Chemical erosion in the eastern Himalaya: Major ion composition …sunil/41__Sunil_Brahmaputra_major...

Geochimica et Cosmochimica Acta, Vol. 69, No. 14, pp. 3573–3588, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2005.02.033

Chemical erosion in the eastern Himalaya: Major ion composition of the Brahmaputraand �13C of dissolved inorganic carbon

SUNIL K. SINGH,1* M. M. SARIN,1 and CHRISTIAN FRANCE-LANORD2

1Physical Reasearch Laboratory, Navrangpura, Ahmedabad-380009, India2CRPG-CNRS, Vandoeuvre Les Nancy, Cedex, France

(Received September 27, 2004; accepted in revised form February 18, 2005)

Abstract—Major ion composition of waters, �13C of its DIC (dissolved inorganic carbon), and the claymineral composition of bank sediments in the Brahmaputra River System (draining India and Bangladesh)have been measured to understand chemical weathering and erosion and the factors controlling these processesin the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputramainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index ofalteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in theGanga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputrabasin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicateerosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosionrates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rateof �300 t km�2 y�1, one of the highest among the world river basins and comparable to those reported forsome of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes muchslower chemical erosion (�40 t km�2 y�1). The abundance of total dissolved solids (TDS, 102–203 mg/L)in the time-series samples collected over a period of one year shows variations in accordance with the annualdischarge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cycliccomponent) shows a strong positive correlation with Si, indicating their common source: silicate weathering.Estimates of silicate cations (Nasil�Ksil�Casil�Mgsil) suggest that about half of the dissolved cations in theBrahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO2

consumption rate due to silicate weathering in the Brahmaputra watershed is �6 � 105 moles km�2 y�1;whereas that in the Eastern Syntaxis subbasin is �19 � 105 moles km�2 y�1, similar to the estimates for someof the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of mostintensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling

0016-7037/05 $30.00 � .00

factors of chemical erosion in the eastern Himalaya. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

The Ganga-Brahmaputra (G-B) is one of the major globalriver systems, ranking first in sediment supply and fourth inwater discharge (Milliman and Meade, 1983; Milliman andSyvitski, 1992; Ludwig and Probst, 1998). The sediment fluxfrom the G-B may account for as much as �15% of the globalsediment discharge if its bed-load equals that of its suspendedload (Galy and France-Lanord, 2001). This makes the G-Bsystem a significant component of the sediment budget of theoceans. In terms of water discharge, G-B accounts for �3% ofglobal riverine water supply (Hay, 1998 and referencestherein). Among these two major rivers, the Ganga has beenstudied in far greater detail for its chemical and isotopic com-position and their impact on ocean chemistry. These studieshave brought out the role of these rivers, particularly theGanga, in contributing to Sr and Os isotope composition ofseawater during the Cenozoic and on the CO2 drawdown onmillion-year time scales (Galy and France-Lanord, 1999;Krishnaswami et al., 1992; Palmer and Edmond, 1989; Harriset al., 1998; Blum et al., 1998; Galy et al., 1999; Richter et al.,1992; Bickle et al., 2003; Dalai et al., 2002).

It is well documented that water and sediment delivery via

* Author to whom correspondence should be addressed ([email protected]).

3573

Brahmaputra, to the Bay of Bengal, exceeded that of the Ganga(Hay, 1998). The limited studies on the Brahmaputra (Sarin etal., 1989; Galy and France-Lanord, 2001) indicate that theeastern Himalaya, encompassing the Brahmaputra drainage, iseroding more rapidly than the central and western Himalayawhich make up the Ganga drainage. This difference has beenattributed to higher rainfall and runoff in the eastern sector(Sarin et al., 1989; Galy and France-Lanord, 2001). Morerecently, chemical, isotopic, and mineralogic data on sedimentsfrom the Brahmaputra watershed show that the basin is char-acterized by differential erosion and that the sediment budget ofthe Brahmaputra is dominated by material derived from theEastern Syntaxis region (Singh and France-Lanord, 2002;Singh et al., 2003; Garzanti et al., 2003). These results are alsosupported by model calculations (Finlaysan et al., 2002). Togain better understanding of the relative erosion rates within theBrahmaputra basin, a detailed investigation on the chemicaland isotope composition of the water and sediments was un-dertaken. The source apportionment of the solutes in the Brah-maputra River and its tributaries from silicates and carbonatesis an integral part of this study for determining the CO2 con-sumption rates. Recently, several studies have been carried outto quantify the sources of solutes in the Ganga drainage (Singhet al., 1998; Galy et al., 1999; Krishnaswami et al., 1999; Dalai
et al., 2002; Bickle et al., 2003). In this context, studies in the

n the inrious su

3574 S. K. Singh, M. M. Sarin, and C. France-Lanord

Brahmaputra system are rather limited (Krishnaswami et al.,1999; Galy and France-Lanord, 1999). In this study, attemptshave been made to determine the contribution of solutes to theBrahmaputra River System from silicate weathering and toestimate the associated CO2 consumption rates. In addition,these data represent a first attempt to document the temporalvariation in the major ion chemistry of the Brahmaputra main-stream based on a biweekly sampling carried out over a periodof one year, including the monsoon. Thus, these data providebetter estimates of annual fluxes and seasonal variations insilicate weathering rates.

2. BRAHMAPUTRA RIVER SYSTEM:HYDROLOGY AND GEOLOGY

2.1. The Brahmaputra River

The Brahmaputra River is known by different names alongits course. It originates in the Kailash Mountain in the Trans-himalaya and flows with a very gentle slope eastward �1200

Fig. 1. Map of the Brahmaputra watershed with samplinYamuna Bridge in Bangladesh. Area of study is shown ione season, and all the numbers are preceded by BR. Va

km in Tibet as Yarlung Tsangpo or Tsangpo. The Tsangpo

takes a U-turn after Pai (Fig. 1) around Namche Barwa, theEastern Syntaxis, where it makes the deepest gorge of the worldand turns south to enter Arunachal Pradesh, where it acquiresthe name Siang or Dihang. This part of the river with thedeepest gorge has a very steep slope (�30 m/km) causing veryturbulent and rapid flow and intense physical erosion (Singhand France-Lanord, 2002). Immediately after Pasighat, theSiang River turns in SW direction and enters the Assam Plain,where it is called the Brahmaputra and flows in WSW directionas a wide and deep braided river. The Brahmaputra acquires awidth of �20 km and depth of �35 m at some locations in theAssam Plain. The Brahmaputra turns south near Dhubri at theIndo-Bangladesh border and flows as the river Jamuna until itmeets the Ganga at Arichaghat.

The Brahmaputra River receives many tributaries along itscourse. In Tibet, the Tsangpo receives the Lhasa He (Zangbo),Doilung, and Nyang Qu (Fig. 1; Guan and Chen, 1981; Hu etal., 1982) in addition to tributaries from the northern slope of

ions. Brahmaputra was sampled from Pasighat in India toset. Sample numbers are indicated based on sampling inbbasins are shown by dashed lines.

g locat

the Himalaya. After Pai, the river Parlung Zangbo (Fig. 1; Guan

3575Chemical erosion in the eastern Himalaya

and Chen, 1981) merges with it. The slope of this tributary isvery high and comparable to that of the Siang in this section. Inthe Assam plain the Brahmaputra receives the Dibang and theLohit from the east and the Subansiri, the Ranganadi, the JiaBhareli, the Puthimari, the Manas, and the Tipkai from thenorth and the Burhi Dihing, the Dhansiri, and the Kopili fromsouth (Fig. 1). The Tista is another northern tributary of theBrahmaputra which merges with it in Bangladesh (Fig. 1).

The water discharge of the Brahmaputra mainstream and manyof its tributaries are given in Table 1. The major source of waterin the Brahmaputra is rainfall, though meltwater and groundwatercontributions are also important. In the Tsangpo in Tibet, forexample, meltwater, groundwater and rainfall contributions areroughly the same (Guan and Chen, 1981). The runoff in theTsangpo drainage is �300 mm yr�1 which increases by more thanorder of magnitude, to �5000 mm yr�1, for the Siang inArunachal Pradesh. Runoff in the Himalayan drainage for thenorthern tributaries in the Assam Plain is �1000–2000 mm yr�1

and for the eastern tributaries it is �3000 mm yr�1. The southerndrainage is exposed to heavy rainfall and the runoff in this regionis �4000 mm yr�1. The major contributor to the Brahmaputradischarge is rainfall during SW monsoon (July to September).Monthly discharge of the Brahmaputra at Bahadurabad, based onthe average of many years, is shown in Figure 2. The monthlywater discharge pattern of the Brahmaputra at Bahadurabad re-

Table 1. Sample details: Locations, Wa

Sample code River (location)

Longitude (E)

deg min d

Brahmaputra MainstreamTsangpo at Pai — —

BR-59 Siang or Dihang (Pasighat) 95 20.15BR-18 Brahmaputra (Dibrugarh) 94 51.11BR-28 Brahmaputra (Tezpur bg.) 92 51.22BR-65 Brahmaputra (Tezpur bg.) 92 51.18BR-5 Brahmaputra (Guwahati) 91 44.22BR-51 Brahmaputra (Guwahati) 91 44.22BR-73 Brahmaputra (Dubri) 89 59.76

Brahmaputra (Chilmari)b — —BR 200 Brahmaputra(Jamuna bg.) 89 47.83Eastern TributariesBR-14 Dibang 95 35.15BR-16 Lohit 95 36.34Himalayan TributariesBR-20 Subansiri 94 15.10BR-61 Subansiri 94 15.42BR-24 Ranga Nadi 94 3.53BR-57 Ranga Nadi 94 3.64BR-26 Jia Bhareli 92 52.57BR-63 Jia Bhareli 92 52.77BR-75 Tipkai 90 8.28BR-32 Manas Biki 90 54.95BR-71 Manas Biki 90 55.17BR-34 Puthimari 91 39.22BR-69 Puthimari 91 39.20Southern TributariesBR-10 Dhansiri 93 43.88BR-12 Buri Dihing 94 53.02BR-30 Kopili 92 21.27BR-67 Kopili 92 21.22

a Rao, 1979; Goswami, 1985; and GRDC website (www.grdc.sr.unhb Galy and France-Lanord, 1999.

flects the monsoon with significant temporal variation. It varies

from �3300 m3/s in February to �59000 m3/s in July. Thedischarge in February is the lowest owing to paucity of rain andless meltwater contribution. This trend is almost similar to that at

Fig. 2. Monthly discharge of the Brahmaputra at Bahadurabad in

harge, Drainage, Temperature and pH.

e (N) Month &year of

sampling

Dischargea,109 m3

y�1Areaa, 103

km2Temp,

°C pHmin

— — 59 220 — —4.67 July 2000 178 246 23.2 7.9

30.03 Oct 1999 323 298 21.1 8.436.73 Oct 1999 — 376 24.7 8.336.66 July 2000 — 376 32.1 8.111.62 Oct 1999 455 384 24.7 8.611.62 July 2000 455 384 31.2 7.81.12 July 2000 510 455 29.6 7.8— Aug 1996 666 636 29.0 7.6

23.35 July 2002 — — — —

47.84 Oct 1999 22 13 21.3 8.647.90 Oct 1999 49 24 18.6 8.4

26.90 Oct 1999 54 26 21.0 8.426.74 July 2000 54 26 23.9 7.312.24 Oct 1999 58 2 20.8 8.412.32 July 2000 58 2 27.4 7.248.60 Oct 1999 26 11 22.0 8.248.65 July 2000 26 11 24.5 7.613.11 July 2000 — — 29.4 7.629.70 Oct 1999 32 28 21.8 8.429.73 July 2000 32 28 21.9 8.322.04 Oct 1999 — — 27.3 8.222.01 July 2000 4 2 28.1 7.8

37.87 Oct 1999 20 8 26.3 7.718.74 Oct 1999 14 3 25.5 8.09.91 Oct 1999 32 15 26.9 8.09.93 July 2000 32 15 32.2 7.4

The values are average of several years of data.

ter disc

Latitud

eg

—28272626262626—24

2727

2727272726262626262626

26272626

.edu).

Bangladesh. Most of the discharge in the Brahmaputra occurs duringmonsoon season, i.e., June to September.


Guwahati in India, where biweekly samples have been collected.The Brahmaputra System drains a total area of �630,000 km2. Ofthe total drainage, about one third is in Tibet with an averageelevation of �5000 m. The Tibetan drainage contributes �10% ofthe water discharge of the Brahmaputra at its mouth. The Brah-maputra drains total area of �200,000 km2 in the plains of Assamand the Bangladesh and its Himalayan tributaries occupy an areaof �120,000 km2 in the Himalaya. The two eastern tributaries, theLohit and the Dibang flowing through the Mishmi Hills havedrainage area �50,000 km2.

2.2. Geology

The drainage basin of the Brahmaputra System can be di-vided into five geologically and climatically different subbasins(Fig. 1). These are (1) the Tibet, the drainage of the Tsangpo,(2) the Eastern Syntaxis, the drainage of Siang and ParlungTsangpo rivers between Pai and Pasighat, (3) the Mishmi Hills,the drainage of the two eastern tributaries, the Lohit and theDibang, (4) the Himalaya, the drainage of the northern tribu-taries of the Brahmaputra in the Assam and the BangladeshPlains (the Renganadi, the Jia Bhareli, the Puthimari, the Ma-nas, the Tipkai), and (5) the Indo-Burmese Ranges, the drain-age of southern tributaries (the Burhi, the Dihing, the Dhansiri,and Kopili) of the Brahmaputra in the Assam and the Bang-ladesh plains.

1. Tibet: In upper reaches the Tsangpo drains turbidites andophiolites of the Indus-Tsangpo Suture Zone. The tributar-ies from the northern slope of the Himalaya drain theTethyan Sedimentary Sequences and the gneiss zone. Thetributaries from Tibetan Plateau, the Doilung, Zangbo, andNyang Qu predominantly drain Transhimalayan gabbroicto granodioritic batholiths. The basins of these tributariesalso contain evaporite deposits (Hu et al., 1982; Pande etal., 1993; Pascoe, 1963).

2. The Eastern Syntaxis: The rocks near the Eastern Syntaxisare highly metamorphosed. At its core, gneisses of theIndian Plate have been exhumed from below the Transhi-malayan Plutonic Belt (TPB; Burg et al., 1998). In this zonethe calc-alkaline plutons of the TPB are surrounded byquartzites, phyllites, and marbles (Burg et al., 1998). Dis-crete lenses of metabasites and serpentinites occur in theseareas, which indicates the continuation of the Indus-Tsangpo Suture in the eastern section (Burg et al., 1998).These are drained by the Dibang, Parlung Tsangpo, andLohit.

3. The Mishmi Hills: The two eastern tributaries, the Lohitand the Dibang, flow through the Mishmi Hills composedof calc-alkaline diorite-tonalite-granodiorite complexes andtholeiitic metavolcanic rocks (Kumar, 1997). It representsthe eastern continuation of the TPB. The Tiding Suturepresent in this area marks the boundary between the TPBand the Himalaya in this section.

4. The Himalaya: The geology of the eastern Himalaya,through which the northern tributaries of the BrahmaputraSystem in Assam Plain, such as the Subansiri, Renganadi,Jia Bhareli, Puthimari, and Manas, flow, is similar to thoseof its central and western sections, which form the Ganga
basin. It comprises of the Higher and the Lesser Himalaya
and the Siwaliks (Thakur, 1986; Gansser, 1964). In general,the proportion of the Lesser Himalaya increases from eastto west in this watershed (Singh and France-Lanord, 2002;Robinson et al., 2001). The Higher Himlayan rocks consistmainly of schists and marbles with amphiboles at somelocations. In Bhutan and Sikkim, the Manas and the Tistadrain through metamorphic rocks of the Higher Himalaya.The Lesser Himalaya in the Brahmaputra System drainageis composed mainly of quartzites and schists. Precambrianlimestones, dolostones, shales, and quartzites along withorthogneiss bodies and dolerite sills are exposed in theLesser Himalaya. The Siwalik is discontinuous in the east-ern section of the Himalaya. It includes a thick section ofNeogene molasses. Continuing uplift and deformation areevident in this section by the presence of tilted gravelterraces and steep fault scarps (Nakata, 1989). Apart fromthese rocks of the Himalaya the basalts of the Abor volca-nics are present in the Himalayan drainage of the Siang(Jain and Thakur, 1978). The northern tributaries of theBrahmaputra in the Assam plain drain through the southernslope of the Himalaya. Only a few of them, the Subansiri,have their drainage in the Tethys Himalaya (Kumar, 1997).

5. Indo-Burmese Ranges: These ranges are made of pelagicsediments overlain by thick turbidites associated withophiolites. The Dhansiri and the Kopili also drain theIndian basement of the Shillong Plateau and the Mikir Hills(Kumar, 1997).

3. SAMPLING AND METHODOLOGY

Water and sediment samples from the Brahmaputra mainstream andits major tributaries draining between Pasighat in Arunachal Pradesh toDhubri at the Indo-Bangladesh border (Fig. 1) were collected duringtwo seasons: the SW monsoon and the postmonsoon. The monsoonsamples were collected during the month of July, representing peakdischarge, and the postmonsoon samples were collected in October(median flow). The Brahmaputra mainstream was sampled at fivestations: Pasighat, Dibrugarh, Tezpur, Guwahati, and Dhubri (Fig. 1).One monsoon sample of the Brahmaputra (BR 200) was collected atJamuna Bridge in Bangladesh. Samples were collected from the mid-channels, accessing either from a boat or from road bridges. Thecollection procedures for sediment have been described in detail inSingh and France-Lanord (2002), Singh et al. (2003), and Garzanti etal. (2004). Soon after their collection, two separate aliquots of 500 mLwater were filtered using 0.2-�m nylon membrane Millipore filters.One of the filtered aliquots was acidified with double-distilled HNO3

for cations, trace metal, and Sr analysis, and the other aliquot waspreserved unacidified for anion measurements. In addition, one sampleof �250 mL water was collected and stored unfiltered for alkalinitymeasurements. The Brahmaputra mainstream was also sampled atGuwahati (Fig. 1) at an interval of 15 days over a period of �10 monthsto assess the temporal variations in its major ion composition.

Temperature and pH of the water samples were measured at site.The water and sediment samples were brought to the laboratory forfurther analysis. Alkalinity was measured by acid titration; Cl, NO3,and SO4 by ion chromatography; K and Na by flame AAS; and Ca,Mg, and Si using ICP-AES. The precision of these measurements,based on earlier studies in this laboratory (Dalai et al., 2002), isabout �5%. Accuracy of measurements for various elements waschecked by measuring dilute solutions of USGS rock standards, W-1and G-2, and also by analyzing river waters of known elementalabundances (Sarin et al., 1992). Clay mineral analyses of selectedsediment samples were done at CRPB-CNRS, France (Bartoli,1991). Quantification of clay minerals was done by analyzing

13
�2-�m size fraction by granulometry. � C of dissolved inorganiccarbon was measured using a modified VG 602D isotope ratio mass


spectrometer following the procedure of Galy and France-Lanord(1999).

4. RESULTS AND DISCUSSION

Sampling details, temperature, pH, discharge, and drainageare given in Table 1. Major ion composition of the Brahmapu-tra mainstream and its various tributaries are presented in Table2, temporal data for the Brahmaputra (at Guwahati) in Table 3,and the clay mineral composition of sediments in Table 4.

All samples were slightly alkaline in nature (pH 7.2–8.6) andtemperature varied from 19°C to 32°C depending on the seasonand on the time of sampling. In some of the following discus-sions, the dissolved Na has been corrected for contributionfrom rainwater and halites by subtracting Na equivalent todissolved Cl from them (Na* � Nariv � Clriv). Other cationsand anions are not corrected for rainwater contribution because

Table 2. Major ion composition and �13C o


Na� Na* K�

Brahmaputra mainstreamTsangpo (South Lhasa)a 396 240 32Tsangpo (South Lhasa)a 446 248 37Siang (Pai)b 387 244 28

BR-59 Siang or Dihang (Pasighat) 78 61 37BR-18 Brahmaputra (Dibrugarh) 106 72 48BR-28 Brahmaputra (Tezpur bg.) 110 86 50BR-65 Brahmaputra (Tezpur bg.) 78 64 49

Brahmaputra (Guwahati)a 159 52 79BR-5 Brahmaputra (Guwahati) 86 58 50BR-51 Brahmaputra (Guwahati) 90 69 67BR-73 Brahmaputra (Dhubri) 107 88 50

Brahmaputra (Chilmari)c 104 79 52BR 200 Brahmaputra (Jamuna bg.) 77 53 62Tibetan Tributaries to Tsangpo

Zangbo (Lhasa)a 380 180 36Doilung (Lhasa)a 300 38 48

Eastern TributariesBR-14 Dibang 47 33 42BR-16 Lohit 59 41 50Himalayan TributariesBR-20 Subansiri 79 59 24BR-61 Subansiri 71 62 26BR-24 Ranga Nadi 137 120 33BR-57 Ranga Nadi 105 94 28BR-26 Jia Bhareli 101 86 32BR-63 Jia Bhareli 90 80 32BR-75 Tipkai 124 106 27BR-32 Manas Biki 97 78 31BR-71 Manas Biki 59 49 27BR-34 Puthimari 133 109 43BR-69 Puthimari 99 80 36Southern TributariesBR-10 Dhansiri 288 205 63BR-12 Buri Dihing 189 147 31BR-30 Kopili 137 104 40BR-67 Kopili 118 88 35

a Hu et al., 1982.b Chen and Guan, 1981.c Galy and France-Lanord, 1999.Na* � (Nariv � Clriv).

they are insignificant (e.g., Galy and France-Lanord, 1999).

4.1. Spatial and Temporal Variability inTotal Dissolved Solids

The total dissolved solids (TDS � Na�K�Ca�Mg�Cl�SO4�HCO3�SiO2 in mg L�1) in the Brahmaputra mainstream(Table 2, Fig. 3) measured in this study shows a narrow range,from 91 mg L�1 at Tezpur during monsoon to 128 mg L�1 atDibrugarh during postmonsoon. In general, downstream variationin TDS of the Brahmaputra mainstream during the current sam-pling is small, and center around 110 � 15 mg L�1 (Fig. 3). Threedata points for the Tsangpo in Tibet show higher TDS, 150–185mg L�1 (Hu et al., 1982; Chen and Guan, 1981). This is anindication that in the Tsangpo the salinity is higher and it getsdiluted downstream by rainfall. The tributaries of the Brahmapu-tra, in contrast, show factors of 3–4 variability in their TDS, from50 to 182 mg L�1 (Table 2). The monsoon samples, as expected,are dilute in terms of their TDS compared to those of postmon-

f waters of the Brahmaputra River System.

Ca2� Cl� NO3� SO4

2� HCO3� SiO2

TDS,mgL1

�13C(DIC),

‰�mol L�1

752 156 — 255 1670 127 185 —717 198 — 223 1740 125 187 —500 143 — 188 984 125 150 —424 17 14 119 854 126 95 —540 34 — 158 1197 152 128 �11.5458 24 — 110 1154 189 119 �14.6378 14 9 86 845 146 91 —425 107 — 100 884 123 101 —395 28 — 87 1005 137 102 �13.6475 21 17 114 1051 140 112 —396 19 11 73 1018 200 106 —393 25 — 55 1114 155 105 —433 24 — 78 1060 127 105 �10.1

270 200 — 75 751 134 90 —224 262 — 60 558 88 73 —

353 14 — 82 780 141 82 �12.4440 18 — 91 996 139 101 �12.6

323 20 — 104 849 169 92 �11.1303 9 15 95 667 148 77 —158 17 — 38 550 294 67 �15.5126 11 20 34 373 218 50 —235 15 — 59 682 206 75 �14.3191 10 12 33 537 185 60 —284 18 7 28 994 258 100 —470 19 — 148 1096 158 117 �12.0416 10 8 102 844 105 90 —698 24 — 143 1877 218 182 �13.6551 19 19 112 1336 166 135 —

230 83 — 129 881 225 106 �13.6228 42 — 85 1084 310 116 �17.2158 33 — 65 591 217 70 �16.0148 30 3 64 425 208 58 —

f DIC o

Mg2�

209191103100144140101119115111153168120

3935

4978

13610845377060

195148101291219

2262939978

soon.


Table 5 is a summary of available data on TDS of theBrahmaputra along its course. These data provide informationon spatial and temporal variation in TDS over about twodecades. The TDS of the Brahmaputra measured in this studyat Jamuna Bridge and Dhubri during monsoon (105 and106 mgL�1, respectively) is identical to the value of 105 mg L�1

reported by Galy and France-Lanord (1999) at Chilmari a fewkilometers downstream of Dhubri collected during monsoon of1996. Similarly, the TDS of 101 mg L�1 at Guwahati (Hu et al.,1982) during monsoon 1979 compares well with value of 112mg L�1 obtained in this study. Sarin et al. (1989) measured thechemical composition of the Brahmaputra mainstream in sam-ples collected from four sites during 1982–1983. The TDSmeasured at Guwahati during April 1982 was 91 mg L�1

compared to values of 140 and 111 mg L�1 measured in thisstudy for the same month. Similarly, the value for December in1982 was 144 mg L�1, marginally lower than the values of 164and 171 mg L�1 in 1999. These results show that over twodecades the TDS in the Brahmaputra does not show any sys-tematic and major variations, the variability being �35% orless; such interannual variations can result from associatedchanges in runoff. TDS of the Brahmaputra river system isgenerally lower than those in samples from the lower reaches ofthe Ganga, the Yamuna, the Karnali, and the Narayani riversbut similar to those in the headwaters of these rivers (Sarin et

Table 4. Abundance and composition of cla


BR 19 Brahmaputra (Dibrugarh)BR 29 Brahmaputra (Tezpur)BR 9 Brahmaputra (Guwahati)BR 21 SubansiriBR 25 RenganadiBR 33 Manas

Table 3. Temporal variation in chemical

Date,month/day/

year

Na� Na* K� Mg2� Ca2�

10/24/1999 86 58 50 115 39511/15/1999 141 108 54 193 58811/30/1999 163 119 56 209 61312/15/1999 171 130 56 228 65812/31/1999 188 142 57 236 6801/16/2000 229 150 66 246 7111/31/2000 212 146 62 236 6792/15/2000 220 149 64 233 6863/1/2000 269 171 67 245 7153/15/2000 195 131 61 208 6324/1/2000 192 117 63 171 5534/19/2000 99 71 52 120 4574/30/2000 88 60 55 122 4515/15/2000 107 75 58 119 4555/31/2000 80 57 54 115 4396/15/2000 95 73 154 192 10047/1/2000 78 60 85 117 5507/26/2000 90 70 67 111 475

Na* � Nariv � Clriv.

BR 21 Kopili

al., 1989; Sarin et al., 1992; Dalai et al., 2002; Galy andFrance-Lanord, 1999). This is most likely due to differences indrainage lithology and runoff among these basins. For example,a lower proportion of the Lesser Himalayan sedimentariescontaining more easily weatherable carbonates and evaporitesin the drainage basin of the Brahmaputra System can causelower TDS in these rivers. Similarly, higher runoff in theBrahmaputra watershed can affect weathering intensity andalso act as a diluent.

The TDS of the Tsangpo in the Tibetan Plateau (Hu et al.,1982) is 185 mg L�1, the highest for the mainstream. Thehigher TDS in the Tibetan region is most likely due to thepresence of easily weatherable evaporites in the basin (Pascoe,1963). The chemistry of its tributaries, e.g., the Zangbo and theDoilung (Hu et al., 1982), from this region supports this infer-ence (Table 2 and discussion). Also the lower runoff of thisregion helps maintain higher TDS. Comparison of TDS of theTsangpo at Pai (Chen and Guan, 1981), before it enters theEastern Syntaxis, and at Pasighat after its exit shows that TDShas decreased from 150 mg L�1 to 95 mg L�1 (Fig. 3). Thisdecrease can result from changes in both geology and climate.Between Pai and Pasighat the Brahmaputra predominantlydrains silicates and its runoff increases by an order of magni-tude which can cause dilution and also affect weathering in-tensity. Downstream of Pasighat, the two tributaries, Dibang

diments of the Brahmaputra River System.

y, wt% Clay composition

1.7 Vermicullite, illite, chlorite2.6 Vermiculite, illite, chlorite3.2 Vermiculite, illite, chlorite1.4 Vermiculite, illite, montmorillonite3.9 Illite, chlorite, Montmorillonite3 Illite, chlorite, montmorillonite

sition of the Brahmaputra at Guwahati.

Cl� NO3� SO4

2� HCO3� SiO2

TDS,mg L�1�1

28 — 90 1005 137 10233 — 146 1395 204 14644 — 154 1470 211 15441 — 169 1560 217 16446 — 175 1634 221 17179 — 177 1694 238 18066 — 179 1618 233 17271 — 179 1589 229 17198 4 185 1676 232 18164 3 185 1460 200 15875 1 156 1272 187 14028 — 108 1066 155 11129 — 85 1081 138 10932 — 101 1067 159 11122 — 96 1025 121 10421 — 456 1585 149 20318 — 112 1238 150 12621 17 114 1051 140 112

y in se

Cla

compo

�mol L

21 Montmorillonite, illite, chlorite


and Lohit meet the Brahmaputra from the east (Fig. 1). AtDibrugarh, the TDS of the Brahmaputra is �35% more (128mg L�1) than at Pasighat (95 mg L�1; Fig. 3, Table 2). Thisincrease in TDS can be due to temporal variation in TDS,because the Pasighat was sampled in monsoon and Dibrugarhduring postmonsoon, or dissolution of detrital carbonates in the

Fig. 3. Downstream variation in TDS of the Brahmaputra. TDS ishighest in Tibet and decreases after the river crosses the EasternSyntaxis. In the Assam plain TDS is almost constant despite receivingmany tributaries with variable TDS. More than one value of TDS at thesame location represents samples from different seasons (Table 2).

Table 5. Decadal variation in TDS of the Brahmaputra along itscourse.

Location Date TDS (mg L�1) Ref.

Lhasa Jul 1979 185 1Jun 1980 187 1

Pai 150 2Dibrugarh Apr 1982 107 3

Oct 1999 128 This studyGuwahati July 1979 101 1

July 2000 112 This studyApr 1982 91 3Apr 2000 111 This studyDec 1982 144 3Dec 2000 164 This studyOct 1999 102 This study

Goalpara Apr 1982 92 3Dec 1982 147 3

Dhubri July 2000 106 This studyChilmari Aug 1996 105 4

Mar 1997 145 4Jamuna Bridge July 2000 105 This studyAricha Ghat Feb 1997 154 4

1. Hu et al., 1982.2. Chen, and Guan, 1981.
3. Sarin et al., 1989.4. Galy and France-Lanord, 1999.
Brahmaputra during its transit from Pasighat to Dibrugarh. Thedecrease in abundance of detrital carbonates in sediments from� 3–4 wt% in the Siang, the Dibang, and the Lohit to �0 wt%at Dibrugarh (Singh and France-Lanord, 2002) supports thelater hypothesis. This is further strengthened from the dissolvedand particulate Ca fluxes of the Brahmaputra at Pasighat andDibrugarh (see Appendix). These observations showed that ina stretch of �50–60 km, the Brahmaputra is dissolving all thedetrital carbonate present in the bed load. This interpretationcan be misleading in view of different season of sampling asthe Siang was sampled during monsoon and the others post-monsoon; however, the carbonate content of sediments at Tej-pur, further downstream of Dibrugarh, shows no variabilitybetween monsoon and nonmonsoon sampling and therefore itcan be safely assumed that detrital carbonate would not havechanged between two seasons at Dibrugarh, too.

The TDS of the Brahmaputra remains nearly constant fromDibrugarh to Jamuna Bridge though it mixes with many trib-utaries from the Himalaya (Fig. 1). The TDS of the Himalayantributaries increase from east to west. The Subansiri, the Ran-ganadi and the Jia Bhareli have lower TDS as they primarilydrain silicates of the Higher Himalaya and the Puthimari, theManas and the Tipkai have higher TDS, resulting from theweathering of Lesser Himalaya having higher proportion ofcarbonates (Thakur, 1986; Gansser, 1964). The decrease inrunoff from east to west over the Himalayan drainage can alsobe an additional contributing factor to this trend. TDS of thesouthern tributaries are comparable to those of the Himalayandrainage.

Temporal variation in the TDS of the Brahmaputra Riverwas measured at Guwahati at 15-day intervals over a period of10 months (Table 3). TDS over the ten months showed about afactor of two variation, from 104 mg L�1 to 203 mg L�1 (Fig.4); the range is more pronounced than spatial variations. Inter-estingly the minimum and maximum TDS occurred in a span of15 days during end of May to mid-June (Table 3). The maxi-mum in TDS seems to be a result of flash flooding in theBrahmaputra (see discussion later). The minimum duringApril–May is attributable to increase in glacier melt water

Fig. 4. Temporal variation of various major ions of the Brahmaputraat Guwahati. All the concentrations are in �M except for TDS in mgL�1. One sample collected during SW monsoon has elevated concen-tration owing to flash flooding in the Tsangpo.

component in the discharge. The second broad maximum of


170–180 mg L�1 during January–March (Fig. 4) can be ex-plained in terms of lean flow and increased contribution fromground water. It is important to note that water discharge duringmonsoon is a factor of �10 higher compared to that of duringlean flow where as the TDS during monsoon is slightly lower(�0.6�) with respect to lean flow. This shows that the kineticsof chemical weathering and supply of solutes of river is notsignificantly affected despite an order of magnitude increase inrunoff.

4.2. Major Ion Composition

Downstream variation in the total cation charge in the Brah-maputra mainstream ranges from �2350 �Eq L�1 in Tibet to1085 �Eq L�1 at Tezpur (Table 2). This is quite similar to thetemporal variability in total cation (TZ�) at Guwahati, 1242 to2641 �Eq L�1, and the range in the tributaries, 604 to 2154�Eq L�1. The TZ� charge balances total anions (TZ�) in mostsamples within analytical uncertainties. The normalized inor-ganic charge balance (NICB) is within �5% for most of thesamples. In four samples TZ� exceeds TZ� by 5% to 11% withthe maximum deviation for the Ranganadi sample. The “ex-cess” anions can be due to the presence of organic ligands suchas oxalate, acetate, and humic/fulvic acids.

Fig. 5. Ternary plots of cations and anions of the Brahmaputra.Cations are dominated by Ca whereas HCO3 dominates the anionbudget. Data are in �Eq L�1.

The cation budget (Fig. 5a) is dominated by Ca and Mg in

most of the samples. On average about three-fourths of thecations are Ca and Mg for the Brahmaputra river system. In theDhansiri, the Kopili, and the Ranganadi tributaries, Ca and Mgaccount for �60% of the total cation budget. If all the Ca andMg in all these river waters are derived from carbonate weath-ering, �75% of the cations (molar) in the Brahmaputra water-shed will be of carbonate origin. This places an upper limit onthe carbonate weathering contribution of cations, because partof Ca and Mg will also be derived from silicates, and Ca alsocomes from evaporites. In the Tsangpo tributaries, Ca and Mgaccount only for 43% of cations (Hu et al., 1982) owing tosupply of Na (or K) by dissolution of saline and alkaline saltspresent in the Tibetan drainage and contribution from salinelakes.

Ca/Mg molar ratios in the Brahmaputra river system average�3, with a range of 0.8 to 7.1. The Ca/Mg ratio in rivers woulddepend on Ca and Mg supplied from various sources—silicates,carbonates, and evaporites—and the behaviour of Ca in therivers. In the Brahmaputra river system, calculations showedthat Ca is undersaturated with respect to calcite in all samplesanalyzed except in six of them. Even in these six samples, theextent of supersatuartion is marginal, a factor of two or less.Therefore the Ca/Mg variation seems to be more dependent ontheir sources: Low Ca/Mg can result from silicate/dolomiteweathering and high Ca/Mg from calcite weathering. Figure 6is a plot of Ca/Mg vs. Mg concentration which shows distinctdecrease with increase in Mg. This is an indication of the roleof dolomite weathering in contributing to Ca and Mg. Theregional lithology and detrital carbonate composition of theDibang, the Lohit, and the Manas rivers attest to this inference.

The anion budget is dominated by alkalinity (Fig. 5b), whichvaries from 425 to 1877 �M (Table 2), contributing �90%(molar) to the anion budget. The temporal variation in alkalin-ity of the Brahmaputra mainstream at Guwahati (Table 3), from1025 to1694 �M, is less than its spatial variation along itscourse. SO4 is the next abundant anion. Analogous to alkalin-

Fig. 6. Scatter diagram of Ca/Mg vs. Mg. Decrease in Ca/Mg is due
to increase in Mg, likely due to dissolution of dolomite of the LesserHimalaya; exposure of such lithologies increases from east to west.


ity, temporal variation in SO4 of the Brahmaputra mainstreamat Guwahati (Table 3, Fig. 4), 85 to 185 �M (barring onesample with 456 �M SO4; see discussion), is also less than itsspatial variation. The SO4 content of the Himalayan tributaries,in general, is lower compared to that of the Ganga and theYamuna (Sarin et al., 1992; Dalai et al., 2002). This is consis-tent with the lithology of their basins, which have lesser expo-sure of the Lesser Himalaya sedimentaries and hence lesspyrites and evaporites in their watershed. SO4 content of theHimalayan tributaries of the Brahmaputra in the Assam plainincreases from east to west following the increase in the pro-portions of the Lesser Himalayan sedimentaries. The SO4 con-tent of the Brahmaputra is generally higher than that of theGanga (Sarin et al., 1992; Galy and France-Lanord, 1999),contributed from Tibetan tributaries.

Cl in the Brahmaputra River System is quite low. It rangesfrom 9 to 83 �M (Table 2), with most samples having valuescentering around 15 �M, similar to those reported for theHimalayan glaciers (Nijampurkar et al., 1993; Sarin and Rao,2002). In general, Cl in the southern tributaries is higher. Thehighest Cl is in the Tsangpo (Hu et al., 1982; Chen and Guan,1981), indicating salt dissolution. In the Brahmaputra main-stream, temporal variation in Cl, 18 to 98 �M, is quite signif-icant, with higher concentrations during lean flow.

Silica in the Brahmaputra river system varies from 105 to310 �M (Table 2). In Tibet, the Tsangpo has �125 �M ofsilica (Hu et al., 1982) which increases to �200 �M as theBrahmaputra crosses the Himalaya. The silica in the Himalayantributaries, 105 to 294 �M, is comparable to those in theheadwaters of the Ganga and the Yamuna (Sarin et al., 1992;Dalai et al., 2002). The southern tributaries, the Burhi Dihing,the Dhansiri and the Kopili, have higher silica, 208–310 �M,indicating more intense silicate weathering. Silica on averagecontributes �12% to the TDS. The silica contribution to TDSin the Tsangpo in Tibet is as low as 4%, indicating lowersilicate weathering in its drainage in Tibet.

The results on temporal variations in the major ion chemistryof the Brahmaputra show that the sample collected in the monthof June has elevated concentrations of most of the ions (Table3, Fig. 5). It has the highest TDS, Ca, and SO4 concentrationsand Ca/Mg ratio. The contribution of Si to TDS is quite low(4%) in this sample, similar to that in the Tsangpo (Hu et al.,1982). The major ion chemistry of this sample shows a closeresemblance to those of the Tibetan samples. There were re-ports of a flash flood in the Brahmaputra during the time periodwhen this sample was collected. The flash flood was a result ofthe bursting of a naturally built dam in Tibet. This dam hadrestricted water discharge from the Tibetan region into theBrahmaputra. Its break-up enhanced the contribution of waterfrom the Tibetan drainage to the Brahmaputra, which is re-flected in the major ion chemistry. Further, damming increasesreaction time of water with basin sediments and rocks, elevat-ing the concentrations of major elements in the dissolved phase.

Samples collected during the postmonsoon and one monsoonsample were analyzed for �13C of DIC. The samples were notpoisoned to arrest biologic activity between sampling and anal-ysis; however, no fungus or any deposit was seen during thetime of measurement. The �13C of the Brahmaputra mainchannel varies from �14.3‰ to �10.1‰. The �13C ranges for
the eastern, the Himalayan, and the southern tributaries are
�12.4‰ to �12.6‰, �11.1‰ to �14.3‰, and �13.6‰ to�17.2‰, respectively. These results have been used to assessthe role of silicate/carbonate weathering in the budget of DIC inthese waters.

4.3. Chemical Weathering in the BrahmaputraWatershed

In this study an attempt has been made to quantify thechemical erosion rate in this drainage, based on major ionchemistry of rivers waters. In addition, the measured mineral-ogic and clay composition of sediments has been used toprovide independent assessment of the chemical weathering inthe basin.

4.3.1. Clay composition and CIA of Sediments

The clay content of Brahmaputra mainstream, from Dibru-garh to Guwahati, is only 1.7 to 3 wt% of total sediments(Table 5). The Himalayan tributaries (the Subansiri, the Ran-ganadi, and the Manas) of the Brahmaputra have similar clayabundance, 1.4 to 3.9 wt%. In contrast, the southern tributary,Kopili, has �21 wt% clay. A direct interpretation of the claycontent of these river sediments is that in the Brahmaputramainstream and in the Himalayan drainage the sediments arepoorly weathered, whereas for the sediment of the southerntributary the chemical weathering is relatively more intense. Animportant uncertainty, however, in using the clay mineral abun-dance to assess the degree of chemical weathering is sizesorting in river sediments, which can significantly alter the clayabundance. In drainage basins subject to high-energy flow,such as in the Brahmaputra, clay minerals and fine particles canbe transported out of sediments. Therefore, in addition toabundance of clays, other proxies such as composition of clayshave to be used to assess the intensity of chemical weathering.In the Brahmaputra mainstream most of the clay is vermiculite,a mineral resulting from less intense chemical weathering (Mil-lot, 1970). Thus, the clay content of the Brahmaputra sedimentsand the abundance of vermiculite in them is an indicationwhich shows that these sediments are poorly weathered. Thiscan be attributed to rapid transport of sediments along thecourse of the river; making this drainage weathering limited(Stallard, 1995). In the southern tributaries, where the claycontent is higher and is made of montmorillonite, illite, andchlorite, the chemical weathering is high, a conclusion alsoattested by the higher concentrations of Si and Na* in theirwaters.

The chemical weathering in a basin can also be gauged fromthe change in mineralogy and chemical composition of sedi-ments along the river course. Sediment composition can bemodified by a number of processes occuring during erosion,transport, recycling, and diagenesis. The mineralogy of sedi-ments of the Brahmaputra River System (Garzanti et al., 2004)show that (1) there is a marginal decrease in the plagioclase/feldspars (P/F) ratio from the mountain streams to the Assamplains, (2) there is no indication of selective dissolution ofplagioclase over the more resistive K-feldspar, (3) clinopy-roxenes and amphiboles show similar extent of alteration, and(4) quartz/feldspar (Q/F) ratio, P/F ratio, and hornblende-dom-
inated dense-mineral assemblages remain constant. All these


observations infer minimal chemical weathering of the Brah-maputra sediments. In contrast the sediments of the southerntributaries contain pitted and embayed quartz grains, have lowP/F ratio, and have abundant microcline, etched clinopyroxene,and laterite clasts. These observations support a more intenseweathering in this drainage. This can be attributed to highertemperature in the region and higher residence time of sedi-ments in the basin because of lower relief.

The chemical weathering of sediments can also be assessedusing their chemical index of alteration (CIA; Nesbitt andYoung, 1982):

CIA � 100 � �(Al2O3 ⁄ �Al2O3 � CaO * � Na2O � K2O)�where Al2O3, CaO*, Na2O, and K2O are molar abundances.CaO* is CaO corrected for carbonates present in the sediments(Nesbitt and Young, 1982). CIA of fresh granites is �50, whichon weathering increases to �100 in soils. CIA for rocks fromthe Higher and Lesser Himalaya are �65–70 as these arerecycled crust whereas those for the Transhimalayan Plutonicrocks are �55 (calculated from Debon et al., 1986), being therecent calc-alkaline rocks of mantle origin. Bedloads of theBrahmaputra mainstream have CIA ranging from 58 to 64 (datafrom Singh and France-Lanord, 2002). (These CIA calculationsare based on total Ca, as these sediments have negligibleamount of carbonate, Singh and France-Lanord, 2002. In theSiang, the Lohit, the Dibang, and the Manas sediments, whichhave carbonate content of �3%–4%, CaO is appropriatelycorrected for carbonates to calculate CIA. One gneiss samplecollected from Guwahati has CIA �58. These lower CIAvalues indicate that they are less weathered. The lower CIA ofthe Brahmaputra can arise due to the presence of Transhima-layan Plutonic rocks in the basin of the Siang and the easterntributaries which have CIA values of �55 (from data Debon etal., 1986). The eastern and the Himalayan tributaries have CIAvalues 56–59 and 65–75, respectively, which underscores theimportance of Transhimalayan Plutonic rocks in lowering theCIA of the Brahmaputra. The CIA of the southern tributariesrange from 65 to 82, indicating that sediments of these tribu-taries are more weathered. This is also consistent with theinferences based on mineralogy and clay content of sedimentsand dissolved major ion composition of corresponding rivers.

4.3.2. Sources of Solutes in the Brahmaputra River System

Determining the sources of solutes, particularly silicate de-rived solutes, in the Brahmaputra river system is one of themajor objectives of this study as it has relevance to CO2

consumption and hence to global change. Rivers receive solutesfrom several sources, precipitation and weathering of silicateand carbonate rocks and dissolution of evaporites. To deter-mine the silicate and carbonate contributions to the major ionchemistry, inputs from rain and evaporites have to be properlyaccounted for. This is commonly achieved using Cl as index(Sarin et al., 1989; Dalai et al., 2002; Krishnaswami et al.,1999; Singh et al., 1998) and assuming Na � Cl. The Nacorrected for Cl (Na* � Nariv � Clriv) is generally taken to beof silicate origin. In the case of the Tsangpo, as mentionedearlier, however, there can be other sources of Na such assodium carbonate or borax in the evaporite deposits (Pascoe,
1963). In such cases, correction only for evaporites using Cl as
an index can yield an upper limit of Na from silicates. Rain-water contribution to other cations and anions are insignificant(e.g., Galy and France-Lanord, 1999) and therefore no correc-tion is applied.

Figure 7 presents a covariation between Na* and Si in therivers analysed. All data, except five, fall in a line, showinggood correlation (r2 � 0.86), attesting to the idea that Na* inthe rivers can serve as a silicate weathering index. The five datapoints which fall off the general trend and have “excess” Na*are for the Tsangpo, Tibetan tributaries, and one of the southerntributaries; in these rivers there could be additional sources forNa, such as evaporite minerals. Si/Na* for most of the rivers is�2.0 which is similar to the slope of the line, 1.84 (Fig. 7),indicating plagioclase weathering to kaolinite. The eastern trib-utaries, the Dibang, and the Lohit have Si/Na* �4. HighSi/Na* in these rivers is possibly due to weathering of sodium-deficient pyroxenes. These inferences, in general, are supportedby the mineralogy of sediments. It is assumed that all K is ofsilicate origin because evaporites and carbonates have verylittle K. Determination of silicate-derived Ca and Mg is proneto more uncertainties because these elements are supplied torivers from multiple sources: from silicates, carbonates, andevaporites. Even among silicates, Ca and Mg contributions canbe significantly different depending on their composition andweatherability. Therefore, to obtain silicate Ca and Mg contri-bution it is necessary to know the composition of silicate rocksof the basins. Singh and France-Lanord (2002) have shown,based on Sr and Nd isotopes, that the Brahmaputra sedimentsare a mixture of the Transhimalayan Plutonic Belt rocks andthose from the Himalaya in the proportion 30:70. Using the Ca,Mg, and Na abundances of these endmembers and above mix-ing proportion, the Ca/Na and Mg/Na ratios for the silicates of

Fig. 7. Si vs. Na*. Good correlation between Si and Na* (Nariv � Clriv)indicates that Na* can be used as a proxy of silicate weathering. Si/Na* formost of the rivers is 2.0, indicating plagioclase weathering to kaolinite,whereas for the Eastern tributaries it is �4.0, owing to weathering ofmafic/ultramafic rocks in their drainage. Rivers influenced by evaporitedissolution such as the Tsangpo, its tributaries in Tibet, and Dhansiri of thesouthern drainage fall off the line, indicating contribution of Na fromevaporites.

the Brahmaputra watershed were calculated. The elemental


ratios of various endmembers and the rivers which drain themare given in the Table 6. The endmember composition havebeen assigned an uncertainty of �50% owing to variation intheir mixing proportions and the approaches adopted for cal-culation (Krishnaswami et al., 1999). The Ca�Mg derivedfrom the silicates, (Ca�Mg)sil, is calculated using these end-member ratios (Table 6) and Na* of the river waters (Table 2)using the relation (Singh et al., 1998)

�Ca � Mg�sil � ��Ca � Mg� ⁄ Na� � Na*

The results show that (Ca�Mg)sil in these waters varies from 9%to 59% of total Ca�Mg (molar), with a mean of 31%. The totalcations derived from silicate, (�Cat)sil, is calculated by summingNa*, K, and (Ca�Mg)sil. These vary between 21% and 77% in therivers of the Brahmaputra River System, with a mean of 45%,nearly the same as (�Cat)sil of the Brahmaputra mainstream atChilmari, its outflow. The (�Cat)sil will be �45% � 11% if the50% uncertainties in the (Ca�Mg)/Na ratio of silicate endmember(Table 6) is considered. The (�Cat)sil generally follows the lithol-ogy of the individual drainage. The major uncertainty in the(�Cat)sil is from Ca/Na of the silicate endmember. Galy andFrance-Lanord (1999) assumed a value of 0.2 for Ca/Na of Hi-malayan silicate, whereas in this study it has been taken as 0.7. Ifthe value of 0.2 is used for Ca/Na for the Himalaya, the (�Cat)sil

will vary between 16% and 60%, with a mean of 38%. While thevalue of 0.2 is based on the Ca/Na of silicates from HH, LH, andleucogranites (Galy and France-Lanord, 1999), a value of 0.7 isderived for Ca/Na based on the Ca/Na of silicates (crystallines andsedimentaries) of LH and HH, of the soil profiles developed overLH silicates, and of stream flowing exclusively through silicateterrains (Krishnaswami et al., 1999).

Calculating (�Cat)sil in the Tsangpo (Tibet) following theabove approach has been hampered, because the validity ofusing Cl as an index for rain and evaporite contribution of Nais in doubt. In this drainage, in addition to rain and halites, Namay also be derived from sulphates, carbonates, and borates(Pascoe, 1963). Therefore in this drainage silicate cations havebeen calculated based on the overall relation (Fig. 7) betweenSi and Na*. Na* from silicates is estimated from the Na-Sirelation (Fig. 7) and the SiO2 concentration of �125 �M forthe Tsangpo. The calculated Na* is �45 �M. Based on thisNa*, a (�Cat)sil of 150 � 25 �M has been estimated for theTsangpo; this represents �17% of the total cations, indicatingsilicate cation contribution to total cation is quite low. This, asmentioned earlier, is because of supply of higher proportion of

Table 6. Endmember of

River

Lohit, DibangRanganadi, Jia Bhareli, Manas, Puthimari, Burhi Dihing, Dhansiri,

Kopili, TipkaiTsangpo, Siang, Subansiri, Brahmaputra mainstream

a Debon et al. 1986.b Krishnaswami et al., 1999.

alkalis from alkaline and saline salts of the basin.

4.3.3. Erosion in Various Subbasins

Singh and France-Lanord (2002), based on Sr and Nd isotopecomposition of sediments from the Brahmaputra drainage, in-ferred that this drainage undergoes differential physical ero-sion. As a part of the current study, chemical erosion in thevarious subbasins of the Brahmaputra watershed have beendetermined to quantify their total chemical and silicate erosionrates and to evaluate the factors influencing chemical erosion.Erosion rates of various subbasins of the Brahmaputra RiverSystem are given in Table 7 based on the TDS and (�Cat)sil

fluxes. For comparison, the erosion rates of a few other selectedbasins of the Himalaya and of other global basins (Sarin et al.,1989) are also given in this table. For the Eastern Syntaxis zoneof the Siang, TDS and (�Cat)sil fluxes are taken to be thedifference in TDS and (�Cat)sil fluxes from Tibet at Pai andthose at Pasighat. Table 7 shows that chemical erosion in theBrahmaputra watershed is not uniform and that it varies bymore than an order of magnitude among the various subbasins.Both total chemical erosion and silicate erosion rates in theEastern Syntaxis zone are the highest compared to other sub-

s for various drainages.

FormationCa/Na(molar)

Mg/Na(molar)

(Ca � Mg)/Na(molar)

imalaya 2.7 � 1.4a 2.5 � 1.3a 5.2 � 2.6aya 0.7 � 0.4b 0.3 � 0.2b 1.0 � 0.5

imalaya � Himalaya70)

1.3 � 0.7 1.0 � 0.5 2.25 � 1.1

Table 7. Chemical erosion and CO2 consumption rates in varioussub-basins of the Brahmaputra and selected basins of the worlda

Basin

TDS Flux

SilicateCationFlux

CO2

consumptionby silicate

weathering,105 moleskm�2 y�1(t km�2 y�1)

BrahmaputraTibet 40 1.3 0.7Eastern Syntaxis 304 38.0 19Eastern 185 18.1 9.5Himalaya 149 10.7 5.1Southern 237 22 12Brahmaputra 120 11.8 6Other Himalayan RiversGanga 72 7.9 3.8Indus 42 1.8 0.6Mekong 72 6.2 2.4Global RiversAmazon 35 2.2 0.5World Average 36 2.0 0.9

a Data for TDS of other rivers are from Sarin et al. (1989) andreferences therein, and those for Silicate Cation fluxes and CO2 con-

silicate

TranshHimal

Transh(30:

sumption due to silicate weathering are from Gaillardet et al. (1999)and Krishnaswami et al. (1999).


basins. The chemical erosion rate in this zone is even higherthan those reported for some of the basaltic terrains such as theReunion (Louvat and Allegre, 1997) and Iceland (Gislason etal., 1996). This zone is also characterized by very high physicalerosion, contributing about half of the sediments to the Brah-maputra at its outflow (Singh and France-Lanord, 2002). Thesouthern tributaries are also eroding quite rapidly but their totalimpact on the Brahmaputra River System is not significant,contributing only �8% of the discharge. The higher chemicalerosion in the Eastern Syntaxis zone is attributable to the higherphysical erosion in this region brought about by higher runoffand steep gradient of the Siang River. This inference attests tothe dependence of chemical erosion on runoff probably througha corresponding enhancement in physical erosion (Bluth andKump, 1994). The runoff as well as gradient of the Tsangpo arelow in Tibet, leading to lower physical and hence correspond-ingly lower chemical and silicate erosion rates.

4.3.4. �13C of Dissolved Inorganic Carbon (DIC)

A major uncertainty in calculating the silicate contribution ofmajor ions to rivers is associated with (Ca/Na) and (Mg/Na)ratios supplied to dissolved phase from various silicate end-members. Therefore, to determine silicate weathering contribu-tion to major ions of rivers, the potential of �13C has beenexploited (Galy and France-Lanord, 1999; Karim and Veizer,2000). Sources of alkalinity in river waters were quantifiedusing �13C of DIC. �13C of DIC depends on the source ofcarbon, which are CO2 from atmosphere and soil and fromcarbonates released during their weathering. The contributionof alkalinity from solution of atmospheric CO2 should benegligible in the Brahmaputra waters, which have �300 �MHCO3

�. The �13 of soil CO2 in the Brahmaputra basin is about�26‰ because of the dominance of C3 plants in this region(Burbank et al., 1993). This soil CO2 during weathering ofsilicates will produce alkalinity with �13C ��18% (Solomonand Cerling, 1987) and with ��9‰ in carbonate weathering,because half of the alkalinity during carbonate weathering isfrom carbonate and the other half from soil CO2. The BurhiDihing has the most depleted �13C, indicating highest silicate-derived alkalinity. This is also consistent with its low Ca/Na*showing higher contribution of silicate cations. In contrast, theManas has more enriched �13C and high Ca/Na*, suggestinghigher contribution from carbonates. Based on calculated end-member �13C values for silicate- and carbonate-derived DICand the measured �13C of DIC in the Brahmaputra RiverSystem (�10.1‰ to �17.2‰, Table 2), it can be estimated thaton average about half of the alkalinity in these waters is derivedfrom silicate weathering and the balance is supplied by carbon-ate weathering. The silicate alkalinity estimated using �13C ofriver water roughly balances the silicate cations calculatedbased on the major ion data in earlier section.

4.3.5. CO2 consumption by silicate weathering

One of the objectives of this study is to calculate CO2

consumption due to silicate weathering, because this is a keyparameter to understand the coupling between chemical weath-ering in the Himalaya and drawdown of atmospheric CO
2
(Raymo and Ruddiman, 1992). The rates of CO2 consumption

by silicate weathering in the Brahmaputra drainage and for itsvarious subbasins have been calculated based on the cationsderived from silicates and assuming that all weathering resultsonly from CO2. The CO2 consumption by silicate weatheringfor the entire Brahmaputra basin is calculated to be �6 � 105

moles km�2 y�1 based on the monsoon sample from Dhubri.This is comparable to the estimate based on Si (1 mol Si � 2mol of CO2; Huh et al., 1998) of �4.5 � 105 moles km�2 y�1.Further, the discharge-weighted CO2 consumption at Guwahaticalculated from the time series data of chemical composition is�7 � 105 moles km�2 y�1, similar to estimate of �6.4 � 105

moles km�2 y�1 based on �13C of DIC and marginally higherthan the value of 5.3 � 105 moles km�2 y�1 for the monsoonsample and (Table 2). These values of CO2 consumption for theBrahmaputra, though, seem higher than those obtained for theAlaknanda, Bhagirathi, and the G-B (Krishnaswami et al.,1999); they fall within the range for the rivers in the Himalaya,2–7 moles km�2 y�1 (Dalai et al., 2002). Further, the CO2

consumption due to silicate weathering in the Himalayan drain-age of the Brahmaputra is closer to those reported for theAlaknanda and the Bhagirathi (Krishnaswami et al., 1999). Thevalue �6 � 105 moles km�2 y�1, in this study may be anupper limit as part of silicates may also be weathered by theH2SO4 produced from pyrite oxidation. However the observa-tion that HCO3/SO4 (equivalent) averages �6, and that Ca/SO4

is high, �4, it seems that weathering mediated by H2SO4 maynot be significant in this basin. Negligible contribution ofsilicate alkalinity to these rivers due to weathering by H2SO4 isalso supported by the CO2 consumption rate estimate of �6.4� 105 moles km�2 y�1 based on �13C of DIC at Guwahatiwhich is similar to the estimate by major ion. The annual CO2

consumption by silicate weathering in the Brahmaputra basin is�2.72 � 1011 moles y�1 at Dhubri. This estimate is comparableto those reported for the Brahmaputra using Si content by Galyand France-Lanord (1999), however, it is higher than their esti-mate based on major ions due to difference in endmember esti-mates of Ca/Na. The silicate weathering in the Brahmaputra RiverSystem consumes � 2%–3% of the global consumption of CO2

by silicate weathering (Amiotte Suchet et al., 2003; Gaillardet etal., 1999) with only �1.6% of the global water discharge.

Chemical erosion among the different subbasins of the Brah-maputra is highly variable and therefore their CO2 consumptionrates also are expected to show similar variability. The CO2

consumption for subbasins (Table 7) vary between 0.7 and 19� 105 moles km�2 y�1, with the Eastern Syntaxis basin havingthe highest consumption, 19 � 105 moles km�2 y�1. This ratefor the eastern syntaxis is much higher than the Brahmaputraaverage but is similar to the CO2 consumption rates reportedfor the basaltic terrains such as the Reunion (Louvat andAllegre, 1997) and the Iceland (Gislason et al., 1996; Fig. 8).This is an important result and suggests that granite/gneissesand mafic rocks of the Brahmaputra river basin, can also erodeas rapidly as basalts from other regions under “favorable”conditions. A similar observation is also made by Das et al.(2005). The lowest CO2 consumption by silicate weatheringamong the subbasins of the Brahmaputra is for the Tibetansubbasin, attributable to factors such as its flat topography,
lower runoff, and colder climate.


4.4. Controls on Chemical Erosion in theBrahmaputra Drainage

It has been suggested that several factors control chemicalerosion. These include temperature, physical erosion, run off,lithology, elevation, vegetation ,and tectonic activities (Velbel,1993; White and Blum, 1995; Berner and Berner, 1997; Ed-mond and Huh, 1997; Huh and Edmond, 1999; Millot et al.,2002; France-Lanord et al., 2003; Dalai et al., 2002).

4.4.1. Runoff vs. Chemical Erosion

Runoff has been suggested as a major factor controllingchemical erosion, (Bluth and Kump, 1994; France-Lanord,2003; Gaillardet et al., 1999). The eastern Himalaya receivesheavy rainfall during both SW and NE monsoons, and thereforerunoff in this region varies both in space and time. This makesthe Brahmaputra drainage a suitable location to assess the impactof runoff on chemical weathering. The TDS flux (t km�2 y�1), anindex of chemical erosion, has been plotted against runoff inFigure 9a. TDS data during SW monsoon has been considered forthis plot because most of the annual river discharge occurs in thisperiod. The results show a good positive correlation betweenrunoff and the TDS flux (r2 � 0.81), attesting to the earliersuggestions that runoff exerts significant control over chemicalerosion. A similar trend is also observed if (�Cat)sil flux isplotted against runoff (Fig. 9b). In the Brahmaputra basin,higher runoff causes higher physical erosion exposing fresh

Fig. 8. CO2 consumption rate due to silicate weathering for varioussubbasins and for the entire Brahmaputra basin. Values for the Ganga,the Yamuna, Reunion, and Iceland are taken from Dalai et al. (2002),Louvat and Allegre (1997), and Gislason et al. (1996), respectively.The rate of CO2 consumption due to silicate weathering for the EasternSyntaxis region is notably high and comparable to the basaltic terrainsuch as those for Reunion and Iceland. Higher CO2 consumption in thisriver section is due to higher physical weathering caused by higherrunoff and stream gradient (stream power) of this zone.

surfaces for chemical erosion. Thus, runoff and physical ero-

sion significantly influence chemical erosion of the Brahmapu-tra basin.

It can be argued that the above inference is not unexpected,because runoff is a multiplication factor for calculating chem-ical erosion rates (CER � TDS � runoff). In samples plottedin Figure 9a, the runoff varies by more than an order ofmagnitude among the various river basins whereas the TDSabundance variation is only within a factor of �2, indicatingthe importance of runoff in controlling CER.

4.4.2. Physical vs. Chemical Erosion

The role of physical erosion as a “driver” of chemical ero-sion has been established in many river basins (Bluth andKump, 1994; Millot et al., 2002). Physical erosion is quite highin the rivers of the Himalaya and in the Brahmaputra in par-ticular. Figure 10 is a scatter plot of physical erosion rates ofthe various subbasins of the Brahmaputra vs. their chemicalerosion rates. Physical erosion rates (PER) have been calcu-

Fig. 9. TDS (a) and (�Cat)sil (b) fluxes vs. runoff. The strongcorrelation between TDS flux, (�Cat)sil flux and runoff indicate thatrunoff is major controlling factor of the chemical erosion.

lated based on the proportion of sediment supplied by these


basins (Singh and France-Lanord, 2002) and a sediment dis-charge of 700 million t y�1 for the Brahmaputra. Sediment fluxfrom the southern subbasin is not significant and therefore notplotted. Figure 10 shows a strong coupling between physicaland chemical erosion, with the following power law:

CER � 4.61(PER)0.44

Millot et al. (2002) also report a power law relation between thechemical and physical erosion rates for a few granitic andbasaltic river basins; however, values of the constant and ex-ponent of their relation are different from those derived abovefor the Brahmaputra. Comparison of these relations indicatesthat for a given PER the CER will be more in the case of theBrahmaputra compared to other river systems (Millot et al.,2002). In the lower PER range this difference is particularlysignificant, possibly owing to the higher temperature of thetropical region.

From the above discussion and positive correlation betweenrunoff and PER (Singh and France-Lanord, 2002) it can beinferred that higher runoff causes higher physical erosionwhich provides more fresh material for chemical erosion.Therefore runoff exerts the first-order control on the chemicalerosion. Further, the data show that enhanced runoff and higherrelief are the primary variables that promote physical erosion,which in turn increases the chemical erosion.

4.5. Chemical Fluxes from the Brahmaputra

The bimonthly chemical compositions of the Brahmaputra atGuwahati coupled with monthly discharge data for Pandu (nearGuwahati, GRDC website) have been used to calculate theannual chemical fluxes from the Brahmaputra at Guwahati. Thecalculated fluxes of various cations and TDS are given in Table 8.

Chemical composition of the water of the Brahmaputra at

Fig. 10. Physical vs. chemical erosion. A significant coupling be-tween chemical and physical erosion is observed, which is related by apower law. The chemical erosion in the Brahmaputra watershed iscontrolled by the physical erosion, which is primarily controlled byrunoff and relief.

Guwahati are not measured for August and September. To

calculate the discharge-weighted annual fluxes of cations andTDS, July concentrations are assumed for the waters of Augustand September. Similar to July, August and September fall inSW monsoon season; therefore this assumption is expected tobe valid. The estimated fluxes are higher for K, Ca, and TDSand lower for Na and Mg than those reported by Sarin et al.(1989) and Galy and France-Lanord (1999). In our study fluxeshave been calculated at Guwahati, whereas in Sarin et al.,(1989) and in Galy and France-Lanord (1999) it is at theoutflow. The difference arises from the temporal variabilityobserved in the concentration of cations, TDS, and dischargeand reinforces the need for temporal data of the river waterchemistry and its discharge for calculating fluxes. Based onthese fluxes, it can be concluded that the Brahmaputra drainageis eroding fast chemically, �2 to 3 times faster than the Gangadrainage (Sarin et al., 1989; Galy and France-Lanord, 1999)and �5 times higher than the global average (Sarin et al.,1989).

5. CONCLUSIONS

Chemical erosion in the Brahmaputra drainage transports�73 million tons of dissolved material annually to the worldoceans; representing �4% of the total dissolved load via worldrivers. About half of the dissolved cations are of silicate origin,which is higher than that reported for the Ganga basin. Suchcharacteristic difference could arise owing to diverse litholo-gies of the two basins: Ganga basin is relatively dominated bysedimentaries (containing more easily weatherable carbonates),whereas the Transhimalayan mafic/ultramafic rocks occur inthe Brahmaputra drainage. The chemical erosion rate in theBrahmaputra (mainly governed by runoff and physical erosion)is �4–5 times higher than the global average. However, thereexists a significant spatial variability in the chemical erosion;some of the regions are eroding relatively faster, particularlythe region around the Eastern Syntaxis.

Despite the fact that higher chemical erosion rates are rep-resentative in the Brahmaputra basin compared to that in theGanga, the intensity of chemical weathering, is relatively lowerin the Brahmaputra basin. The cause for this is attributed toenhanced physical erosion and rapid sediment transport in theBrahmaputra drainage. CO2 consumption due to silicate weath-ering in the Brahmaputra drainage is �6.0 � 105 moles km�2

Table 8. Annual fluxes of cations and TDS fromthe Brahmaputra.

Cations andTDS

Flux

a b

Na 45 116K 41 107Mg 77 202Ca 308 803TDS 73 189

a Na, K, Mg, and Ca in 109 mol y�1; TDS in 106

t y�1.b Na, K, Mg, and Ca in 103 mol km�2 y�1; TDS

in t km�2 y�1.

y�1, which is a few times higher than the global average. The


Brahmaputra drainage accounts for �2%–3% of global CO2

consumption due to silicate weathering, marginally higher thanits contribution to water discharge �1.6%. Analogous to chem-ical and silicate erosion rates, higher CO2 consumption rate inthe Eastern Syntaxis zone is comparable to some of the globalbasaltic terrains.

Acknowledgments—We thank S. Krishnaswami for the useful discus-sion throughout the course of this study. The analytical help providedby A. K. Sudheer was extremely valuable. We are greatly benefitedfrom the comments provided by the three anonymous reviewers. Theevaluation report provided by Lee Kump was helpful.

Associate editor: Lee Kump

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APPENDIX

Ca Flux ofSiang at Pasighat � 3 � 106 t y�1

Dibang at Sadeya � 0.3 � 106 t y�1

Lohit at Sadeya � 0.7 � 106 t y�1

Total Ca flux � 4 � 106 t y�1 (i)Ca flux leaving Dibrugarh � 7 � 106 t y�1 (ii)Ca flux gained between Pasighat, Sadeya and Dibrugarh � �3

� 106 t y�1 (iii)Particulate Ca flux (following data from Singh and France-Lanord,

2002)Detrital carbonate (calcite and dolomite) content ofSiang at Pasighat � 4 wt%Dibang at Sadeya � 2 wt%Lohit at Sadeya � 3 wt%Brahmaputra at Dibrugarh � �0 wt%Flux of sediment from Siang � �50%Flux of sediment from Dibang and Lohit � �10%Total sediment flux from the Brahmaputra � 700 � 106 t/yrCa flux at Dibrugarh gained due to dissolution of detrital carbonate

during transport from Pasighat and Sadeya to Dibrugarh � �3–4� 106 t y�1 (iv)

Therefore, from (i), (ii), (iii), and (iv),Ca flux leaving Dibrugarh Total Ca flux coming from Siang,

Dibang, and Lohit � Ca flux gained due to dissolution of detrital
carbonate between Pasighat, Sadeya, and Dibrugarh.

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