of poorly radiogenic calcite in the Bowser Basin because it is highly possible that the leaching procedure (acetic acid and HCl) released Sr from the silicates of the suspended matter. Finally, the preferential dissolution of calcite has been shown to explain the enrichment in Ca found in the streams draining the foreland of a retreating glacier in south –central Alaska, draining cretaceous metasediments and basalts deposited in a continental margin trench of Andean type (Anderson et al., 1999).

6. Model and results The method of calculation for the contribution of each endmember is briefly described in this section but has been detailed elsewhere (Ne´grel et al., 1993; Millot et al., 2003). This calculation is basically an inverse method constituted of several mass budget equations. We consider that each major dissolved element within the river water could have four different origins: igneous silicate, sedimentary silicate, rainwater and carbonate. This calculation is based on mass budget equations between the river water and the four above reservoirs. The general form of mixing equations are the following (for X = Cl, Ca, Mg, HCO3 and Sr) and are normalized to Na concentrations.


X Na

 ¼

X
X 

river

Na

i

ai ðNaÞ

ð1Þ

i

where ai represent the mixing proportions of Na in the four different reservoirs. For Sr isotopic composition, the mixing equation is the following:
87

Sr

86 Sr

 river




Sr Na

 ¼ river

X
87 Sr 
Sr  i

86 Sr

i

Na

269

This method is particularly adapted for the purpose of this paper where we want to extract the silicate component from the dissolved load of rivers. Due to the uncertainty of which minerals actually dissolve and the origin of the Ca and Mg enrichment in the Stikine river waters, we have used the inversion procedure to test two main scenarios: 

Following what we presented in Section 4, the first hypothesis is that the silicate endmember has the chemical signature of the suspended sediment. This approach will give low estimate of silicate denudation rates, because, as already mentioned, Na normalized ratios are low in the suspended phase of these rivers (Ca/Na between 0.22 and 0.4).  In the second hypothesis, we consider that the silicate endmember is characterized by mean elemental ratios (Ca/Na, Mg/Na and Sr/Na) reported by Dessert et al. (2003) from a compilation of data on basaltic rivers at the world scale (Table 3). This global survey led to Ca/Na, Mg/Na and Sr/Na of 1.3, 1 and 2.10 3, respectively, for mean values (Table 3 and Fig. 1). Finally, in a third scenario, we will assume that all solutes, except the part derived from atmospheric contribution, will be derived from silicate weathering. Silicate weathering rates are estimated taken into account the entire the dissolved load, once corrected from atmospheric inputs. According to this hypothesis, the enrichment in Ca, Mg and Sr compared to Na is not due to carbonate contribution but is rather due to the preferential weathering and the release of Ca, Mg and Sr by silicate minerals. Such an estimate will give the upper estimate of the silicate denudation rates

Table 3 Chemical molar ratios of rainwater, silicate, carbonate and saltrock endmembers used in the inversion procedure

ai ðNaÞ i

ð2Þ From these mass budget equations, we are able to calculate the proportion of each element delivered by each weathering source.

Ca/Na HCO3/Na Mg/Na 1000
Sr/Na

Silicate

Carbonate

Evaporite

Rain

1.30 F 0.20 5F1 1.0 F 0.20 2F1

50 F 20 100 F 40 20 F 8 75 F 25

0.17 F 0.09 0.3 F 0.3 0.02 F 0.01 3F2

0.022 0.004 0.12 0.19

The silicate endmembers comes from a compilation of rivers draining basalts (Dessert et al., 2003).

0.7 0.5 0.1 0.1 0.02 0.01

0.4 0.2 0.05 8.7

15.9 11.7 5.0 4.1 3.1 2.8 29 – 36 33 – 41 64 – 71 59 – 67 0.2 0.1 1285 811

Hypothesis 2

4.2 2.3 41 – 56 44 – 59

Hypothesis 2 Hypothesis 1 Hypothesis 1

Hypothesis 3

CO2 consumption rate (106 mol/km2/year) Silicate weathering rate (tons/km2/year) % Silicate % Carbonate % Rain

0.2 719 30.36

24.68 23.83 19 200 29 300 CAN99-30 CAN99-39

The symbol U describes specific fluxes. Silica concentrations are not taken into account for this calculation in order to be consistent with our previous studies in northern Canada (Millot et al., 2002, 2003). Silica concentrations are excluded from the silicate denudation rate calculation because of the potential

42 200

ð3Þ

Skeena at Kitwanga Nass Stikine

¼ UNasil þ UKsil þ UMgsil þ UCasil

CAN99-28

Cationic Silicate Weathering Rate

Runoff (mm/year)

The inversion procedure presented above allows us to calculate the contributions of each weathering source for the above hypotheses. From these results, we estimate silicate weathering rates from concentrations of major cations elements delivered by silicate weathering according to Eq. (3).

Discharge (km3/year)

6.2. Silicate denudation rates

Area (km2)

The contributions of each reservoir are calculated for the three main river basins of the Canadian Cordillera. Results (in %) are listed in Table 4 and correspond to the contributions of each weathering reservoir for the TDS load considering the three above hypotheses. From the two first estimates, we find that the chemical weathering load is greatly influenced by the weathering of carbonate. Carbonate weathering contribution is ranging from 44% to 71% for the Skeena River and the Nass River, respectively. From these calculations, it appears that the Skeena River presents the most important contribution of silicate rock dissolution (between 41% and 56%). For the Nass and the Stikine River, silicate weathering contributions are lower and comprised between 29% and 36%, and between 33% and 41%, respectively. From the inversion calculation, we also find that other contributions are low and represent a very little input to the chemical weathering mass budget of these river basins. Rainwater input is negligible, ranging from 0.1% to 0.2%, and no evaporite contribution was detected.

River name

6.1. Chemical weathering contributions

Sample number

considering that the entire dissolved load is delivered by silicate weathering.

Hypothesis 3

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279 Table 4 Proportions of each reservoir contributing to the dissolved load of the Stikine rivers and silicate weathering rates for cations calculated according to the three hypothesis explained in text

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biogenic uptake of silica in river waters. For example, the rivers from the Grenville Province (Que´bec Province) are 20 times enriched in Si compared those of the Slave Province (NWT) what seems to be essentially due to biological processes (Millot et al., 2002). The so-calculated weathering flux is directly equivalent to the alkalinity produced by silicate weathering. For potassium, we have considered the total concentration of K in the river water. From Table 1, we observe a mean K/Na ratio close to 0.1, which is in a good agreement with the ratio obtained for river waters draining pure silicate (see Gaillardet et al., 1999 for discussion). Specific chemical fluxes (U) are calculated from runoff and discharge database (Hydat CD-ROM, 1998, Environment Canada). Because of the variability of runoff data over time, we estimate a mean error of F 20% for this calculation. Silicate denudation rates are given in Table 4, these rates are normalized to the total surface area of each river basin and are thus chemical denudation specific fluxes. Due to the method of calculation in the first estimate, values are the lowest, ranging from 2.3 to 3.1 tons/km2/year. Silicate weathering rates are intermediate in the second case (ranging from 4.1 to 5.0 tons/km2/year). Silicate denudation rates are higher in the third case, with values comprised between 8.7 and 15.9 tons/ km2/year, respectively, for the Skeena and the Nass River. However, even if silicate weathering rates span over a large range depending on the origin postulated for Ca and Mg, we only observe a 4 –5 factor of variation, this lead to a first order assessment of silicate denudation rates of the Canadian Cordillera. It is interesting to note the excellent agreement between the rates calculated under hypothesis 3 (all solutes derived from silicates) and the rates estimated by Dethier (1986) in streams of the Pacific Northwest, USA based on the total cation load.

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mentary gypsum and oxidative weathering of sulfides. In the presence of oxygen, sulfide weathering produces protons that are used for rock weathering. In such a scheme, part of the cations delivered by rock weathering are not balanced by bicarbonate ions (whose carbon is derived from the atmosphere) but by sulfate ions. The input of atmospheric sulfate is probably around 10– 15 Amol/l, as indicated by the sulfate concentrations found in the rivers of the Slave Province, where no crustal source of sulfur exists (Millot et al., 2002). First order atmospheric CO2 consumption rates associated with chemical weathering are given in Table 4. They have been estimated by calculating the sum (in eq/l) of Na, K, Ca and Mg ion concentrations and subtracting the atmospheric corrected sulfate concentration. This assumes that no sulfate is derived from the dissolution of sedimentary sulfates. Bicarbonate concentrations derived from water-silicates interaction range from 15 to 70 Amol/l for the first hypothesis, 100– 200 Amol/l for the second hypothesis and 550 – 600 Amol/l if the only source of cations is silicate weathering. Although imprecise, these results show that the production of protons by oxidative weathering is potentially very important for the determination of CO2 consumption rates by silicate weathering in the Stikine river waters. According to hypothesis 1, almost no atmospheric CO2 is consumed from the atmosphere during water – rock interactions of the Stikine basin. A closer examination of sulfate provenance would therefore be needed to refine the mass budgets of CO2 consumption rates. The CO2 consumption fluxes are given in Table 4; they range from 0.1– 0.2
106 mol/km2/year for the first hypothesis to 0.01 to 0.05
106 mol/km2/year for the second hypothesis and 0.4 – 0.7
106 mol/km2/ year for the third hypothesis. Errors propagated on the CO2 consumption rates are much higher than those propagated on weathering fluxes due to the uncertainty on the origin of SO4 concentrations.

6.3. Alkalinity and CO2 consumption rates As indicated by Table 1, the calculation of the amount of HCO3 equilibrating the different cations delivered by silicate weathering is complicated by the presence of SO4 in river waters. In this study, we are not able to constrain the source of sulfate ions. Three different origins are classically invoked for SO4 in river waters: atmospheric input, dissolution of sedi-

7. Discussion The methods of calculation investigated in the present work for the estimation of silicate denudation rates are based on important assumptions. In the first scenario tested above, we assume that the suspended sediments transported by the Stikine rivers represent

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the residue of chemical weathering of the silicates that released the major solutes in the river waters. The excess of base cations observed in the Stikine River dissolved load (Fig. 2) are thus due to the dissolution of carbonates. The important issue has to be discussed: is this hypothesis realistic or could a certain type of silicate weathering have provided such Ca and Mg excesses? In the following, the different scenarios passed through the inversion procedure are discussed. 7.1. The use of suspended sediments to constrain silicate weathering (hypothesis 1) In the first hypothesis, it is assumed that the river dissolved load is the river suspended load complement. The data on the chemistry of dissolved and suspended load of volcanic rivers from different settings of basaltic lithology (Java, Iceland, Reunion, Azores) reported by Louvat (1997), Louvat and Alle`gre (1997, 1998) support this hypothesis. In a Mg/Na vs. Ca/Na plot, suspended and dissolved loads plot symmetrically relative to the bedrock. In the case of Iceland and Re´union, for which physical erosion fluxes are much greater than chemical erosion fluxes, the chemical signature of the suspended load is close to that of the bedrock, while the base cation over Na ratios are much lower than that of the bedrock. For a similar mass budget reason, it is likely that in the Stikine Province, where mechanical erosion is very active, the chemistry of the suspended load will be close to that of the average bedrock. It is interesting to note that the Ca/Na ratios observed in the suspended loads of the Stikine rivers are low compared to the other volcanic settings investigated by Louvat (1997) for which ratios of 3 – 4 are classically observed. If, as currently reported, Ca and Mg are less soluble than Na, the Ca/Na and Mg/Na ratios of the suspended sediments are then maximum values for the Ca/Na and Mg/Na ratios in the water interacting with silicates. Applying the observations made by Louvat (1997) on the Ca/Na ratios in dissolved and suspended loads of Icelandic rivers (respectively 0.3 and 1.5) to the Stikine rivers, then proportionally, a mean value of 0.1 is expected for the silicate draining waters. A sensitivity test showed that taking a Ca/Na of 0.1 (and corresponding Mg, Sr and HCO3/Na ratios) does not lead to significantly different results from those of Table 4. Therefore, we consider that the estimates of

chemical weathering rates and CO2 consumption rates found with the first scenario are close to minimum possible values. The other possibility that has to be envisaged is that the suspended sediments and the dissolved load are not complementary reservoirs. This could be especially the case in the Bowser Basin due to the presence of volcaniclastic sediments. If the sedimentary rocks erode physically faster than the associated volcanic lithologies, then the dissolved load would reflect the weathering of volcanics while the particulates would come predominantly from sedimentary materials. As shown earlier, the Sr isotopic ratios in the dissolved load of the rivers of Bowser Basin are lower than that of the associated suspended sediments. Our main interpretation is that this difference comes from the contribution of non-radiogenic calcite. It is however possible that these low values reflect the preferential contribution of Sr derived from volcanic rocks. The rivers of the southern part of the Stikine would be closer to this endmember just because the proportion of clastics is lower than in the Bowser Basin. This scheme is plausible although it does not explain the relationships of Fig. 6 between Sr isotopic ratios and Ca/Na ratios. In terms of chemical denudation rates, it fully justifies the use of the mean basaltic endmember (hypothesis 2). 7.2. Could the excess ca be due to silicate weathering? If, as proposed in the third hypothesis of Section 6, we allocate all the Ca, Mg, Na in the river dissolved load to silicate weathering, we implicitly assume that, in the weathering conditions of the Western Cordillera, Ca is far more soluble than Na. Molar ratios of 0.3 – 0.4 (for mean continental average) and 1 – 2 (basaltic formations) are usually assumed for the river waters interacting with silicate minerals (Gaillardet et al., 1999; Dessert et al., 2003). In order to explain the Ca/Na values comprised between 5 and 10 found for the rivers of the Northern Stikine region, the preferential dissolution of Ca-rich minerals has to be invoked. A number of Ca-rich silicate minerals can be proposed as a source of Ca including either major phases such as calcic plagioclases, calcic amphiboles and pyroxenes, or trace minerals such epidote, prehnite, apatite, calcic zeolites and laumontite. A number of studies have reported high Ca/Na molar ratios

J. Gaillardet et al. / Chemical Geology 201 (2003) 257–279

for waters draining granitic watersheds, especially in high altitude regions (Mast et al., 1990; Drever and Hurcomb, 1986; April et al., 1986; White et al., 1999). Most often, these high base cation enrichments are attributed to disseminated calcite dissolution. In a recent paper on an alpine catchment in the Pyrene´es (France), draining a monzogranite, Oliva et al. (2003) have reported molar Ca/Na ratios in the river waters close to 3 – 4. A very close examination of thin sections of bedrocks and soils did not confirm the idea that such high Ca/Na ratios were due to the dissolution of trace calcite. Rather, the high Ca/Na ratios have been attributed in their study to the preferential dissolution of trace apatite, epidote and prehnite minerals. In a global review on basaltic rivers, Dessert et al. (2003) shown that the Ca/Na ratios of waters interacting with basalts (once corrected from marine input) range from 0.3 to 2.5 – 3, with no relation to climate, but rather related to the bulk chemical composition of the bedrock. In each case, no disseminated calcite was reported in the bedrock, indicating that the relatively high Ca/Na ratios result from the weathering of silicate minerals and/or glassy material. When the data of the Stikine rivers are compared to the global database compiled by Dessert et al. (2003), they appear as an endmember of a Ca/Na vs. Mg/Na correlation. Therefore, if the high Ca/Na and Mg/Na ratios found in the rivers of the Stikine terrane were to be derived from silicates, they would represent a quite extreme regime of silicate weathering or a particular mineralogical setting. In the compilation of Dessert et al. (2003), arcvolcanism is only represented by the data of Java that display Ca/Na ratios comprised between 1 and 2.5, only consistent with the lower values found here (Fig. 2). The preferential weathering of calcic-rich cores in zoned plagioclases is classically invoked to explain the relatively high Ca/Na ratios of water chemistry of streams draining granites. As shown by Oliva et al. (2003), the calcic cores of plagioclase are Mg-depleted and the Ca/Mg ratios observed here would not be compatible with such an incongruent dissolution. Epidotes, prehnite and apatite are also characterized by a more than 100 times enrichment of Ca relative to Mg and their dissolution cannot be invoked to account for the excess of Ca and Mg. Other good candidates could be pyroxenes and amphiboles. Amphiboles are widespread in arc volcanism lavas and therefore a

273

plausible explanation for the relatively high Ca/Na and Mg/Na ratios reported in the river waters of the Stikine Province could be the preferential dissolution of amphibole. If it is the case, the excess of Ca observed in the rivers draining the Stikine Province is due to silicate weathering and we have to explain the differences outlined above between the more calcic character of the northern rivers (Stikine and Nass drainage basin) compared to those of the southern part of the province (Skeena basin). A possible explanation could be that the northern part of the Stikine Province being more mountainous, the production of fresh bedrock material is more easy, due to active mechanical denudation, particularly the occurrence of glaciers. As fresh bedrock is continually exposed, fresh mineral surfaces react with the water and it would explain the most important contribution of reactive Ca- and Mg-rich minerals. In that case, the non-stoichiometric release of Ca and Mg in certain Stikine rivers would therefore be a transient effect, due to the exposure of fresh mineral surfaces created by the glaciers (Anderson et al., 1999). To conclude, it is plausible to attribute the excess of Ca and Mg in the rivers of the Stikine Province to the chemical weathering of pure silicate minerals. However, high Ca/Na and Mg/Na ratios, such as those reported here, have never been observed before for silicate draining watershed (the maximum values observed in granite or basalt-draining waters are close to 4), we think that the highest ratios (5 –14) observed here for the rivers of northern part of the Stikine Province are probably due to the contribution of carbonates. To be tested, this idea would need a refined examination of what minerals actually weather at the scale of the entire drainage basin. A restricted case study focused on a small watershed would be necessary, similar to what has been done for granitic catchments by numerous authors (see reference above). Such a level of detail is beyond the scope of our study but remains necessary to refine the budgets of CO2 consumption by basaltic and associated rocks. 7.3. Susceptibility of Canadian geological provinces to chemical weathering The above discussion shows how difficult it is to exactly determine the rates at which silicate rocks weather in the Stikine Basin. Our preferred estimates

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will be that using the endmembers estimated by Dessert et al. (2003) for basalt weathering and that using the suspended sediments as an upper value of Na normalized ratios. This study on the Stikine Province is part of a survey of continental rock weathering fluxes in Canada (Millot, 2002). In the following, we compare the different weathering rates obtained in different parts of Canada, classified according to their dominant rock type. Alternatively, these regions also correspond to different geomorphic settings. These regions are the Mackenzie and Rocky Mountains, separated from the Stikine Province by a series of major faults and characterized by the absence of volcanic and plutonic rocks. Shales, blackshales, limestone and dolostones are the dominant rock type. Most of the Western Cordillera in Canada is made of Proterozoic to Cretaceous folded and faulted rocks. Further to the East, the low lying region of the Interior Platform is a vast plateau made of undeformed marine sedimentary rocks that accumulated from the Cambrian to the Cenozoic. In contrast to the Rocky and Mackenzie Mountains, these rocks are calcareous shales and blackshales. Applying a technique similar to the one described above, Millot et al. (2003) quantified the weathering rates of carbonates and (sedimentary) silicates for the Rockies and the sedimentary Platform. They showed that the weathering of silicate minerals is strongly enhanced in the lowland part of the Mackenzie Basin compared to the Rocky and Mackenzie Mountains. Finally, the northern part of the Mackenzie River Basin is made of old granitoid rocks, forming the so-called Slave Province. Similar terranes have been investigated in the Grenville Province, northern Que´bec (Millot et al., 2002). All chemical denudation data are compiled in Table 5 for the different lithologies investigated and the chemical denudation rates obtained. They have been represented as a function of runoff in Fig. 7 to show a global picture of weathering rates in Canada. Cationic denudation rates in Canada appear to vary over more than two orders of magnitude while runoff values span about one order of magnitude, showing that runoff variations do not explain the whole variability of weathering rates. The nature of the rocks is a crucial parameter. Under the subarctic climate of northern Canada, carbonates weather about 10 –20 times faster than silicates. If we except the silicates of the Interior

Platform, the order of susceptibility to chemical weathering is ranked as follows: granite, volcanicdominated rocks from the Stikine (3 times granite) and carbonates from the Rockies (20 times granite). This order confirms the order previously established under temperate climate based on the analysis of French unpolluted streams (Meybeck, 1987), but shows that in northern Canada, the differences of weathering rates are higher. The remarkable feature of weathering rates in northern Canada is the high rate of sedimentary silicates weathering in the plain. A factor of 3– 4 is observed between granites and the shales (silicate part only) of the lowlands, implying that the region of the interior plains weather at least as fast as the western volcanic dominated Cordillera. The reason of such high rates of weathering in the lowlands is discussed in detail by Millot et al. (2003). Three main related factors combine to promote chemical weathering: high levels of dissolved organic matter (up to 20 – 40 mg/l) favor chemical degradation of silicates by complexing a number of elements, the partly volcanic-derived nature of sedimentary rocks in the Cenozoic formations (as indicated by REE and Sr isotopic ratios) and finally the sustained high mechanical erosion favored by the immature nature of the rocks and recent river incision. To conclude, in addition to variations due to runoff variability, the order of weatherability of Canadian formations is as follows: granitoids < volcaniclastics
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Table 5 Synthesis of the chemical denudation rates (of cations) for different lithologies within Canada Lithology

Reference and location

Sample number

River name

Runoff (mm/year)

Cation weathering rate (tons/km2/year)

Volcanics and volcanoclastics

This study Western Cordillera

Shales and blackshales

Millot et al., 2003 Mackenzie River Basin

Granitoids

Millot et al., 2002 Slave Province Canadian Shield

Granitoids

Millot et al., 2002 Grenville Province Canadian Shield

Carbonates

Millot et al., 2003 Mackenzie River Basin

CAN99-28 CAN99-30 CAN99-39 CAN96-14 CAN96-33 CAN96-39 CAN96-40 CAN99-1 CAN99-79 CAN99-80 CAN96-30 CAN96-31 CAN99-84 CAN99-85 CAN99-86 CAN99-87 CAN99-88 CAN99-90 CAN99-91 CAN99-92 CAN95-3 CAN95-7 CAN95-8 CAN95-10 CAN95-11 CAN98-12 CAN98-14 CAN98-15 CAN98-16 CAN98-18 CAN98-19 CAN98-20 CAN96-2 CAN96-3 CAN96-19 CAN96-20 CAN96-21 CAN99-5 CAN99-8 CAN99-10 CAN99-11 CAN99-15 CAN99-46 CAN99-54 CAN99-57 CAN99-60 CAN99-64 CAN99-74

Skeena at Kitwanga Nass Stikine Little Smoky Hay at mouth Hay Lesser Slave Pembina at Evansburg Simonette Little Smoky Tibbitt Yellowknife Wecho Emile Wopmay Indin Mc Crea Yellowknife Cameron Tibbitt Gattineau Saguenay Peribonka Mistassini St. Maurice Wessoneau Saint Maurice La Croche Rivie`re des Bostonnais Mistassini Mistassibi Rivie`re aux Rats Pelly Stewart Racing Toad Liard at Liard River Athabasca at Hinton Rocky Maligne Athabasca at Jasper Whirlpool Liard at Upper Liard Smith Trout Toad Racing Peace at Hudson’s Hope

719 1285 811 257 75 97 97 386 229 257 115 71 90 103 115 163 115 71 49 115 512 631 717 626 531 480 531 612 480 626 677 480 251 374 1209 532 351 558 410 563 716 1747 351 207 432 532 1209 497

4.2 5.0 4.1 2.42 1.34 1.76 1.28 4.33 3.94 2.38 0.40 0.27 0.39 0.49 0.33 0.43 0.33 0.28 0.21 0.41 1.07 1.82 1.23 1.62 2.18 1.25 1.18 1.76 1.01 1.49 2.11 1.95 10.14 19.72 57.97 29.67 15.32 20.65 28.10 18.86 19.96 48.65 11.87 14.12 21.22 25.31 51.89 17.25

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Fig. 7. Chemical weathering rates of cations plotted as a function of runoff for different lithologies: volcanics of the Cordillera (this study), shales and blackshales weathering of the Interior Plain within the Mackenzie River Basin (Millot et al., 2003), carbonate weathering for river basins within the Mackenzie and Rocky Mountains (Millot et al., 2003) and silicate weathering rates for granitoid rocks of the Canadian Shield in the Slave and the Grenville Province (Millot et al., 2002).

granites of Puerto Rico weather at a higher rates (10 tons/km2/year). Dessert et al. (2003) have reported global relationships between silicate denudation fluxes and runoff or temperature. The two lower estimates calculated here (hypotheses 1 and 2) for

the Stikine Province fall perfectly for what is expected for river basins having runoff close to 1000 mm/year and mean temperature close to 0– 4 jC (Fig. 8). Note that our highest estimate (hypothesis 3) would fall in the domain of Azores Island (15 jC of mean annual

Fig. 8. Cationic weathering rates of different basaltic provinces as a function of runoff. This database has been compiled and established by Dessert et al. (2003). The data from the Stikine Province are from this study.

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temperature). Compared to the Skeena and Stikine rivers, the Nass, that entirely drains the sedimentary Bowser Basin, shows slightly lower denudation rates that the Stikine and Skeena. This difference might be due to the greater abundance of recycled crustal material (thus depleted in cations during previous weathering cycles) in the drainage basin of the Nass. The lower Ca concentrations found in the suspended sediments of the Nass river (6000 ppm) compared to the suspended sediments of the Skeena and Stikine (11 300 and 10 600 ppm, respectively) support this conclusion. A global correlation linking HCO3 concentrations in rivers draining basalts to temperature is also given by Dessert et al. (2003). The concentrations found here, although quite uncertain, are much lower than those predicted by a global law of CO2 consumption during basalt weathering. Only the highest estimate (hypothesis 3) is in the range of predicted HCO3 concentration at the temperature of the Stikine Province (0– 5 jC). The good agreement between predicted and measured fluxes of cationic weathering therefore contrasts with the difference between predicted and calculated HCO3 concentrations. This difference is clearly due to the relative abundance of sulfate ions in the rivers of the Stikine Province. As stated above, it is not possible, in the present work, to conclude whether the abundance of sulfur is a particularity of the Stikine Province setting or if the low values of CO2 consumption rates found here are an artifact due to an overestimation of the oxidative weathering of sulfur. A closer examination of the origin of sulfur, based on its isotopic composition is in progress.

8. Conclusion The Stikine, Nass and Skeena rivers are draining juvenile igneous and volcaniclastic rocks of Mesozoic and Cretaceous age from an uplifted accretionary prism. These basins offer a unique opportunity to determine the rate at which igneous and volcaniclastic sedimentary rocks weather under cold and humid climatic conditions. The rivers of the Stikine Province in the Western Canadian Cordillera are dilute and their major feature is to present relative enrichments in base cations

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compared to sodium. The associated suspended sediments and sands transported at the bottom of the rivers do not present such enrichments. With an inversion technique, we tested different possibilities that can explain the origin of the Ca and Mg enrichment. The two extreme are that the excess Ca and Mg are due to carbonate dissolution or that Ca and Mg in excess are derived from the preferential weathering of calcic silicate minerals, most probably amphiboles. We showed that discriminating the different hypothesis is not easy without an extensive study of the fresh minerals involved in chemical weathering. A number of arguments tend to show however that the excess of Ca and Mg is related to the presence of carbonate minerals of probable hydrothermal origin. With these limitations in mind, we calculated chemical weathering rates of cations for the three main rivers of the Stikine Province: the Nass, the Skeena and the Stikine River. The numbers found (4 – 5 tons/km2/year) are entirely consistent with the global picture established recently for regions draining volcanic areas by previous authors (Louvat, 1997; Dessert et al., 2003), although deduced from different geodynamic settings. The weathering of the accreted terranes of the Stikine Province thus seems to follow the general picture established for basaltic rocks. This result is important because it shows that the volcaniclastic sedimentary rocks weather chemically about at the same rate as volcanic rocks, although they have been submitted to previous weathering cycles. The relatively low chemical denudation rates found in the Stikine Basin are mainly due to the low mean air temperature. At the scale of Canada, the comparison with our previous estimates of silicate chemical denudation rates (Millot et al., 2003) shows that the Stikine Province, due to its volcanic derived nature, weather about three times faster than the granitic provinces. The silicate denudation rates of the Stikine are therefore among the highest in Canada. It is more difficult to draw clear conclusions from this study on the long-term alkalinity production and CO2 consumption rates by the Stikine Province. This is essentially due to the abundance of sulfate ions in the river waters and our lack of constraints for their origin. If all sulfate ions present in the rivers are to be attributed to oxidative weathering of sulfides, then clearly no correlation will any longer exist between

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cationic denudation rates and CO2 consumption rates in such geological settings. Obviously, more work is to be done in that direction. We hope that this work will encourage similar studies in subduction and collision contexts in particular in order to confirm whether similar cations enrichments are observed, to identify their origin and to refine the atmospheric CO2 consumption budget of subduction/collision regions.

Acknowledgements S.P. Anderson and S.R. Gislason are thanked for constructive reviews and T. Horscroft for editorial handling. Caroline Gorge is thanked for major element analysis. We also thank Michel Valladon for ICP-MS measurements. D. Lemarchand and C. Dessert are acknowledged for their help in the field and for fruitful discussions. C. Gariepy from GEOTOP in Montre´al is greatly thanked for assistance in Canada and K. Telmar for helpful phychological information. This work was supported by the French program funded by the INSU-CNRS (PNSE contribution no. 344). RM benefited from a grant of the Ministe`re of Education Nationale. This is IPGP contribution no. 1915. [LW]

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