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

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1 doi: /j.gca Chemical erosion in the eastern Himalaya: Major ion composition of the Brahmaputra and 13 C of dissolved inorganic carbon SUNIL K. SINGH, 1 *M.M.SARIN, 1 and CHRISTIAN FRANCE-LANORD 2 1 Physical Reasearch Laboratory, Navrangpura, Ahmedabad , India 2 CRPG-CNRS, Vandoeuvre Les Nancy, Cedex, France Geochimica et Cosmochimica Acta, Vol. 69, No. 14, pp , 2005 Copyright 2005 Elsevier Ltd Printed in the USA. All rights reserved /05 $ (Received September 27, 2004; accepted in revised form February 18, 2005) Abstract Major ion composition of waters, 13 C of its DIC (dissolved inorganic carbon), and the clay mineral 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 processes in the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputra mainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index of alteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in the Ganga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputra basin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicate erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rate of 300tkm 2 y 1, one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes much slower chemical erosion ( 40tkm 2 y 1 ). The abundance of total dissolved solids (TDS, mg/l) in the time-series samples collected over a period of one year shows variations in accordance with the annual discharge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cyclic component) shows a strong positive correlation with Si, indicating their common source: silicate weathering. Estimates of silicate cations (Na sil K sil Ca sil Mg sil ) suggest that about half of the dissolved cations in the Brahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO 2 consumption rate due to silicate weathering in the Brahmaputra watershed is moles km 2 y 1 ; whereas that in the Eastern Syntaxis subbasin is moles km 2 y 1, similar to the estimates for some of the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of most intensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern Himalaya. Copyright 2005 Elsevier Ltd 1. INTRODUCTION The Ganga-Brahmaputra (G-B) is one of the major global river systems, ranking first in sediment supply and fourth in water discharge (Milliman and Meade, 1983; Milliman and Syvitski, 1992; Ludwig and Probst, 1998). The sediment flux from the G-B may account for as much as 15% of the global sediment discharge if its bed-load equals that of its suspended load (Galy and France-Lanord, 2001). This makes the G-B system a significant component of the sediment budget of the oceans. In terms of water discharge, G-B accounts for 3% of global riverine water supply (Hay, 1998 and references therein). Among these two major rivers, the Ganga has been studied in far greater detail for its chemical and isotopic composition and their impact on ocean chemistry. These studies have brought out the role of these rivers, particularly the Ganga, in contributing to Sr and Os isotope composition of seawater during the Cenozoic and on the CO 2 drawdown on million-year time scales (Galy and France-Lanord, 1999; Krishnaswami et al., 1992; Palmer and Edmond, 1989; Harris et 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 (sunil@prl. ernet.in) Brahmaputra, to the Bay of Bengal, exceeded that of the Ganga (Hay, 1998). The limited studies on the Brahmaputra (Sarin et al., 1989; Galy and France-Lanord, 2001) indicate that the eastern Himalaya, encompassing the Brahmaputra drainage, is eroding more rapidly than the central and western Himalaya which make up the Ganga drainage. This difference has been attributed to higher rainfall and runoff in the eastern sector (Sarin et al., 1989; Galy and France-Lanord, 2001). More recently, chemical, isotopic, and mineralogic data on sediments from the Brahmaputra watershed show that the basin is characterized by differential erosion and that the sediment budget of the Brahmaputra is dominated by material derived from the Eastern Syntaxis region (Singh and France-Lanord, 2002; Singh et al., 2003; Garzanti et al., 2003). These results are also supported by model calculations (Finlaysan et al., 2002). To gain better understanding of the relative erosion rates within the Brahmaputra basin, a detailed investigation on the chemical and isotope composition of the water and sediments was undertaken. The source apportionment of the solutes in the Brahmaputra River and its tributaries from silicates and carbonates is an integral part of this study for determining the CO 2 consumption rates. Recently, several studies have been carried out to quantify the sources of solutes in the Ganga drainage (Singh et al., 1998; Galy et al., 1999; Krishnaswami et al., 1999; Dalai et al., 2002; Bickle et al., 2003). In this context, studies in the

2 3574 S. K. Singh, M. M. Sarin, and C. France-Lanord Fig. 1. Map of the Brahmaputra watershed with sampling locations. Brahmaputra was sampled from Pasighat in India to Yamuna Bridge in Bangladesh. Area of study is shown in the inset. Sample numbers are indicated based on sampling in one season, and all the numbers are preceded by BR. Various subbasins are shown by dashed lines. Brahmaputra system are rather limited (Krishnaswami et al., 1999; Galy and France-Lanord, 1999). In this study, attempts have been made to determine the contribution of solutes to the Brahmaputra River System from silicate weathering and to estimate the associated CO 2 consumption rates. In addition, these data represent a first attempt to document the temporal variation in the major ion chemistry of the Brahmaputra mainstream based on a biweekly sampling carried out over a period of one year, including the monsoon. Thus, these data provide better estimates of annual fluxes and seasonal variations in silicate weathering rates. 2. BRAHMAPUTRA RIVER SYSTEM: HYDROLOGY AND GEOLOGY 2.1. The Brahmaputra River The Brahmaputra River is known by different names along its course. It originates in the Kailash Mountain in the Transhimalaya and flows with a very gentle slope eastward 1200 km in Tibet as Yarlung Tsangpo or Tsangpo. The Tsangpo takes a U-turn after Pai (Fig. 1) around Namche Barwa, the Eastern Syntaxis, where it makes the deepest gorge of the world and turns south to enter Arunachal Pradesh, where it acquires the name Siang or Dihang. This part of the river with the deepest gorge has a very steep slope ( 30 m/km) causing very turbulent and rapid flow and intense physical erosion (Singh and France-Lanord, 2002). Immediately after Pasighat, the Siang River turns in SW direction and enters the Assam Plain, where it is called the Brahmaputra and flows in WSW direction as a wide and deep braided river. The Brahmaputra acquires a width of 20 km and depth of 35 m at some locations in the Assam Plain. The Brahmaputra turns south near Dhubri at the Indo-Bangladesh border and flows as the river Jamuna until it meets the Ganga at Arichaghat. The Brahmaputra River receives many tributaries along its course. In Tibet, the Tsangpo receives the Lhasa He (Zangbo), Doilung, and Nyang Qu (Fig. 1; Guan and Chen, 1981; Hu et al., 1982) in addition to tributaries from the northern slope of the Himalaya. After Pai, the river Parlung Zangbo (Fig. 1; Guan

3 Chemical erosion in the eastern Himalaya Table 1. Sample details: Locations, Water discharge, Drainage, Temperature and ph Sample code River (location) Longitude (E) Latitude (N) Month & year of deg min deg min sampling Discharge a, 10 9 m 3 Area a,10 3 y 1 km 2 Temp, C ph Brahmaputra Mainstream Tsangpo at Pai BR-59 Siang or Dihang (Pasighat) July BR-18 Brahmaputra (Dibrugarh) Oct BR-28 Brahmaputra (Tezpur bg.) Oct BR-65 Brahmaputra (Tezpur bg.) July BR-5 Brahmaputra (Guwahati) Oct BR-51 Brahmaputra (Guwahati) July BR-73 Brahmaputra (Dubri) July Brahmaputra (Chilmari) b Aug BR 200 Brahmaputra(Jamuna bg.) July 2002 Eastern Tributaries BR-14 Dibang Oct BR-16 Lohit Oct Himalayan Tributaries BR-20 Subansiri Oct BR-61 Subansiri July BR-24 Ranga Nadi Oct BR-57 Ranga Nadi July BR-26 Jia Bhareli Oct BR-63 Jia Bhareli July BR-75 Tipkai July BR-32 Manas Biki Oct BR-71 Manas Biki July BR-34 Puthimari Oct BR-69 Puthimari July Southern Tributaries BR-10 Dhansiri Oct BR-12 Buri Dihing Oct BR-30 Kopili Oct BR-67 Kopili July a Rao, 1979; Goswami, 1985; and GRDC website ( The values are average of several years of data. b Galy and France-Lanord, and Chen, 1981) merges with it. The slope of this tributary is very high and comparable to that of the Siang in this section. In the Assam plain the Brahmaputra receives the Dibang and the Lohit from the east and the Subansiri, the Ranganadi, the Jia Bhareli, the Puthimari, the Manas, and the Tipkai from the north and the Burhi Dihing, the Dhansiri, and the Kopili from south (Fig. 1). The Tista is another northern tributary of the Brahmaputra which merges with it in Bangladesh (Fig. 1). The water discharge of the Brahmaputra mainstream and many of its tributaries are given in Table 1. The major source of water in the Brahmaputra is rainfall, though meltwater and groundwater contributions are also important. In the Tsangpo in Tibet, for example, meltwater, groundwater and rainfall contributions are roughly the same (Guan and Chen, 1981). The runoff in the Tsangpo drainage is 300 mm yr 1 which increases by more than order of magnitude, to 5000 mm yr 1, for the Siang in Arunachal Pradesh. Runoff in the Himalayan drainage for the northern tributaries in the Assam Plain is mm yr 1 and for the eastern tributaries it is 3000 mm yr 1. The southern drainage is exposed to heavy rainfall and the runoff in this region is 4000 mm yr 1. The major contributor to the Brahmaputra discharge is rainfall during SW monsoon (July to September). Monthly discharge of the Brahmaputra at Bahadurabad, based on the average of many years, is shown in Figure 2. The monthly water discharge pattern of the Brahmaputra at Bahadurabad reflects the monsoon with significant temporal variation. It varies from 3300 m 3 /s in February to m 3 /s in July. The discharge in February is the lowest owing to paucity of rain and less meltwater contribution. This trend is almost similar to that at Fig. 2. Monthly discharge of the Brahmaputra at Bahadurabad in Bangladesh. Most of the discharge in the Brahmaputra occurs during monsoon season, i.e., June to September.

4 3576 S. K. Singh, M. M. Sarin, and C. France-Lanord Guwahati in India, where biweekly samples have been collected. The Brahmaputra System drains a total area of 630,000 km 2.Of the total drainage, about one third is in Tibet with an average elevation of 5000 m. The Tibetan drainage contributes 10% of the water discharge of the Brahmaputra at its mouth. The Brahmaputra drains total area of 200,000 km 2 in the plains of Assam and the Bangladesh and its Himalayan tributaries occupy an area of 120,000 km 2 in the Himalaya. The two eastern tributaries, the Lohit and the Dibang flowing through the Mishmi Hills have drainage area 50,000 km Geology The drainage basin of the Brahmaputra System can be divided 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 Parlung Tsangpo rivers between Pai and Pasighat, (3) the Mishmi Hills, the drainage of the two eastern tributaries, the Lohit and the Dibang, (4) the Himalaya, the drainage of the northern tributaries of the Brahmaputra in the Assam and the Bangladesh Plains (the Renganadi, the Jia Bhareli, the Puthimari, the Manas, the Tipkai), and (5) the Indo-Burmese Ranges, the drainage of southern tributaries (the Burhi, the Dihing, the Dhansiri, and Kopili) of the Brahmaputra in the Assam and the Bangladesh plains. 1. Tibet: In upper reaches the Tsangpo drains turbidites and ophiolites of the Indus-Tsangpo Suture Zone. The tributaries from the northern slope of the Himalaya drain the Tethyan Sedimentary Sequences and the gneiss zone. The tributaries from Tibetan Plateau, the Doilung, Zangbo, and Nyang Qu predominantly drain Transhimalayan gabbroic to granodioritic batholiths. The basins of these tributaries also contain evaporite deposits (Hu et al., 1982; Pande et al., 1993; Pascoe, 1963). 2. The Eastern Syntaxis: The rocks near the Eastern Syntaxis are highly metamorphosed. At its core, gneisses of the Indian Plate have been exhumed from below the Transhimalayan Plutonic Belt (TPB; Burg et al., 1998). In this zone the calc-alkaline plutons of the TPB are surrounded by quartzites, phyllites, and marbles (Burg et al., 1998). Discrete lenses of metabasites and serpentinites occur in these areas, 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, and Lohit. 3. The Mishmi Hills: The two eastern tributaries, the Lohit and the Dibang, flow through the Mishmi Hills composed of calc-alkaline diorite-tonalite-granodiorite complexes and tholeiitic metavolcanic rocks (Kumar, 1997). It represents the eastern continuation of the TPB. The Tiding Suture present in this area marks the boundary between the TPB and the Himalaya in this section. 4. The Himalaya: The geology of the eastern Himalaya, through which the northern tributaries of the Brahmaputra System in Assam Plain, such as the Subansiri, Renganadi, Jia Bhareli, Puthimari, and Manas, flow, is similar to those of 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 east to west in this watershed (Singh and France-Lanord, 2002; Robinson et al., 2001). The Higher Himlayan rocks consist mainly of schists and marbles with amphiboles at some locations. In Bhutan and Sikkim, the Manas and the Tista drain through metamorphic rocks of the Higher Himalaya. The Lesser Himalaya in the Brahmaputra System drainage is composed mainly of quartzites and schists. Precambrian limestones, dolostones, shales, and quartzites along with orthogneiss bodies and dolerite sills are exposed in the Lesser Himalaya. The Siwalik is discontinuous in the eastern section of the Himalaya. It includes a thick section of Neogene molasses. Continuing uplift and deformation are evident in this section by the presence of tilted gravel terraces and steep fault scarps (Nakata, 1989). Apart from these rocks of the Himalaya the basalts of the Abor volcanics are present in the Himalayan drainage of the Siang (Jain and Thakur, 1978). The northern tributaries of the Brahmaputra in the Assam plain drain through the southern slope 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 pelagic sediments overlain by thick turbidites associated with ophiolites. The Dhansiri and the Kopili also drain the Indian basement of the Shillong Plateau and the Mikir Hills (Kumar, 1997). 3. SAMPLING AND METHODOLOGY Water and sediment samples from the Brahmaputra mainstream and its major tributaries draining between Pasighat in Arunachal Pradesh to Dhubri at the Indo-Bangladesh border (Fig. 1) were collected during two seasons: the SW monsoon and the postmonsoon. The monsoon samples were collected during the month of July, representing peak discharge, and the postmonsoon samples were collected in October (median flow). The Brahmaputra mainstream was sampled at five stations: Pasighat, Dibrugarh, Tezpur, Guwahati, and Dhubri (Fig. 1). One monsoon sample of the Brahmaputra (BR 200) was collected at Jamuna Bridge in Bangladesh. Samples were collected from the midchannels, accessing either from a boat or from road bridges. The collection procedures for sediment have been described in detail in Singh and France-Lanord (2002), Singh et al. (2003), and Garzanti et al. (2004). Soon after their collection, two separate aliquots of 500 ml water were filtered using 0.2- m nylon membrane Millipore filters. One of the filtered aliquots was acidified with double-distilled HNO 3 for cations, trace metal, and Sr analysis, and the other aliquot was preserved unacidified for anion measurements. In addition, one sample of 250 ml water was collected and stored unfiltered for alkalinity measurements. The Brahmaputra mainstream was also sampled at Guwahati (Fig. 1) at an interval of 15 days over a period of 10 months to 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 for further analysis. Alkalinity was measured by acid titration; Cl, NO 3, and SO 4 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), is about 5%. Accuracy of measurements for various elements was checked by measuring dilute solutions of USGS rock standards, W-1 and G-2, and also by analyzing river waters of known elemental abundances (Sarin et al., 1992). Clay mineral analyses of selected sediment samples were done at CRPB-CNRS, France (Bartoli, 1991). Quantification of clay minerals was done by analyzing 2- m size fraction by granulometry. 13 C of dissolved inorganic carbon was measured using a modified VG 602D isotope ratio mass

5 Chemical erosion in the eastern Himalaya 3577 Table 2. Major ion composition and 13 C of DIC of waters of the Brahmaputra River System. Sample code River (location) Na Na* K Mg 2 Ca 2 Cl NO 3 SO 4 2 HCO 3 mol L 1 SiO 2 TDS, mgl 1 13 C (DIC), Brahmaputra mainstream Tsangpo (South Lhasa) a Tsangpo (South Lhasa) a Siang (Pai) b BR-59 Siang or Dihang (Pasighat) BR-18 Brahmaputra (Dibrugarh) BR-28 Brahmaputra (Tezpur bg.) BR-65 Brahmaputra (Tezpur bg.) Brahmaputra (Guwahati) a BR-5 Brahmaputra (Guwahati) BR-51 Brahmaputra (Guwahati) BR-73 Brahmaputra (Dhubri) Brahmaputra (Chilmari) c BR 200 Brahmaputra (Jamuna bg.) Tibetan Tributaries to Tsangpo Zangbo (Lhasa) a Doilung (Lhasa) a Eastern Tributaries BR-14 Dibang BR-16 Lohit Himalayan Tributaries BR-20 Subansiri BR-61 Subansiri BR-24 Ranga Nadi BR-57 Ranga Nadi BR-26 Jia Bhareli BR-63 Jia Bhareli BR-75 Tipkai BR-32 Manas Biki BR-71 Manas Biki BR-34 Puthimari BR-69 Puthimari Southern Tributaries BR-10 Dhansiri BR-12 Buri Dihing BR-30 Kopili BR-67 Kopili a Hu et al., b Chen and Guan, c Galy and France-Lanord, Na* (Na riv Cl riv ). spectrometer following the procedure of Galy and France-Lanord (1999). 4. RESULTS AND DISCUSSION Sampling details, temperature, ph, discharge, and drainage are given in Table 1. Major ion composition of the Brahmaputra mainstream and its various tributaries are presented in Table 2, 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 ) and temperature varied from 19 C to 32 C depending on the season and on the time of sampling. In some of the following discussions, the dissolved Na has been corrected for contribution from rainwater and halites by subtracting Na equivalent to dissolved Cl from them (Na* Na riv Cl riv ). Other cations and anions are not corrected for rainwater contribution because they are insignificant (e.g., Galy and France-Lanord, 1999) Spatial and Temporal Variability in Total Dissolved Solids The total dissolved solids (TDS Na K Ca Mg Cl SO 4 HCO 3 SiO 2 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 at Dibrugarh during postmonsoon. In general, downstream variation in TDS of the Brahmaputra mainstream during the current sampling is small, and center around mg L 1 (Fig. 3). Three data points for the Tsangpo in Tibet show higher TDS, mg L 1 (Hu et al., 1982; Chen and Guan, 1981). This is an indication that in the Tsangpo the salinity is higher and it gets diluted downstream by rainfall. The tributaries of the Brahmaputra, in contrast, show factors of 3 4 variability in their TDS, from 50 to 182 mg L 1 (Table 2). The monsoon samples, as expected, are dilute in terms of their TDS compared to those of postmonsoon.

6 3578 S. K. Singh, M. M. Sarin, and C. France-Lanord Table 3. Temporal variation in chemical composition of the Brahmaputra at Guwahati. Date, month/day/ year Na Na* K Mg 2 Ca 2 Cl NO 3 mol L 1 SO 4 2 HCO 3 SiO 2 TDS, mg L 1 10/24/ /15/ /30/ /15/ /31/ /16/ /31/ /15/ /1/ /15/ /1/ /19/ /30/ /15/ /31/ /15/ /1/ /26/ Na* Na riv Cl riv. Table 5 is a summary of available data on TDS of the Brahmaputra along its course. These data provide information on spatial and temporal variation in TDS over about two decades. The TDS of the Brahmaputra measured in this study at Jamuna Bridge and Dhubri during monsoon (105 and106 mg L 1, respectively) is identical to the value of 105 mg L 1 reported by Galy and France-Lanord (1999) at Chilmari a few kilometers downstream of Dhubri collected during monsoon of Similarly, the TDS of 101 mg L 1 at Guwahati (Hu et al., 1982) during monsoon 1979 compares well with value of 112 mg L 1 obtained in this study. Sarin et al. (1989) measured the chemical composition of the Brahmaputra mainstream in samples collected from four sites during The TDS measured at Guwahati during April 1982 was 91 mg L 1 compared to values of 140 and 111 mg L 1 measured in this study for the same month. Similarly, the value for December in 1982 was 144 mg L 1, marginally lower than the values of 164 and 171 mg L 1 in These results show that over two decades the TDS in the Brahmaputra does not show any systematic and major variations, the variability being 35% or less; such interannual variations can result from associated changes in runoff. TDS of the Brahmaputra river system is generally lower than those in samples from the lower reaches of the Ganga, the Yamuna, the Karnali, and the Narayani rivers but similar to those in the headwaters of these rivers (Sarin et al., 1989; Sarin et al., 1992; Dalai et al., 2002; Galy and France-Lanord, 1999). This is most likely due to differences in drainage lithology and runoff among these basins. For example, a lower proportion of the Lesser Himalayan sedimentaries containing more easily weatherable carbonates and evaporites in the drainage basin of the Brahmaputra System can cause lower TDS in these rivers. Similarly, higher runoff in the Brahmaputra watershed can affect weathering intensity and also 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. The higher TDS in the Tibetan region is most likely due to the presence of easily weatherable evaporites in the basin (Pascoe, 1963). The chemistry of its tributaries, e.g., the Zangbo and the Doilung (Hu et al., 1982), from this region supports this inference (Table 2 and discussion). Also the lower runoff of this region helps maintain higher TDS. Comparison of TDS of the Tsangpo at Pai (Chen and Guan, 1981), before it enters the Eastern Syntaxis, and at Pasighat after its exit shows that TDS has decreased from 150 mg L 1 to 95 mg L 1 (Fig. 3). This decrease can result from changes in both geology and climate. Between Pai and Pasighat the Brahmaputra predominantly drains silicates and its runoff increases by an order of magnitude which can cause dilution and also affect weathering intensity. Downstream of Pasighat, the two tributaries, Dibang Table 4. Abundance and composition of clay in sediments of the Brahmaputra River System. Sample code River (location) Clay, wt% Clay composition BR 19 Brahmaputra (Dibrugarh) 1.7 Vermicullite, illite, chlorite BR 29 Brahmaputra (Tezpur) 2.6 Vermiculite, illite, chlorite BR 9 Brahmaputra (Guwahati) 3.2 Vermiculite, illite, chlorite BR 21 Subansiri 1.4 Vermiculite, illite, montmorillonite BR 25 Renganadi 3.9 Illite, chlorite, Montmorillonite BR 33 Manas 3 Illite, chlorite, montmorillonite BR 21 Kopili 21 Montmorillonite, illite, chlorite

7 Chemical erosion in the eastern Himalaya 3579 Fig. 4. Temporal variation of various major ions of the Brahmaputra at Guwahati. All the concentrations are in M except for TDS in mg L 1. One sample collected during SW monsoon has elevated concentration owing to flash flooding in the Tsangpo. Fig. 3. Downstream variation in TDS of the Brahmaputra. TDS is highest in Tibet and decreases after the river crosses the Eastern Syntaxis. In the Assam plain TDS is almost constant despite receiving many tributaries with variable TDS. More than one value of TDS at the same location represents samples from different seasons (Table 2). and Lohit meet the Brahmaputra from the east (Fig. 1). At Dibrugarh, the TDS of the Brahmaputra is 35% more (128 mg L 1 ) than at Pasighat (95 mg L 1 ; Fig. 3, Table 2). This increase in TDS can be due to temporal variation in TDS, because the Pasighat was sampled in monsoon and Dibrugarh during postmonsoon, or dissolution of detrital carbonates in the Table 5. Decadal variation in TDS of the Brahmaputra along its course. Location Date TDS (mg L 1 ) Ref. Lhasa Jul Jun Pai Dibrugarh Apr Oct This study Guwahati July July This study Apr Apr This study Dec Dec This study Oct This study Goalpara Apr Dec Dhubri July This study Chilmari Aug Mar Jamuna Bridge July This study Aricha Ghat Feb Hu et al., Chen, and Guan, Sarin et al., Galy and France-Lanord, Brahmaputra during its transit from Pasighat to Dibrugarh. The decrease 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 the later hypothesis. This is further strengthened from the dissolved and particulate Ca fluxes of the Brahmaputra at Pasighat and Dibrugarh (see Appendix). These observations showed that in a stretch of km, the Brahmaputra is dissolving all the detrital carbonate present in the bed load. This interpretation can be misleading in view of different season of sampling as the Siang was sampled during monsoon and the others postmonsoon; however, the carbonate content of sediments at Tejpur, further downstream of Dibrugarh, shows no variability between monsoon and nonmonsoon sampling and therefore it can be safely assumed that detrital carbonate would not have changed between two seasons at Dibrugarh, too. The TDS of the Brahmaputra remains nearly constant from Dibrugarh to Jamuna Bridge though it mixes with many tributaries from the Himalaya (Fig. 1). The TDS of the Himalayan tributaries increase from east to west. The Subansiri, the Ranganadi and the Jia Bhareli have lower TDS as they primarily drain silicates of the Higher Himalaya and the Puthimari, the Manas and the Tipkai have higher TDS, resulting from the weathering of Lesser Himalaya having higher proportion of carbonates (Thakur, 1986; Gansser, 1964). The decrease in runoff from east to west over the Himalayan drainage can also be an additional contributing factor to this trend. TDS of the southern tributaries are comparable to those of the Himalayan drainage. Temporal variation in the TDS of the Brahmaputra River was measured at Guwahati at 15-day intervals over a period of 10 months (Table 3). TDS over the ten months showed about a factor of two variation, from 104 mg L 1 to 203 mg L 1 (Fig. 4); the range is more pronounced than spatial variations. Interestingly the minimum and maximum TDS occurred in a span of 15 days during end of May to mid-june (Table 3). The maximum in TDS seems to be a result of flash flooding in the Brahmaputra (see discussion later). The minimum during April May is attributable to increase in glacier melt water component in the discharge. The second broad maximum of

8 3580 S. K. Singh, M. M. Sarin, and C. France-Lanord Fig. 5. Ternary plots of cations and anions of the Brahmaputra. Cations are dominated by Ca whereas HCO 3 dominates the anion budget. Data are in Eq L mg L 1 during January March (Fig. 4) can be explained in terms of lean flow and increased contribution from ground water. It is important to note that water discharge during monsoon is a factor of 10 higher compared to that of during lean flow where as the TDS during monsoon is slightly lower ( 0.6 ) with respect to lean flow. This shows that the kinetics of chemical weathering and supply of solutes of river is not significantly affected despite an order of magnitude increase in runoff. most of the samples. On average about three-fourths of the cations are Ca and Mg for the Brahmaputra river system. In the Dhansiri, the Kopili, and the Ranganadi tributaries, Ca and Mg account for 60% of the total cation budget. If all the Ca and Mg in all these river waters are derived from carbonate weathering, 75% of the cations (molar) in the Brahmaputra watershed will be of carbonate origin. This places an upper limit on the carbonate weathering contribution of cations, because part of Ca and Mg will also be derived from silicates, and Ca also comes from evaporites. In the Tsangpo tributaries, Ca and Mg account only for 43% of cations (Hu et al., 1982) owing to supply of Na (or K) by dissolution of saline and alkaline salts present in the Tibetan drainage and contribution from saline lakes. 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 would depend on Ca and Mg supplied from various sources silicates, carbonates, and evaporites and the behaviour of Ca in the rivers. In the Brahmaputra river system, calculations showed that Ca is undersaturated with respect to calcite in all samples analyzed except in six of them. Even in these six samples, the extent of supersatuartion is marginal, a factor of two or less. Therefore the Ca/Mg variation seems to be more dependent on their sources: Low Ca/Mg can result from silicate/dolomite weathering and high Ca/Mg from calcite weathering. Figure 6 is a plot of Ca/Mg vs. Mg concentration which shows distinct decrease with increase in Mg. This is an indication of the role of dolomite weathering in contributing to Ca and Mg. The regional lithology and detrital carbonate composition of the Dibang, the Lohit, and the Manas rivers attest to this inference. The anion budget is dominated by alkalinity (Fig. 5b), which varies from 425 to 1877 M (Table 2), contributing 90% (molar) to the anion budget. The temporal variation in alkalinity of the Brahmaputra mainstream at Guwahati (Table 3), from 1025 to1694 M, is less than its spatial variation along its course. SO 4 is the next abundant anion. Analogous to alkalin Major Ion Composition Downstream variation in the total cation charge in the Brahmaputra mainstream ranges from 2350 Eq L 1 in Tibet to 1085 Eq L 1 at Tezpur (Table 2). This is quite similar to the temporal variability in total cation (TZ ) at Guwahati, 1242 to 2641 Eq L 1, and the range in the tributaries, 604 to 2154 Eq L 1. The TZ charge balances total anions (TZ ) in most samples within analytical uncertainties. The normalized inorganic charge balance (NICB) is within 5% for most of the samples. In four samples TZ exceeds TZ by 5% to 11% with the maximum deviation for the Ranganadi sample. The excess anions can be due to the presence of organic ligands such as oxalate, acetate, and humic/fulvic acids. The cation budget (Fig. 5a) is dominated by Ca and Mg in 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 Lesser Himalaya; exposure of such lithologies increases from east to west.

9 Chemical erosion in the eastern Himalaya 3581 ity, temporal variation in SO 4 of the Brahmaputra mainstream at Guwahati (Table 3, Fig. 4), 85 to 185 M (barring one sample with 456 M SO 4 ; see discussion), is also less than its spatial variation. The SO 4 content of the Himalayan tributaries, in general, is lower compared to that of the Ganga and the Yamuna (Sarin et al., 1992; Dalai et al., 2002). This is consistent with the lithology of their basins, which have lesser exposure of the Lesser Himalaya sedimentaries and hence less pyrites and evaporites in their watershed. SO 4 content of the Himalayan tributaries of the Brahmaputra in the Assam plain increases from east to west following the increase in the proportions of the Lesser Himalayan sedimentaries. The SO 4 content of the Brahmaputra is generally higher than that of the Ganga (Sarin et al., 1992; Galy and France-Lanord, 1999), contributed from Tibetan tributaries. Cl in the Brahmaputra River System is quite low. It ranges from9to83 M (Table 2), with most samples having values centering around 15 M, similar to those reported for the Himalayan glaciers (Nijampurkar et al., 1993; Sarin and Rao, 2002). In general, Cl in the southern tributaries is higher. The highest Cl is in the Tsangpo (Hu et al., 1982; Chen and Guan, 1981), indicating salt dissolution. In the Brahmaputra mainstream, temporal variation in Cl, 18 to 98 M, is quite significant, with higher concentrations during lean flow. Silica in the Brahmaputra river system varies from 105 to 310 M (Table 2). In Tibet, the Tsangpo has 125 M of silica (Hu et al., 1982) which increases to 200 M as the Brahmaputra crosses the Himalaya. The silica in the Himalayan tributaries, 105 to 294 M, is comparable to those in the headwaters 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, M, indicating more intense silicate weathering. Silica on average contributes 12% to the TDS. The silica contribution to TDS in the Tsangpo in Tibet is as low as 4%, indicating lower silicate weathering in its drainage in Tibet. The results on temporal variations in the major ion chemistry of the Brahmaputra show that the sample collected in the month of June has elevated concentrations of most of the ions (Table 3, Fig. 5). It has the highest TDS, Ca, and SO 4 concentrations and 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 close resemblance to those of the Tibetan samples. There were reports of a flash flood in the Brahmaputra during the time period when this sample was collected. The flash flood was a result of the bursting of a naturally built dam in Tibet. This dam had restricted water discharge from the Tibetan region into the Brahmaputra. Its break-up enhanced the contribution of water from the Tibetan drainage to the Brahmaputra, which is reflected in the major ion chemistry. Further, damming increases reaction time of water with basin sediments and rocks, elevating the concentrations of major elements in the dissolved phase. Samples collected during the postmonsoon and one monsoon sample were analyzed for 13 C of DIC. The samples were not poisoned to arrest biologic activity between sampling and analysis; however, no fungus or any deposit was seen during the time of measurement. The 13 C of the Brahmaputra main channel varies from 14.3 to The 13 C 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 assess the role of silicate/carbonate weathering in the budget of DIC in these waters Chemical Weathering in the Brahmaputra Watershed In this study an attempt has been made to quantify the chemical erosion rate in this drainage, based on major ion chemistry of rivers waters. In addition, the measured mineralogic and clay composition of sediments has been used to provide independent assessment of the chemical weathering in the basin Clay composition and CIA of Sediments The clay content of Brahmaputra mainstream, from Dibrugarh to Guwahati, is only 1.7 to 3 wt% of total sediments (Table 5). The Himalayan tributaries (the Subansiri, the Ranganadi, and the Manas) of the Brahmaputra have similar clay abundance, 1.4 to 3.9 wt%. In contrast, the southern tributary, Kopili, has 21 wt% clay. A direct interpretation of the clay content of these river sediments is that in the Brahmaputra mainstream and in the Himalayan drainage the sediments are poorly weathered, whereas for the sediment of the southern tributary the chemical weathering is relatively more intense. An important uncertainty, however, in using the clay mineral abundance to assess the degree of chemical weathering is size sorting in river sediments, which can significantly alter the clay abundance. In drainage basins subject to high-energy flow, such as in the Brahmaputra, clay minerals and fine particles can be transported out of sediments. Therefore, in addition to abundance of clays, other proxies such as composition of clays have 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 (Millot, 1970). Thus, the clay content of the Brahmaputra sediments and the abundance of vermiculite in them is an indication which shows that these sediments are poorly weathered. This can be attributed to rapid transport of sediments along the course of the river; making this drainage weathering limited (Stallard, 1995). In the southern tributaries, where the clay content is higher and is made of montmorillonite, illite, and chlorite, the chemical weathering is high, a conclusion also attested by the higher concentrations of Si and Na* in their waters. The chemical weathering in a basin can also be gauged from the change in mineralogy and chemical composition of sediments along the river course. Sediment composition can be modified by a number of processes occuring during erosion, transport, recycling, and diagenesis. The mineralogy of sediments 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 Assam plains, (2) there is no indication of selective dissolution of plagioclase over the more resistive K-feldspar, (3) clinopyroxenes and amphiboles show similar extent of alteration, and (4) quartz/feldspar (Q/F) ratio, P/F ratio, and hornblende-dominated dense-mineral assemblages remain constant. All these

10 3582 S. K. Singh, M. M. Sarin, and C. France-Lanord observations infer minimal chemical weathering of the Brahmaputra sediments. In contrast the sediments of the southern tributaries contain pitted and embayed quartz grains, have low P/F ratio, and have abundant microcline, etched clinopyroxene, and laterite clasts. These observations support a more intense weathering in this drainage. This can be attributed to higher temperature in the region and higher residence time of sediments in the basin because of lower relief. The chemical weathering of sediments can also be assessed using their chemical index of alteration (CIA; Nesbitt and Young, 1982): CIA 100 (Al 2 O 3 Al 2 O 3 CaO * Na 2 O K 2 O) where Al 2 O 3, CaO*, Na 2 O, and K 2 O are molar abundances. CaO* is CaO corrected for carbonates present in the sediments (Nesbitt and Young, 1982). CIA of fresh granites is 50, which on weathering increases to 100 in soils. CIA for rocks from the Higher and Lesser Himalaya are as these are recycled crust whereas those for the Transhimalayan Plutonic rocks are 55 (calculated from Debon et al., 1986), being the recent calc-alkaline rocks of mantle origin. Bedloads of the Brahmaputra mainstream have CIA ranging from 58 to 64 (data from Singh and France-Lanord, 2002). (These CIA calculations are based on total Ca, as these sediments have negligible amount of carbonate, Singh and France-Lanord, In the Siang, the Lohit, the Dibang, and the Manas sediments, which have carbonate content of 3% 4%, CaO is appropriately corrected for carbonates to calculate CIA. One gneiss sample collected from Guwahati has CIA 58. These lower CIA values indicate that they are less weathered. The lower CIA of the Brahmaputra can arise due to the presence of Transhimalayan Plutonic rocks in the basin of the Siang and the eastern tributaries which have CIA values of 55 (from data Debon et al., 1986). The eastern and the Himalayan tributaries have CIA values and 65 75, respectively, which underscores the importance of Transhimalayan Plutonic rocks in lowering the CIA of the Brahmaputra. The CIA of the southern tributaries range from 65 to 82, indicating that sediments of these tributaries are more weathered. This is also consistent with the inferences based on mineralogy and clay content of sediments and dissolved major ion composition of corresponding rivers Sources of Solutes in the Brahmaputra River System Determining the sources of solutes, particularly silicate derived solutes, in the Brahmaputra river system is one of the major objectives of this study as it has relevance to CO 2 consumption and hence to global change. Rivers receive solutes from several sources, precipitation and weathering of silicate and carbonate rocks and dissolution of evaporites. To determine the silicate and carbonate contributions to the major ion chemistry, inputs from rain and evaporites have to be properly accounted 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 Na corrected for Cl (Na* Na riv Cl riv ) is generally taken to be of silicate origin. In the case of the Tsangpo, as mentioned earlier, however, there can be other sources of Na such as sodium carbonate or borax in the evaporite deposits (Pascoe, 1963). In such cases, correction only for evaporites using Cl as Fig. 7. Si vs. Na*. Good correlation between Si and Na* (Na riv Cl riv ) indicates that Na* can be used as a proxy of silicate weathering. Si/Na* for most of the rivers is 2.0, indicating plagioclase weathering to kaolinite, whereas for the Eastern tributaries it is 4.0, owing to weathering of mafic/ultramafic rocks in their drainage. Rivers influenced by evaporite dissolution such as the Tsangpo, its tributaries in Tibet, and Dhansiri of the southern drainage fall off the line, indicating contribution of Na from evaporites. an index can yield an upper limit of Na from silicates. Rainwater contribution to other cations and anions are insignificant (e.g., Galy and France-Lanord, 1999) and therefore no correction is applied. Figure 7 presents a covariation between Na* and Si in the rivers analysed. All data, except five, fall in a line, showing good correlation (r ), attesting to the idea that Na* in the rivers can serve as a silicate weathering index. The five data points which fall off the general trend and have excess Na* are for the Tsangpo, Tibetan tributaries, and one of the southern tributaries; in these rivers there could be additional sources for Na, 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 tributaries, the Dibang, and the Lohit have Si/Na* 4. High Si/Na* in these rivers is possibly due to weathering of sodiumdeficient pyroxenes. These inferences, in general, are supported by the mineralogy of sediments. It is assumed that all K is of silicate origin because evaporites and carbonates have very little K. Determination of silicate-derived Ca and Mg is prone to more uncertainties because these elements are supplied to rivers from multiple sources: from silicates, carbonates, and evaporites. Even among silicates, Ca and Mg contributions can be significantly different depending on their composition and weatherability. Therefore, to obtain silicate Ca and Mg contribution it is necessary to know the composition of silicate rocks of the basins. Singh and France-Lanord (2002) have shown, based on Sr and Nd isotopes, that the Brahmaputra sediments are a mixture of the Transhimalayan Plutonic Belt rocks and those from the Himalaya in the proportion 30:70. Using the Ca, Mg, and Na abundances of these endmembers and above mixing proportion, the Ca/Na and Mg/Na ratios for the silicates of the Brahmaputra watershed were calculated. The elemental

11 Chemical erosion in the eastern Himalaya 3583 Table 6. Endmember of silicates for various drainages. River Formation Ca/Na (molar) Mg/Na (molar) (Ca Mg)/Na (molar) Lohit, Dibang Transhimalaya a a Ranganadi, Jia Bhareli, Manas, Puthimari, Burhi Dihing, Dhansiri, Himalaya b b Kopili, Tipkai Tsangpo, Siang, Subansiri, Brahmaputra mainstream Transhimalaya Himalaya (30:70) a Debon et al b Krishnaswami et al., ratios of various endmembers and the rivers which drain them are given in the Table 6. The endmember composition have been assigned an uncertainty of 50% owing to variation in their mixing proportions and the approaches adopted for calculation (Krishnaswami et al., 1999). The Ca Mg derived from the silicates, (Ca Mg) sil, is calculated using these endmember 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 total cations derived from silicate, ( Cat) sil, is calculated by summing Na*, K, and (Ca Mg) sil. These vary between 21% and 77% in the rivers of the Brahmaputra River System, with a mean of 45%, nearly the same as ( Cat) sil of the Brahmaputra mainstream at Chilmari, its outflow. The ( Cat) sil will be 45% 11% if the 50% uncertainties in the (Ca Mg)/Na ratio of silicate endmember (Table 6) is considered. The ( Cat) sil generally follows the lithology of the individual drainage. The major uncertainty in the ( Cat) sil is from Ca/Na of the silicate endmember. Galy and France-Lanord (1999) assumed a value of 0.2 for Ca/Na of Himalayan silicate, whereas in this study it has been taken as 0.7. If the 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 the value of 0.2 is based on the Ca/Na of silicates from HH, LH, and leucogranites (Galy and France-Lanord, 1999), a value of 0.7 is derived for Ca/Na based on the Ca/Na of silicates (crystallines and sedimentaries) of LH and HH, of the soil profiles developed over LH silicates, and of stream flowing exclusively through silicate terrains (Krishnaswami et al., 1999). Calculating ( Cat) sil in the Tsangpo (Tibet) following the above approach has been hampered, because the validity of using Cl as an index for rain and evaporite contribution of Na is in doubt. In this drainage, in addition to rain and halites, Na may also be derived from sulphates, carbonates, and borates (Pascoe, 1963). Therefore in this drainage silicate cations have been calculated based on the overall relation (Fig. 7) between Si and Na*. Na* from silicates is estimated from the Na-Si relation (Fig. 7) and the SiO 2 concentration of 125 M for the Tsangpo. The calculated Na* is 45 M. Based on this Na*, a ( Cat) sil of M has been estimated for the Tsangpo; this represents 17% of the total cations, indicating silicate cation contribution to total cation is quite low. This, as mentioned earlier, is because of supply of higher proportion of alkalis from alkaline and saline salts of the basin Erosion in Various Subbasins Singh and France-Lanord (2002), based on Sr and Nd isotope composition of sediments from the Brahmaputra drainage, inferred that this drainage undergoes differential physical erosion. As a part of the current study, chemical erosion in the various subbasins of the Brahmaputra watershed have been determined to quantify their total chemical and silicate erosion rates and to evaluate the factors influencing chemical erosion. Erosion rates of various subbasins of the Brahmaputra River System are given in Table 7 based on the TDS and ( Cat) sil fluxes. For comparison, the erosion rates of a few other selected basins of the Himalaya and of other global basins (Sarin et al., 1989) are also given in this table. For the Eastern Syntaxis zone of the Siang, TDS and ( Cat) sil fluxes are taken to be the difference in TDS and ( Cat) sil fluxes from Tibet at Pai and those at Pasighat. Table 7 shows that chemical erosion in the Brahmaputra watershed is not uniform and that it varies by more than an order of magnitude among the various subbasins. Both total chemical erosion and silicate erosion rates in the Eastern Syntaxis zone are the highest compared to other sub- Table 7. Chemical erosion and CO 2 consumption rates in various sub-basins of the Brahmaputra and selected basins of the world a Basin TDS Flux (t km 2 y 1 ) Silicate Cation Flux CO 2 consumption by silicate weathering, 10 5 moles km 2 y 1 Brahmaputra Tibet Eastern Syntaxis Eastern Himalaya Southern Brahmaputra Other Himalayan Rivers Ganga Indus Mekong Global Rivers Amazon World Average a Data for TDS of other rivers are from Sarin et al. (1989) and references therein, and those for Silicate Cation fluxes and CO 2 consumption due to silicate weathering are from Gaillardet et al. (1999) and Krishnaswami et al. (1999).