Silicate Versus Carbonate Weathering in the Himalaya: a Comparison of the Arun and Seti River Watersheds

Chemical Geology 202 (2003) 275–296 www.elsevier.com/locate/chemgeo

Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds

Jay Quade*, Nathan English, Peter G. DeCelles

Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA Received 29 October 2001; received in revised form 23 April 2002; accepted 15 May 2002

Abstract

We studied the water chemistry of two large and geologically differing Himalayan watersheds in order to maximize the contrast between silicate versus carbonate weathering effects on river chemistry. Our previous research involved the Seti River of westernmost Nepal, geologically typical of many rivers in Nepal in draining mixed carbonate/silicate lithologies, including abundant carbonate rocks of the Lesser Himalayan Sequence. For comparison, the Arun River was chosen for study because south of the Himalayan front it drains almost exclusively Greater and Lesser Himalayan silicate rocks. Despite this dominance of silicate rocks, carbonate weathering—probably of metamorphic calcite in Great Himalayan paragneisses—is clearly in evidence in many Arun watersheds. Weathering of silicate rocks exposed all along the Arun south of the range front has a small impact on mainstem river chemistry. The mainstems of both the Seti and Arun systems are dominated by weathering of carbonate rocks, although the contribution of silicate weathering is more visible in Arun mainstem chemistry. The carbonate weathering source to the Arun mainstem is probably both limestone in the Tethyan Sequence widely exposed in northern headwaters of the system, and metamorphic calcite within the Greater Himalayan Sequence. A number of small watersheds along the Arun and Seti appear to be carbonate-free. They probably provide the best constraints to date on the Ca/Na and Mg/Na ratios of waters draining Himalayan silicate rocks, two critical parameters for calculation of CO2 consumption by silicate weathering in the Himalaya. The observed Ca/Na and Mg/Na ratios would produce slightly higher estimates of silicate weathering fluxes than previous studies. The geologic contrasts between the Seti and the Arun produce large differences in the 87Sr/86Sr ratio of each mainstem, unlike the major element chemistry. The Seti mainstem displays much higher 87Sr/86Sr ratios than the Arun mainstem, the opposite of the expected relationship since radiogenic silicate rocks of the Greater and Lesser Himalaya are so widely exposed along the Arun. 87Sr/86Sr ratios of the Arun mainstem never exceed 0.734 and show little downstream change as the mainstem passes through silicate rocks of the Greater and Lesser Himalaya. 87Sr/86Sr ratios of the Seti mainstem increase sharply from 0.725 to 0.785 when the river enters the belt of metacarbonate rocks of the northern Lesser Himalayan Sequence, a pattern also displayed by other Himalayan rivers such as the Kali Gandaki and Bhotse Khola. Metacarbonate rocks, including those of the Lesser Himalaya, are a major source of radiogenic Sr in modern Himalayan Rivers and probably have been key players in elevating marine 87Sr/86Sr ratios since the Early Miocene. D 2003 Elsevier B.V. All rights reserved.

Keywords: Himalayas; Silicate weathering; Carbonate weathering; Strontium isotopes; Arun River; Seti River

* Corresponding author. E-mail address: [email protected] (J. Quade).

1. Introduction western and eastern Nepal (DeCelles et al., 2001).As a result, Lesser Himalayan carbonate rocks are nearly The Himalaya occupies center stage in the debate absent in the Arun drainage, whereas they are abun- over the role that large mountain uplift potentially dant in the Seti drainage, allowing us to isolate the plays in periodically cooling the Earth’s climate. contribution of Lesser Himalayan metacarbonate Weathering of Ca-silicates leads to net drawdown of weathering to riverine Sr. Moreover, the abundance atmospheric CO2, the rates of which are potentially of silicate rocks along the lower Arun watershed— intensified when large volumes of silicate rocks are unique among most large rivers of Nepal where exposed to chemical weathering during major oroge- carbonate rocks are usually exposed—permits the role ny. With the rise of the Himalaya starting in the Early of silicate weathering in determining Himalayan water Cenozoic, it has been suggested that climate cooled as chemistry to be rigorously evaluated. silicate weathering intensified, a process also thought to be reflected in the dramatic increase in marine 87Sr/86Sr ratios since the Late Eocene (Raymo et al., 2. Geologic setting 1988; Raymo and Ruddiman, 1992; Edmond, 1992). Until recently, weathering of silicate minerals in The Arun and Seti Rivers both begin north of the the high-grade metamorphic rocks of the Greater Himalayan ridge crest and flow south across the Himalayan Sequence was viewed as the main CO2 structural grain of the Himalaya, draining f 30,000 sink and the main source of high 87Sr/86Sr ratios in km2 and 5340 km2, respectively (Fig. 1). Both river Himalayan rivers (Raymo and Ruddiman, 1992; systems drain rocks of the Tethyan Sequence along Edmond, 1992; Krishnaswami et al., 1992). This view and north of the high Himalayan ridge crest. Impor- has been challenged recently by studies that attribute tantly, f 75% of the Arun watershed lies north of the most of the weathering flux as well as the elevated Himalayan crest, largely in Tethyan rocks, a higher 87Sr/86Sr ratios in Himalayan rivers to carbonate, not proportion than any other large Nepalese River. The silicate weathering (Palmer and Edmond, 1992; Tethyan Sequence is dominated by Phanerozoic car- Quade et al., 1997; Harris et al., 1998; Blum et al., bonate and siliclastic sedimentary rocks. Gneiss, calc- 1998; English et al., 2000; Karim and Veizer, 2000). silicates, and schist of the Greater Himalayan Se- The distinction is crucial because carbonate weather- quence underlie the middle reaches of the Arun and ing, unlike Ca-silicate weathering, entails no net, Seti drainages (Fig. 1). Along the Arun, paragneiss far long-term drawdown in atmospheric CO2. Disagree- outstrips orthogneiss. Previously viewed as uplifted ment persists, however, over the relative contributions Indian-plate basement, the Greater Himalayan high- to riverine 87Sr/86Sr ratios of carbonate versus silicate grade metamorphic rocks are now known to be part of weathering in the various Himalayan terranes. a much younger terrane that may have accreted onto In this paper, we combine geological and geochem- India during the Early Paleozoic (Parrish and Hodges, ical observations of the Arun drainage in eastern 1996; DeCelles et al., 2000). Both the Arun and Seti Nepal with the previously studied Seti River in Rivers pass out of the Greater Himalayan Sequence western Nepal (Fig. 1) in order to (1) contrast the and into the Lesser Himalayan Sequence south of the impact silicate versus carbonate weathering on major Main Central thrust (Fig. 1). The Lesser Himalayan element chemistry of Himalayan waters, and (2) to Sequence was deposited during the Early to Middle isolate the causes of the elevated 87Sr/86Sr ratios in Proterozoic (DeCelles et al., 2000) and is mainly Himalayan rivers as a whole. Both rivers cross the siliclastic in the lower half and limestone and dolo- same three major Himalayan tectonostratigraphic ter- stone (metamorphosed to lower greenschist grade) in ranes (Tethyan, Greater Himalayan, Lesser Himala- the upper half. Along the Seti River, these metacar- yan), and both rivers receive runoff from many bonate rocks are widely exposed (Fig. 1B; DeCelles et smaller, terrane-specific tributaries that allow us to al., 2001). Along the Arun, only the lower siliclastic define the typical weathering products from each strata of the Lesser Himalayan Sequence are exposed terrane. However, depth of erosion into thrust-stacked and the metacarbonate rocks are virtually absent (Fig. Himalayan terranes is dramatically different between 1A). Thus, the lower Arun watershed encompasses J. Quade et al. / Chemical Geology 202 (2003) 275–296 277

Fig. 1. 278 J. Quade et al. / Chemical Geology 202 (2003) 275–296 J. Quade et al. / Chemical Geology 202 (2003) 275–296 279

Table 1 Temperature, pH, and major element chemistry of Arun waters All mmol/l except TDS

Locality Distance T pH Ca Mg Na* K Si Cl SO4 HCO3 TDS (km) (jC) (mg/l) Arun River Arun above Pikuwa Khola 0.1 9.5 8.1 0.552 0.130 0.220 0.043 0.137 0.071 0.162 1.212 128.3 Arun above Khandapp Khola 4.2 14.5 8.0 0.562 0.151 0.268 0.054 0.137 0.076 0.164 1.175 129.0 Arun above Sabaya Khola 23.9 9.8 8.0 0.496 0.134 0.244 0.052 0.145 0.068 0.151 1.100 119.2 Arun above Yankuwa Khola 33.8 16.8 8.0 0.486 0.123 0.190 0.041 0.160 0.046 0.129 1.012 108.7 Arun below Piluwa Khola 37.0 12.2 8.3 0.687 0.185 0.291 0.077 0.161 0.057 0.253 1.337 154.1 Arun below Lekuwa Khola 51.1 18.9 8.1 0.450 0.109 0.176 0.042 0.151 0.100 0.126 0.987 107.8 Arun above Pikhuwa Khola 63.8 15.4 8.0 0.519 0.138 0.217 0.053 0.162 0.065 0.145 1.200 125.5 Arun above Ulleri Khola 71.3 10.9 7.7 0.461 0.094 0.275 0.035 0.157 0.045 0.126 1.075 111.7 Arun above Sun Kosi 77.5 15.3 8.1 0.514 0.150 0.275 0.056 0.154 0.063 0.158 1.012 116.7

Arun Tributaries Pikuwa Khola 0.1 14.3 7.6 0.090 < 0.022 0.187 0.038 0.160 0.024 0.019 0.312 36.9 Khandapp Khola 17.9 17.9 7.7 0.054 < 0.022 0.242 0.034 0.285 0.034 0.010 0.287 38.3 Sankuwa Khola 5.8 13.5 7.6 0.193 0.027 0.128 0.039 0.145 0.028 0.039 0.437 49.0 Irkuwa Khola 10.2 15.7 7.7 0.130 0.022 0.152 0.041 0.229 0.032 0.029 0.387 45.9 Chirkuwa Khola 12.7 16.3 7.7 0.105 0.046 0.215 0.045 0.285 0.043 0.032 0.387 48.9 Sabaya Khola 30.2 21.7 8.7 0.303 0.117 0.136 0.050 0.215 0.055 0.052 0.912 89.1 Yankuwa Khola 33.9 16.9 8.2 0.149 0.087 0.195 0.037 0.315 0.032 0.054 0.550 63.0 Piluwa Khola 41.4 12.0 7.8 0.167 0.067 0.157 0.052 0.270 0.038 0.070 0.525 61.9 Lekuwa Khola 48.0 18.9 8.1 0.539 0.171 0.222 0.101 0.253 0.056 0.112 1.387 139.2 Mahamatna Khola 52.0 17.8 8.6 0.774 0.484 0.423 0.114 0.254 0.106 0.281 2.312 236.0 Munga Khola 68.7 20.7 8.4 0.687 0.164 0.335 0.106 0.321 0.058 0.178 1.812 181.6 Ulleri Khola 72.7 12.3 8.5 0.667 0.311 0.452 0.138 0.389 0.072 0.585 1.281 198.2

Irkuwa Khola Irkuwa above Sisuwa Khola 16.4 14.6 7.5 0.141 0.022 0.109 0.029 0.196 0.022 0.040 0.331 40.9 Irkuwa above Phedi Khola 12.6 13.7 7.6 0.125 0.023 0.209 0.032 0.202 0.002 0.035 0.337 41.5 Irkuwa above Bankuwa 8.3 15.2 7.8 0.138 0.023 0.160 0.034 0.231 0.010 0.029 0.375 44.0 Irkuwa above Nakla Khola 6.4 15.1 7.7 0.120 < 0.022 0.163 < 0.025 0.222 0.024 0.030 0.387 44.3

Irkuwa Khola Tributaries Sisuwa Khola 16.3 – 7.3 0.046 < 0.022 0.134 < 0.025 0.230 0.013 0.012 0.206 27.8 Phedi Khola 12.5 15.6 7.6 0.170 0.027 0.184 0.032 0.238 0.022 0.028 0.419 49.1 Bankuwa Khola 8.1 – 7.4 0.077 < 0.022 0.123 < 0.025 0.188 0.022 0.025 0.275 33.4 Nakla Khola 6.3 14.2 7.9 0.115 0.040 0.254 0.033 0.430 0.035 0.015 0.581 63.3 Palung Granite (central Nepal) – – – 0.008 0.113 0.292 0.026 0.469 0.054 0.007 0.55 60.8 Na*= Na (Total)-Cl. silicate rocks to a far greater extent then any other The cause of the geologic contrast between the Seti large Himalayan river studied to date south of the and Arun watersheds is that the depth of erosion is Himalayan front. much shallower in eastern Nepal than previously

Fig. 1. Geology and sampling locations in study transects. Inset shows location of Arun, Seti, Kali Gandaki, and Bhotse Khola transect studies, as well as the Sun Kosi River. (A) The Arun River watershed. Sampling sites for tributaries (solid dots) and the mainstem (open circles). Geology of the Arun from Schelling (1992) and our own field observations. MCT—Main Central thrust; MBT—Main Boundary thrust; RT— Ramgarh thrust, and (B) The Seti River watershed. Geology modified from DeCelles et al. (2001). The main carbonate units in the watershed lie in the Lakarphata Group and Syangia, Galyang, and Sangram Formations of the Lesser Himalayan Sequence. The dark line between points A and B marks the sample transect fully described in English et al. (2000) and depicted in Fig. 8A. 280 J. Quade et al. / Chemical Geology 202 (2003) 275–296 studied drainages to the west, including the Seti. The treatment, since crushing could be shown to increase Lesser Himalayan Sequence is exposed along the the release of silicate Sr during even mild acid Arun in a large erosional window (the ‘‘Arun win- leaching (English et al., 2000). Sediments were treated dow’’) cut down through a thick thrust sheet of with 3% H2O2, 0.2 M ammonium acetate, a step overlying Greater Himalayan rocks (Fig. 1A). These shown to minimize acid leaching of the silicate Greater Himalayan rocks are only preserved as iso- fraction (after Montan˜ez et al., 1996), and then dis- lated klippen in central and western Nepal (Fig. 1B, solved with doubly distilled acetic acid to isolate the above the Dadeldhura Thrust). The stage of unroofing carbonate fraction, in the same way as carbonate in eastern Nepal is probably equivalent to depths of rocks. erosion attained in much of western Nepal during Major cation and Sr concentrations (henceforth Early to Middle Miocene time. [Sr]) for waters (Table 1) were determined by induc- tively coupled plasma (ICP-OES), and anion concentrations by ion chromatography. Strontium from 3. Methods waters, sediment and rock digests was separated with ion-specific resin and 87Sr/86Sr ratios were measured Reconnaissance mapping and sample collection on a Micromass Sector 54 thermal ionization mass encompassed the lowest f 70 km of the Arun main- spectrometer. The 86Sr/88Sr ratio was normalized to stem and was performed during a single 10-day foot 0.1194 and seven analyses of the NBS-987 standard traverse in January, 1999. Our sampling included run during the sample analyses yielded a mean ratio of mainstem and tributary waters, bedrock, river pebbles, 0.710267 F 0.000011 (2r). and sand and silt samples. The mainstem has already passed though diverse lithologies of the Tethyan and some Greater Himalayan rocks before arriving at our 4. Results and discussion highest sampling point. From this point below, all the large tributaries head in the Greater Himalayan Se- 4.1. Clast counts and petrography quence, then descend through a narrow strip of Lesser Himalayan silicate rocks before joining the Arun Clast counts of the Arun River bed load verify mainstem. We sampled in some detail along Irkhuwa what reconnaissance mapping (Schelling, 1992; our Khola and its small tributaries that drain only Greater unpublished data) had suggested, that the lower Arun Himalayan Sequence rocks (Fig. 1A). We also sam- watershed is underlain largely by silicate rocks de- pled the Sun Kosi above its confluence with the Arun, rived from the Greater Himalayan and Lesser Hima- which traverses a similar suite of silicate-rich rocks as layan Sequences, and that Lesser Himalayan carbon- the lower Arun. ate rocks are negligible. Gneissic clasts (Fig. 1A, Water samples were passed through a disposable largely Junbesi paragneiss, Schelling, 1992) compose 0.2 Am filter and collected in acid-washed polyethyl- 65% of the 1234 clasts counted in the Arun watershed ene bottles. River waters were titrated to determine (mainstem and tributaries), and Lesser Himalayan, total alkalinity in the field immediately after collection quartzite, phyllite, and orthogneiss (Tumlingtar using a HANNAk HC-9055 electronic pH meter. Group, Khare Phyllite, and Ulleri Augen Gneiss) The percentage of calcite in detrital sand samples compose most of the rest (Fig. 2). Petrographic was determined manometrically, which is accurate to examination of the paragneiss from outcrops shows F 0.05%. it to be dominated by feldspar and quartz (60–85%), Calcite seams in silicate rocks and marble were variable mixtures of biotite and muscovite (15–20%), handpicked to minimize any contamination by leach- and minor garnet, F minor sillimanite, kyanite, and ing of silicates during hydrolysis. These were crushed staurolite. Lesser Himalayan phyllite is composed and dissolved in doubly distilled 1 M acetic acid (e.g. mostly of quartz (50–70%), variable mixtures of Asahara et al., 1995), put in an ultrasonic bath for 30 chlorite, muscovite, and biotite (30–40%), and minor min and left to stand for 14 h at 20 jC. Most sediment plagioclase and garnet and other accessory minerals. samples were left uncrushed prior to chemical pre- Less than 2% of clasts proved calcareous, including J. Quade et al. / Chemical Geology 202 (2003) 275–296 281

Fig. 2. Alluvial clast counts for mainstems and tributaries in the Arun and Seti watersheds. Calc-silicates are present among alluvial clasts along the Seti, but we failed to distinguish them during our survey.

Greater Himalayan calc-gneiss dominated by epidote, wide zone of Lesser Himalayan rocks (Fig. 1B) that diopside, amphibole, and calcite, minor marble, and contribute abundant dolostone and limestone clasts to only four dolomite cobbles of probable Lesser Hima- the bed load of the river, both as cobbles and as fine layan derivation. detritus. Calcareous detritus (up to 1.3%) is present in the sand-size bed load of the Arun mainstem and some 4.2. Major element solute chemistry lower Arun tributaries (Table 2). Marble and epidote/calcite partings in gneiss from the Greater Total dissolved solids (TDS) are 128.3 mg/l in the Himalayan Sequence are the main source of this Arun mainstem once it enters the study area, varying carbonate, as it is virtually the only form carbonate downstream between 107.8 and 154.1 mg/l, and takes in the hundreds of bed load clasts we exam- ending up at 116.7 mg/l just above the confluence ined along the Arun (Fig. 3a–b). Carbonate was not with the Sun Kosi (Table 1). Most tributaries along found in the sand-size load of a number of small the lower Arun are more dilute (avg. TDS = 99 mg/l) watersheds such as the many sampled tributaries to than the mainstem. Only four tributaries (Maha- Irkhuwa Khola draining Greater Himalayan gneiss, matna, Lekuwa, Ulleri, and Munga), all contiguous Khandapp and Pikuwa Khola draining Lesser Hima- along the lower part of the watershed, show TDS layan augen gneiss, and some other Arun tributaries (181.6–236 mg/l) higher than the Arun mainstem, (Table 2). due mainly to much higher concentrations of Ca and In contrast, pebbles and cobbles derived from the Mg than the other Arun tributaries. These four Greater Himalayan Sequence are not abundant in the tributaries stand out as carrying abundant carbonate Seti watershed (English et al., 2000) (Fig. 2). Gneiss as cobbles or in sand-size detritus (Table 2); most composes f 6% of the 2820 clasts counted. The (but not all) of the other study tributaries have little Seti also contains a far higher proportion of carbon- or no bed load carbonate and show much lower TDS ate rocks in the coarse bed load ( f 25% of all (27.8–89.1 mg/l). Total dissolved solids along the mainstem clasts counted) as well as higher (6–27%) Seti mainstem (108.4–155.6 mg/l) are similar to that carbonate in the sand-sized detrital load. The stron- of the Arun mainstem, and Seti tributaries, like most gest contrast in bed load between the Arun and Seti Arun tributaries carrying detrital carbonate, display develops when the Seti passes through a f 50-km- higher TDS (68.5–287.6 mg/l) than those draining 282 J. Quade et al. / Chemical Geology 202 (2003) 275–296

Table 2 Strontium isotope results from Arun waters and carbonate detritus 87 86 87 86 Locality Downstream Sr (Amol/l) Sr/ Sr Sr/ Sr in CaCO3 of Pikuwa water water sand-size (%) Khola (km) carbonate detritus Arun River Arun above Pikuwa Khola 0.0 1.01 0.72422 0.72996 0.2 Arun above Khandapp Khola 4.2 1.07 0.72385 0.73838 0.5 Arun above Sabaya Khola 23.9 1.16 0.72597 0.72355 1.3 Arun above Yankuwa Khola 33.8 0.81 0.72632 0.77713 0.2 Arun below Piluwa Khola 37.0 1.10 0.73422 0.72857 1.0 Arun below Lekuwa Khola 51.1 0.88 0.72894 0.72743 0.6 Arun above Pikhuwa Khola 63.8 0.96 0.73173 – 0 Arun above Ulleri Khola 71.3 0.88 0.72946 0.72387 0.5 Arun above Sun Kosi 77.5 0.85 NA 0.72809 0.5

Arun Tributaries Pikuwa Khola 0.1 0.10 1.02408 – 0 Khandapp Khola 4.3 0.09 1.06374 – 0 Sankuwa Khola 5.8 0.35 0.74262 – < 0.05 Irkuwa Khola 10.2 0.35 0.74155 – < 0.05 Chirkuwa Khola 12.7 0.38 0.75536 – 0 Sabaya Khola 30.2 0.42 0.77933 – < 0.05 Yankuwa Khola 33.9 0.25 0.76864 – 0 Piluwa Khola 41.4 0.41 0.76116 0.73812 0.7 Lekuwa Khola 48.0 0.66 0.76206 0.76916 0.1 Mahamatna Khola 52.0 0.88 0.77292 0.78829 0.3 Munga Khola 68.7 1.65 0.72768 – 0 Ulleri Khola 72.7 1.19 0.74708 – 0

Irkuwa Khola Irkuwa above Sisuwa Khola 16.4 0.20 NA – < 0.05 Irkuwa above Phedi Khola 12.6 0.21 0.73058 – 0 Irkuwa above Bankuwa Khola 8.3 0.25 0.73004 – 0 Irkuwa above Nakla Khola 6.4 0.31 0.73348 – 0

Irkuwa Khola Tributaries Sisuwa Khola 16.3 0.16 NA – 0 Phedi Khola 12.5 0.27 0.72710 – 0 Bankuwa Khola 8.1 0.22 0.74228 – 0 Nakla Khola 6.3 0.34 0.74604 – 0

purely silicate rocks (29.4–63.4 mg/l) (English et al., contrasts hold for most of the sampled tributaries of 2000). the two systems. There are systematic major element differences The cause underlying these differences can be between the mainstem waters of the Arun and Seti tied directly to geological substrate, best illustrated mainstems, although both are Ca–HCO3-dominated by a comparison of solute chemistry of lithologically (Figs. 4 and 5). In general, the proportion of Ca + Mg/ specific watersheds both within and outside the Na + K tends to be lower along the Arun mainstem study area. In this comparison, we used the major (Fig. 4A) compared to the Seti (Fig. 4B). Similarly, element chemistry of small springs draining purely the ratio of HCO3/Si is lower in the Arun mainstem Paleozoic carbonates in the western USA to define (Fig. 5A) compared to the Seti (Fig. 5B). The same end-member carbonate weathering (McKinley et al., J. Quade et al. / Chemical Geology 202 (2003) 275–296 283

Fig. 3. (a–b) Photos and photomicrographs of calcitic partings in Greater Himalayan paragneiss along the Arun, (a) in outcrop, with more rapid dissolution of calcite producing the recesses on the weathered clast surface (knife f 9 cm), (b) photomicrograph of a calcite vein ( f 3mm wide) from Irkhuwa Khola and high-relief epidote phenocrysts on the vein margin. This calcite displays moderate (0.7121) to high (0.8976) 87Sr/86Sr ratios, based on 10 analyses from clasts all over the watershed.

1991). For end-member silicate weathering, we used 1967) as well as that of a small spring draining the the solute chemistry of ephemeral springs draining Ordovician-age Palung granite in central Nepal the Sierra Nevada batholith (Garrels and MacKenzie, (Figs. 4–6). 284 J. Quade et al. / Chemical Geology 202 (2003) 275–296

Fig. 4. Mole fractions of major cations for the (A) Arun, and (B) Seti. Also shown for comparison are the same mole fractions of springs draining purely silicate rocks in the Sierras and the Palung Granite in central Nepal (see Table 1), and draining purely carbonate rocks in southern Nevada. Note that the Seti mainstem plots closer to the carbonate end-member than the mainstem of the Arun, although both mainstems are still dominated by carbonate weathering. J. Quade et al. / Chemical Geology 202 (2003) 275–296 285

Fig. 5. Mole fractions of Si–HCO3 –SO4 + Cl for the (A) Arun, and (B) Seti. Also shown for comparison are the mole fractions of springs draining purely silicate rocks in the Sierras and the Palung Granite in central Nepal (see Table 1), and draining purely carbonate rocks in southern Nevada. Note that the Seti mainstem plots closer to the carbonate end-member than the mainstem of the Arun, although both mainstems are still dominated by carbonate weathering. 286 J. Quade et al. / Chemical Geology 202 (2003) 275–296

Five Arun tributaries (Mahamatna, Lekuwa, Mu- plausible source of the carbonate-dominated main- nga, Irkhuwa, and Sabaya) and 15 of 19 Seti stem chemistry, a pattern consistent with other large tributaries plot toward the carbonate weathering Nepalese rivers with large parts of their watersheds end-member (Figs. 4–6). This includes tributaries in the Tethys Himalaya (Galy and France-Lanord, draining all the major Himalayan terranes, including 1999). Some weathering contribution from carbo- the Greater Himalayan Sequence. The shared litho- nates and silicates in the Greater Himalayan Se- logic feature of nearly all these tributaries is that they quence higher in the drainage is also likely. The carry moderate to abundant detrital carbonate. This exact proportions will only be established by sam- comes in the form of limestone in the Tethys pling of the rugged upper reaches of the Arun Himalaya, metamorphic calcite and marble in the drainage. Greater Himalaya, and limestone and dolostone in The comparison of water chemistry of the Arun the Lesser Himalaya. and Seti to rivers in the rest of the Himalaya yields Fourteen of twenty Arun/Irhkuwa tributaries (Figs. some interesting patterns. In general, large Nepalese 4A and 5A) and four of nineteen Seti tributaries (Figs. rivers west of Kathmandu such as the Narayanya and 4B and 5B) plot on or close to the silicate weathering Karnali (our data and from Galy and France-Lanord, end-member. The drainages with no detectable car- 1999) fall close to the Seti on our ternary diagrams bonate in the bed load tend to show the lowest (Fig. 6), whereas the Sun Kosi, the only other major Ca + Mg/Na + K and HCO3/Si proportions. Along the river in eastern Nepal, falls on the Arun mainstem Arun, these include Khandapp and Pikuwa Khola values. The Arun and the Seti may therefore be draining Lesser Himalayan Khandbari augen gneiss thought of as end-members for the geochemical and several of the small tributaries of the Irhkuwa continuum of Nepalese Himalayan rivers. Evident Khola draining the Junbesi paragneiss of the Greater from the ternary diagrams is that more silicate Himalayan Sequence. Along the Seti, three small weathering occurs along the Arun and Sun Kosi creeks draining the Buri Gandaki and Sani Gad than in any other large rivers in Nepal, although gneisses in the frontal Dadeldhura Thrust sheet were carbonate weathering still dominates the overall free of carbonate and plot on or near end-member mainstem chemistry. The Ganges, Brahmaputra and silicate weathering. The range of Ca + Na/Si ratios of Indus also plot between the Arun and the Seti on all these waters draining apparently carbonate-free ternary plots, suggesting a similar mix of Arun- and silicate rocks is consistent with ratios produced from Seti-style rivers determine the major element chem- alteration of albite/oligoclase to a mix of smectite/ istry of the largest rivers (Fig. 6). kaolinite. The geochemical contrasts between rivers in east- The solute chemistry of the Seti mainstem plots ern versus western Nepal make sense geologically. In close to the carbonate weathering end-member, as do western Nepal, carbonates of the Lesser Himalayan most of its tributaries, consistent with the varied and Sequence are much more widely exposed in western abundant carbonate found in that area and in much than eastern Nepal. Silicate rocks above the Main of Nepal west of Kathmandu. By comparison the Central thrust and its frontal equivalents such as the Arun mainstem displays uniformly lower Ca + Mg/ Dadeldhura Thrust still cover the underlying Lesser Na + K and HCO3/Si proportions than the Seti main- Himalaya Sequence in eastern Nepal, whereas the stem, although the proportions are still consistent same silicate rocks only survive as isolated klippen with dominantly carbonate weathering (Figs. 4 and in the west. 5).ItisclearfromFigs. 4A and 5A that the We can use the geochemical constraints imposed weathering of silicates in the lower Arun tributaries by our silicate- and carbonates-specific end-members makes little contribution to mainstem chemistry over to quantify the fraction silicate weathering in these the final 70 km reach of the mainstem, and that Arun drainages. The calculation requires an estimate of the mainstem chemistry is dominated by weathering of Ca/Nasil and Mg/Nasil of waters derived from pure carbonates exposed higher in the watershed. As silicate weathering. For this we used the Seti and Arun f 75% of the drainage lies in the carbonate-rich tributaries in which no carbonate was detected in Tethyan Sedimentary Sequence, this is the most detrital sand, in clast surveys, or in thin-section. These J. Quade et al. / Chemical Geology 202 (2003) 275–296 287

Fig. 6. Mole fractions of major cations for the major Himalayan rivers. Also shown for comparison are the mole fractions of springs draining purely silicate rocks in the Sierras and the Palung Granite in central Nepal (see Table 1), and draining purely carbonate rocks in southern Nevada. Note that eastern Nepal rivers (Arun and Sun Kosi) plot together, as do western Nepalese rivers (the Seti, Karnali, and Narayanya). The Ganges, Brahmaputra (Galy and France-Lanord, 1999) and Indus (Karim and Veizer, 2000) plot between the clusters of eastern and western Nepalese rivers, and are monsoon season values only. also turn out to be the waters with the highest Na + K/ where Na* = Natotal Cl (corrects for Na from aero- Ca + Mg (Figs. 4A and 5A). These include Khandapp, sols and evaporates) Pikuwa, Sisuwa, Nakla, and Chirkhuwa Kholas on the Arun, Bheri, Kali, Rori Gad on the Seti (English et al., Na* ¼ Nasilði:e: remaining Na comes from silicatesÞ 2000), and a single spring draining the Palung Granite of Central Nepal. From these we obtain (Ca/ ð2Þ Na)sil =0.41F 0.18 and (Mg/Na)sil =0.24F 0.10. The Mg/Nasil should be regarded as a maximum Ksil ¼ Ktotalði:e: all K from silicatesÞð3Þ estimate, as [Mg] content of some Arun tributaries used in this calculation was below the analytical detection limit, 0.5 ppm (Table 1). But since the Ca/ Casil ¼ðCa=NaÞsil Nasil; where ðCa=NaÞsil Mg ratio of most Arun and Seti waters is 2–5, this ¼ 0:41F0:18 ð4Þ uncertainty should have a small effect on our estima- tion of fsil below. We calculate the fraction silicate contribution on an Mgsil ¼ðMg=NaÞsil Nasil; where ðMg=NaÞsil equivalent basis (modified from Galy and France- ¼ 0:24F0:10 ð5Þ Lanord, 1999) using the averages of these ratios as follows: Substitute Eqs. (2)–(5) into Eq. (1): Fraction silicates

¼ fsil ¼ð2Ca þ 2Mg þ K þ Na*Þsilicates fsil ¼ðKsil þ 2:24Na*Þ

=ð2Ca þ 2Mg þ K þ Na*Þtotal ð1Þ =ð2Catotal þ 2Mgtotal þ Ksil þ Na*Þð6Þ 288 J. Quade et al. / Chemical Geology 202 (2003) 275–296

We use the average value of 2.24 in Eq. (6) in our calculation of fsil, and as the basis for discussions below. Use of the average assumes that the same mix of silicate lithologies in our sample population is representative of the Himalaya as a whole. But the large range of values is an important issue for calculating uncertainties in rates of silicate weathering for the Himalaya. The wide range in Ca/Nasil and Mg/Nasil is present (1) because some of our ‘‘silicate end-member’’ drainages are in fact not carbonate- free, as implied by the studies of White et al. (1999) and Blum et al. (1998) for other crystalline silicate terranes, in which case the fraction silicates calculated from Eq. (6) is maximum, or (2) because of heterogeneity in silicates with respect to Ca/Nasil (mainly determined by elemental variations and weathering proportions of plagioclase and epidote) and Mg/Nasil (mainly determined by weathering proportions of garnet + chlorite + biotite versus pla- Fig. 7. The fsil versus %CaCO3 of sand-sized bed load in the gioclase) ratios, since waters draining both the Lesser mainstems and tributaries and the Arun and Seti Rivers. fsil>1 an and Greater Himalayan silicates are represented in artifact of using average rather than the lowest observed values for Ca/Na and Mg/Na (see text) of weathering end-members. our ratio averages. sil sil In general, the fsil is very low along the Seti mainstem and evolves from 0.013 near the headwaters 4.3. Strontium isotope patterns in water, sediment, in the Tethyan Himalaya to 0.069 within silicate rocks and rock of the Lesser Himalaya just above the confluence with the Karnali (Fig. 1B). The Arun mainstem shows a Mainstem waters of the Seti and Arun show very 87 86 much higher fsil (0.334–0.434) and no systematic different Sr/ Sr ratios and downstream patterns of downstream variation. Not surprisingly, fsil is inverse- change. Above the Main Central thrust, the Seti ly correlated with the percentage of carbonate in the mainstem displays low 87Sr/86Sr (0.72–0.73) but high bed load (Fig. 7), but the relationship in nonlinear, Sr. Below the Main Central thrust, the mainstem showing that that a very small fraction of carbonate in composition shifts towards higher 87Sr/86Sr ratios of the system produces a large reduction in fsil. 0.77–0.78, whereas [Sr] decreases slightly (Fig. 8A). Our calculated fsil of the Ganges, Brahmaputra and In contrast, the Arun mainstem shows very little Indus (our data, Galy and France-Lanord, 1999, and downstream change in 87Sr/86Sr ratios or concentra- Karim and Veizer, 2000) is lower in the monsoon tions (Fig. 8B), and 87Sr/86Sr ratios of the Arun are period (0.146–0.192) and higher in the non-rainy considerably lower (0.7242–0.7317) than those of the season (0.361), probably the result of a lower water/ Seti below the Main Central thrust. rock ratio experienced by average river base flow We approach the interpretation of these results in compared to high runoff. Our calculated fsil is some- the same way that we did the major element chemistry what higher than estimates made by Galy and France- of the rivers, by first looking at the Sr isotopic Lanord (1999) using their own estimated ratios for Ca/ characteristics of a few lithologically well-defined, Nasil (0.2 F ?), and Mg/Ksil (0.5 F 0.25) in place of terrane-specific tributaries in each watershed. Mg/Nasil of silicate rocks. We would emphasize the large uncertainties in our Ca/Nasil and Mg/Nasil, even 4.4. Carbonate weathering end-member though we are using what may be the most carefully collected sample set for silicate-dominated watersheds A number of drainages and one spring drain largely in the Himalaya. carbonate rocks along the Seti; these are typified by J. Quade et al. / Chemical Geology 202 (2003) 275–296 289

as marbles and calc-silicates. We found no drainages that are overwhelmingly carbonate in this terrane, especially along the Arun, so this carbonate end- member is harder to define with confidence. Ghad, Ramus, and Gadpai Gad (Seti tributaries) all carry significant (10–12%) detrital metamorphic carbonate and may come closest to defining waters typical of this group at 0.7151–0.7458 for 87Sr/86Sr and 0.16– 0.56 Amol/1 for [Sr]. These patterns in water chemistry can be ex- plained by the 87Sr/86Sr ratios of carbonate rocks

Table 3 Strontium isotope results from Arun River carbonate Sample no. Locality Rocktypea87Sr/86Sr Arun River AR-4Arck Arun above Pikuwa Khola Mbl 0.70660 AR-4Brck Arun above Pikuwa Khola CS 0.83030 AR-15Arck Arun above Khandapp Khola CS 0.72296 AR-15Brck Arun above Khandapp Khola CS 0.77473 AR-15Crck Arun above Khandapp Khola Mbl 0.70571 AR-11Arck Arun above Sabaya Khola CS 0.89763 AR-11Brck Arun above Sabaya Khola Dolo 0.79401 AR-11Crck Arun above Sabaya Khola Ls 0.72086 AR-28Brck Arun above Yankuwa Khola Dolo 0.73145 AR-17Arck Arun above Pikuwa Khola CS 0.82610 AR-17Brck Arun above Pikuwa Khola Ls 0.73886 87 86 Fig. 8. (A) Downstream evolution in Sr/ Srwater ratios of the Seti River mainstem (data from English et al., 2000). Location of Irkuwa Khola transect given in Fig. 1B. The marked increase in mainstem AR-20Arck Irkuwa above Nakla Khola CS 0.72872 87 86 Sr/ Srwater ratios coincides with inflow of tributaries underlain by AR-20Brck Irkuwa above Nakla Khola Mbl 0.70938 Lesser Himalayan metacarbonates. (B) Downstream evolution in AR-22Arck Irkuwa above Bankuwa Mbl 0.71316 87 86 Sr/ Srwater ratios of the Arun River mainstem. Location of AR-22Brck Irkuwa above Bankuwa CS 0.71211 87 86 transect given in Fig. 1A. Sr/ Sr ratios show little change, and AR-23Arck Irkuwa above Phedi Khola CS 0.71962 Lesser Himalayan metacarbonates are nearly absent in the water- AR-23Brck Irkuwa above Phedi Khola CS 0.71927 shed. Solid triangles are from the Arun mainstem, whereas solid diamonds are from the Irkuwa Khola drainage (see Fig. 1A). Arun Tributaries AR-13Arck Irkuwa Khola Silicate 0.73243 AR-13Brck Irkuwa Khola Mbl 0.71582 87 86 high [Sr] but variable Sr/ Sr. The high [Sr] is AR-13Crck Irkuwa Khola Mbl 0.70577 consistent with high weathering rates of carbonate AR-10Arck Sabaya Khola Mbl 0.70578 rocks in general. The variability of 87Sr/86Sr follows AR-10Brck Sabaya Khola ? 0.90651 that of silicates (although absolute ratios differ; see AR-10Crck Sabaya Khola Dolo 0.80096 AR-27rck Yankuwa Khola Dolo 0.73288 below) and breaks out systematically according to AR-7Arck Lekuwa Khola Mbl 0.70674 terrane. On the one hand, streams draining abundant AR-7Brck Lekuwa Khola Mbl 0.70734 carbonate rocks in the Tethyan Himalaya along the Seti AR-5Arck Mahatmana Khola Mbl 0.70696 yield low 87Sr/86Sr (0.72–0.73) and high [Sr] (0.89– AR-5Brck Mahatmana Khola Mbl 0.70612 1.75 Amol/1). At the other extreme, some drainages in AR-3Arck Munga Khola Mbl 0.70667 AR-3Brck Munga Khola Mbl 0.70565 Lesser Himalayan dolostone and limestone yield mod- AR-2Arck Ulleri Khola Ls 0.73083 erate [Sr] (0.16–0.84 Amol/1), but much higher AR-2Brck Ulleri Khola CS 0.72530 87 86 Sr/ Sr (0.800–1.05). As already noted, carbonate a Mbl = marble, Ls = limestone, Dolo = dolostone, CS = calc- can be abundant in the Greater Himalayan Sequence schist/calc-gneiss. 290 J. Quade et al. / Chemical Geology 202 (2003) 275–296

87 86 analyzed from each of the major terranes (Fig. 9A; gneisses ( Sr/ Sraverage = 0.7428), virtually the only Table 3). Tethyan Sequence carbonate rocks were not carbonate-bearing clasts in the lower Arun water- measured in the Arun, but in the Seti drainage they shed. Detrital carbonate in the Seti mainstem is much yield 87Sr/86Sr ratios of 0.7135–0.7221 (English et more variable in 87Sr/86Sr, and is dominated by al., 2000). Calc-gneiss and marble are common in Tethyan carbonates at higher elevation, Greater Hi- the Greater Himalayan Sequence of both watersheds. malayan calc-silicates and marble just above the These rocks display a wide range of 87Sr/86Sr ratios Main Central thrust, and increasing dolostone below (0.7056–0.8976) in the Arun watershed. Marble the Main Central thrust. from the Greater Himalayan Sequence has the lowest ratios (0.7056 to 0.7132; mean = 0.7078; n = 13), 4.5. Silicate weathering end-member whereas calcite bands in Greater Himalayan paragneiss have higher ratios (0.7121–0.8976; mean =0.7627; Khandapp and Pikuwa Kholas along the Arun and n = 10) along the Arun. Along the Seti River, carbonate Talkoti Gad and Unamed Khola along the Seti all in the Greater Himalayan Sequence yielded 87Sr/86Sr appear carbonate-free and drain exclusively silicate ratios of 0.7138–0.7240 (English et al., 2000). The rocks of the Lesser Himalayan Sequence. They only four Lesser Himalayan dolostone clasts encoun- yielded extremely low [Sr] (0.1 Amol/1), consistent tered along the Arun returned high 87Sr/86Sr ratios with the low weathering rates of silicate minerals (0.7315–0.8010; mean = 0.7648; n = 4), like the dolo- compared to carbonates. These drainages also dis- stones of the Lesser Himalayan Sequence along the played very high 87Sr/86Sr ratios of 0.912–1.06 (gen- Seti River (English et al., 2000: 0.7158–0.8208; erally the highest of all the drainages), consistent with mean = 0.7540, n = 12). the very high whole-rock 87Sr/86Sr ratios (0.832– Sand-size carbonate detritus from terrane-specific 1.498) of Lesser Himalayan silicate rocks reported watersheds of both the Arun and Seti display the by France-Lanord and LeFort (1988), a result of the same 87Sr/86Sr patterns as the clasts (Table 2; Fig. great antiquity (>1.8 Ga) and high Rb content of these 9B), with Tethys-specific drainages carrying the least rocks. 87 86 radiogenic detritus ( Sr/ Sraverage = 0.7168) and Tributaries draining silicate rocks of the Greater Lesser Himalayan drainages the most radiogenic Himalaya and frontal crystalline nappes yield very 87 86 87 86 ( Sr/ Sraverage = 0.7767; Fig. 9B). Sr/ Sr ratios low [Sr] (0.05–0.34 Amol/1), similar to drainages in of most carbonate in the sands from the Arun Lesser Himalayan silicates, whereas 87Sr/86Sr ratios mainstem falls between 0.72 and 0.73 and is calcitic. are much lower (0.7315–0.7460). This comparison This calcite probably derives from some combination includes Sisuwa and Nakla Kholas along the Arun and 87 86 Tethyan limestones ( Sr/ Sraverage = 0.7168), and Bheri, Rori, Kali Gad along the Seti (English et al., the marbles and calcite bands in Greater Himalayan 2000). To this we can add a small spring from central

87 86 Fig. 9. Sr/ Srcarbonate ratios from the Seti and Arun drainages sorted by terrane of (a) alluvial class and bedrock, and (b) sand-size river detritus. Arrows denote averages for each group. J. Quade et al. / Chemical Geology 202 (2003) 275–296 291

Nepal draining the Palung granite of 0.7296. Our rock detritus in even trace amounts has been shown to 87 86 Sr/ Srwater results from watersheds (listed above) dominate local river Sr chemistry (e.g. Drever and draining Greater Himalayan gneisses overlap the low Hurcomb, 1986; White et al., 1999; Jacobson and end of the range of reported whole-silicate rock Blum, 2000). 87Sr/86Sr ratios (0.714–1.11; Kai, 1981; France-Lan- Mixing diagrams best define differing Sr end- ord and LeFort, 1988) of Greater Himalayan gneisses. members in the Arun and Seti systems, and indicate the main sources of Sr for the mainstem rivers (Fig. 4.6. Mixing patterns 10). The terrane-specific tributaries of the Seti identify three main contributors to mainstem Sr: Tethyan Having defined the end-members, previous studies limestone and Greater Himalayan calc-gneiss with (e.g. Singh et al., 1998; Galy et al., 1999) of Hima- low 87Sr/86Sr (0.72–0.73) and high [Sr] (0.15–2 layan water chemistry have at this point in their Amol/1), Lesser Himalayan metacarbonate rocks with analysis attempted to calculate the proportion that very high 87Sr/86Sr (0.800–1.05) and moderate to silicate versus carbonate weathering makes to Sr in high [Sr] (0.15–0.5 Amol/1), and Lesser Himalayan Himalayan rivers using the Sr/Ca ratios of silicate and silicate rocks with high 87Sr/86Sr (0.750–0.95) and carbonate end-members. The approach is similar to very low [Sr] (0.1 Amol/1) (Fig. 10B). Above the that used for major elements, and requires the as- Main Central thrust, the Seti mainstem displays low sumption of conservation of Sr/Ca ratios in river 87Sr/86Sr (0.72–0.73) but high [Sr] and overlaps the waters after weathering occurs. We point out in composition of the local tributary rivers draining the English et al. (2000) that this assumption of conser- Tethyan and Greater Himalayan rocks. Below the vation of Sr/Ca is probably seriously violated by Main Central thrust, the mainstem composition shifts preferential uptake of Ca compared to Sr during towards higher 87Sr/86Sr ratios of 0.76–0.78, whereas secondary calcite formation along ground-water flow [Sr] decreases slightly, as tributaries flowing from paths. The relative contributions of carbonate to Lesser Himalayan metacarbonates join the mainstem. silicate weathering to the Sr budget of Himalayan For the lower Arun watershed, the Sr-water chem- rivers can’t be estimated until this effect is better istry of terrane-specific tributaries plotted on mixing understood. As is, this approach yields maximum diagrams show that Sr sources simplify to two end- estimates of the fraction of Sr from silicate weather- members: Greater Himalayan silicates and calc-sili- ing, as noted by Galy et al. (1999). cates with lower 87Sr/86Sr (0.73–0.77) but high [Sr] Although the exact proportion of silicate to car- (0.25–1.0 Amol/l), and Lesser Himalayan silicates bonate weathering contribution to Sr cannot be esti- with very high 87Sr/86Sr (1.02–1.06) but very low mated, the high [Sr] alone in the Arun (Table 2;0.81– [Sr] (0.1 Amol/l) (Fig. 10A). The Arun mainstem falls 1.16 Amol/l) and Seti (0.38–1.38 Amol/l) are most outside the field defined by nearly all the points in consistent with our terrane-specific tributaries domi- these two end-members, displaying higher [Sr] (0.81– nated by carbonate-rock weathering (avg. [Sr] = 0.5 1.01 Amol/l) and lower to comparable 87Sr/86Sr Amol/l; range 0.16–1.74) than by silicate weathering (0.72–0.73). This is the same tributary–mainstem (avg. Sr = 0.20 Amol/l, range 0.11–0.34). The consis- relationship displayed by major element chemistry, tently high [Sr] in the Arun mainstem is much higher suggesting that the mainstem Sr chemistry is deter- than concentrations in all but three tributaries (Maha- mined by yet a third source upstream. Again, we matna, Munga, and Ulleri), and best matches [Sr] in speculate (and cannot test without further sampling) waters flowing out of Tethyan limestone (0.89–1.74 that this source is mostly carbonates of the Tethyan Amol/l) along the Seti. Carbonate minerals are far Sedimentary Series mixed with some weathering of more susceptible to chemical breakdown than silicate metamorphic calcite and silicates in the Greater Hi- minerals (e.g. Lasaga et al., 1994), as locally reflected malayan Sequence. We suggest this based on the (1) by the strong preferential etching in calcite-rich bands the fact that Tethys sediments underlie f 3/4 of the in gneissic cobbles (Fig. 3A), and the deep leaching of Arun drainage, (2) the major element evidence for a carbonate in modern Arun and Seti watershed soils dominantly carbonate weathering source for the main- (based on our observations). Weathering of carbonate stem, and (3) the moderate 87Sr/86Sr and high [Sr] 292 J. Quade et al. / Chemical Geology 202 (2003) 275–296

87 86 Fig. 10. Sr/ Srwater versus 1/Sr (in 1/Amol/L) for the (A) Arun River system and (B) the Seti River system (data from English et al., 2000). 87 86 Boxes enclose mainstem values and arrow denotes the downstream evolution of Seti mainstem Sr/ Srwater. Tethyan carbonate values were measured only along the Seti. typical of waters draining the Tethyan Sedimentary sediments and in some local calc-silicate rocks (Fig. Series (Fig. 10A). This explanation would also fit with 10). Below the Main Central thrust, however, the two 87 86 other major rivers in the Nepal that head in the rivers differ sharply in Sr/ Srwater, a reflection, we Tethyan Himalaya (Galy et al., 1999; English et al., suggest, of the presence or absence of Lesser Hima- 2000). layan carbonates. The Arun mainstem shows little 87 86 In summary, above the Main Central thrust, both change in Sr/ Srwater downstream (Fig. 8B), from 87 86 the Seti and Arun Rivers yield Sr/ Srwater of 0.72– 0.724 to 0.729, whereas [Sr] decreases. In contrast, 87 86 87 86 0.73, overlapping the Sr/ Sr of calcite in Tethyan the Seti River displays a sharp jump in Sr/ Srwater J. Quade et al. / Chemical Geology 202 (2003) 275–296 293 ratios (Fig. 8A) but a slight decrease in [Sr] coinciding to riverine Sr is potentially important in the Himalaya with the point where large tributaries sourced in but remains unquantified. Lesser Himalayan carbonate rocks enter the river.

4.7. Comparison of 87Sr/ 86Sr to other Himalayan 5. Broader implications rivers 5.1. Major element chemistry How does the Sr chemistry of the Arun and Seti compare other Himalayan rivers in Nepal? We are not Ostensibly, the Arun watershed was a good choice aware of a large system quite like the Arun that has for studying the effects of silicate weathering on been previously studied in the Himalaya, perhaps Himalayan river chemistry: south of the range front, because other river studies have been around or west both the Sun Kosi and the Arun pass through the most of Kathmandu where Lesser Himalayan metacarbon- continuous expanses of silicate rocks of any large ate rocks are widely exposed. The Seti belongs to this rivers in Nepal. Despite the contribution of weathering category, as do Bhotse Khola studied by Harris et al. from these silicates, the dissolved load of the main- (1998) and the Kali Gandaki by Galy et al. (1999). stems is still dominated by carbonate weathering. The Seti, Bhotse, and Kali Gandaki rivers all show Galy and France-Lanord (1999) make the important sharp increases in 87Sr/86Sr as they pass below the observation that large Himalayan rivers that head Main Central thrust and into Lesser Himalayan car- within the Tethyan Himalaya (such as the Karnali bonate rocks, suggesting that the Seti River results may and Narayanya) tend to display lower fsil than other be representative of weathering reactions in that re- large Nepalese rivers (such as the Bheri, Rapti) which gion. These increases are from 0.725 to 0.785 along the only drain the frontal areas of the Himalaya. This Seti River (English et al., 2000); from 0.719 to 0.768 reflects, as they point out, the large contribution that along Bhotse Khola (Harris et al., 1998); and from carbonate weathering of Tethyan carbonate rocks 0.717 to 0.748 along Kali Gandaki (Galy et al., 1999). make to the dissolved load of the largest Himalayan The greater contribution of carbonate weathering in rivers. This contrast is even more true in eastern Nepal these three rivers compared to the Arun is illustrated by along the Arun and probably the Sun Kosi. Carbonate silicate weathering fractions below 0.16 along all of weathering is much more evident in the chemistry of them. Lesser Himalayan carbonates are common along the mainstem than in nearly all of the tributaries we the Kali Gandaki, but Galy et al. (1999) attributed the measured south of the Himalayan front. rise in mainstem 87Sr/86Sr ratios to weathering of The contribution of silicate weathering along the Lesser Himalayan silicate minerals. Our Seti water- Arun south of the Himalayan crest is visible but shed studies (English et al., 2000) show that weather- modest, probably due to several factors. First, even ing of Lesser Himalayan silicates has little impact on though most of the watershed lies north of the range mainstem chemistry, nor is the impact of Lesser crest, it is much wetter south of the crest. The ratio of Himalayan silicates evident along the lower Arun. mean annual rainfall between our study area along the Thus, we view the 87Sr/86Sr increases along the Kali Arun and Lhasa, Tibet is f 4:1. However, most of Gandaki below the Main Central thrust as originating the drainages south of the front are more dilute than from weathering of Lesser Himalayan carbonates. the mainstem. And weathering of small amounts of Alternatively, Evans et al. (2001) have identified a metamorphic calcite influences water chemistry in 87 86 hydrothermal contribution to the rise in Sr/ Srwater many watersheds draining the silicate rocks south of as the Marsayandi River passes the Main Central the range front. Elsewhere, the influence of trace thrust in central Nepal. Springs, both hydrothermal calcite on the water chemistry of rivers draining other and otherwise, probably contribute to all major Hi- dominantly crystalline terranes (Drever and Hurcomb, malayan Rivers, although the elevated Cl concentra- 1986; Anderson et al., 1997, 2000; White et al., 1999) tions associated with thermal inputs are not in has been amply demonstrated, especially in areas with evidence along the Arun nor Seti near the Main high physical erosion rates such as the Himalayas. We Central thrust. The overall hydrothermal contribution can now demonstrate the influence of weathering of 294 J. Quade et al. / Chemical Geology 202 (2003) 275–296 metamorphic calcite over a much larger extent of ences between the Greater and Lesser Himalayan Himalayan ‘‘crystalline’’ rocks, especially among the Sequence. In general, neither silicates nor carbonates paragneisses. in the Greater Himalayan Sequence are as radiogenic Harris et al. (1998) and more recently West et al. as their counterparts in the Lesser Himalayan Se- (2002) have argued for greater silicate weathering quence (France-Lanord et al., 1993; English et al., rates in the wet Himalayan foothills compared to the 2000), because of the comparative youth of the Himalayan highlands where physical weathering Greater Himalayan Sequence. Results from the Raikot dominates. These contrasting effects of rainfall and Valley in Pakistan demonstrate the importance of relief make sense, where lithology is held constant in weathering of trace amounts of calcite in producing 87 86 the comparison. In the Arun, however, the influence high Sr/ Srwater (Blum et al., 1998; Jacobson and of relief and rainfall is largely overshadowed by Blum, 2000), but in that case the source is highly lithologic differences. In terms of total weathering metamorphosed Lesser Himalayan rocks (Whitington fluxes (carbonate and silicates), the highlands domi- et al., 1999), which we would expect to yield high nate Arun mainstem chemistry, probably due to the 87Sr/86Sr ratios due to their great age. Metamorphic abundance of Tethyan carbonate in the headwaters, calcite in the crystalline terranes of the Arun is also and to the relative paucity of carbonates south of the moderately radiogenic and a plausible source of some Himalayan crest. In closer analogy to the conclusions of the elevated 87Sr/86Sr ratios in derivative rivers, of West et al. (2002), it is carbonates (largely Lesser although weathering of silicates also likely makes a Himalayan) in the wet Himalayan foothills that dom- contribution. Much of the Ca and Sr in secondary inate total weathering fluxes along the Seti (English et calcite in crystalline terranes of the Himalaya may al., 2000). ultimately derive from high-temperature alteration of silicate minerals, which neutralizes hydrothermal 5.2. Strontium CO2. Such a process would contribute to atmospheric CO2 drawdown (Evans et al., 2001), unlike the low- Our Sr results from the Arun add significantly to temperature weathering of limestone and dolostone the database for watersheds underlain by mainly that we see dominates the Sr chemistry of the Seti, silicate rocks, and underscore the sharp isotopic differ- Bhotse Khola, and probably the Kali Gandaki.

Fig. 11. The fraction of silicate weathering, fsil, (as calculated from equation 6 in text) for the main Himalayan terranes and key rivers (see Fig. 1) 87 86 versus Sr/ Srwater. J. Quade et al. / Chemical Geology 202 (2003) 275–296 295

There is a strong irony in the contrasts in Sr Asahara, Y., Tanaka, T., Kamioka, H., Nishimura, A., 1995. Asian 87 86 chemistry between the Arun and Seti Rivers, because, continental nature of Sr/ Sr ratios in north central Pacific sediments. Earth and Planetary Science Letters 133, 105–116. prior to 1997, most researchers would have predicted Blum, J.D., Gazis, C.A., Jacobson, A.D., Chamberlain, C.P., 1998. the opposite relationships to those observed by us. Carbonate versus silicate weathering in the Raikhot watershed Raymo and Ruddiman (1992), Edmond (1992), Krish- within the High Himalayan crystalline series. Geology 26, naswami et al. (1992) and more recently Singh et al. 411–414. (1998) looked to silicate weathering of Greater and/or DeCelles, P.G., Gehrels, G.E., Quade, J., LaReau, B., Spurlin, M., 2000. Tectonic implications of U–Pb zircon ages of the Hima- Lesser Himalayan rocks as the main source of high 87 86 layan orogenic belt in Nepal. Science 288, 497–499. 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