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Glaciers, while commonly regarded as sensitive indicators of climate change, can also be affected by various factors like atmospheric deposition. Study of major ions (Mg2+, Ca2+, Na+, K+, HCO3-, SO42- and Cl-) of meltwater draining from Chhota Shigri glacier was carried out to understand weathering and geochemical processes controlling the major ion chemistry of the glacier meltwater . Chhota Shigri glacier meltwater was sampled in August 2008. Mg2+ is the dominant cation followed by Ca2+, K+ and Na+. Whereas HCO3- is the dominant anion followed by SO42- and Cl-. The high ratio of Ca+Mg/TZ+ i.e. 0.77 and low ratio of Na+K/TZ+ i.e. 0.23 shows that carbonate weathering is the dominant mechanism controlling major ion chemistry followed by silicate weathering in the study area. In this paper we aim to evaluate the possible effect of various processes that affect the chemistry of meltwater of Chhota Shigri glacier.
Decadal variation in the major ion chemistry of Chhota Shigri glacier meltwater was carried out by comparing the hydrochemistry of August 2008 with previous study carried out in 1987. Comparison between the two data sets shows that cations like Ca2+ increased nearly 6 times, Mg2+ 87 times, Na+ 10 times and K+ 19 times, while anions like Cl- increased nearly 12 times, HCO3- 85 times and SO42- 49 times between 1987 and 2008. This could be attributed to increased weathering rates due to climate warming, atmospheric deposition or a combination of both. Annual specific mass balance of Chhota Shigri glacier was often sharply negative during 2002 to 2010, pointing to glacier recession, which may be also attributed to global climatic change.
Temporal variation, Major ion chemistry, Mass balance, Weathering, Chhota Shigri glacier, Atmospheric deposition
The Himalayas are commonly referred to as "the abode of eternal snow" being the largest storehouse of snow and ice outside the polar region . Indus, Ganga and Brahmaputra basins put together have 32392 glaciers occupying 71182.08 Km2 of glaciated area . Various studies of glaciers in Himalaya such as Ahmad and Hasnain , Kumar et al.  and in Alpine region by Rainwater and Guy , Beherens et al. , Lorrain and Souchez , Collins , Tranter and Raiswell  have suggested that the principal flow component of meltwater coming out of the glacier terminus passes through the subglacial environment. These studies indicate that rate of chemical weathering is high in glaciated catchments resulting from long residence times of meltwater in contact with the bed rock. Hydrochemistry of glacier evolves from precipitation (rainfall and snow) and gets enriched during the traverse through sub-glacial channels and rock ice interface .
Conceptual mixing models have produced by time series analysis of fluctuations in dissolved ion concentrations of meltwaters draining from snout of glacier, which elucidate hydrological characteristics of glacier on seasonal and diurnal scales (Tranter et al 1993, Theakston and Knudsen 1996 a,b, Fairchild et al 1999a, Yde and Knudsen 2004). Various hydrochemical studies on Alpine glaciers carried out by Tranter et al 1993, Rainwater and Guy 1961, Brown et al 1996a,b and in Himalayan glaciers by Hasnain et al 1989, Dhanpal 1990, Singh et al 1998, Ahmad and Hasnain 2000, 2001, Kumar et al. 2009 and Singh et al. 2012 indicate that generally calcium is the dominant cation in glacier meltwater with lesser concentrations of magnesium, potassium and sodium. Whereas bicarbonate and sulphate are dominant anions with varying proportions. Hydrogeochemical characterisation of meltwater draining from glacier helps in identifying the nature and concentration of solute embedded in the underlying bedrock and contribution from atmospheric deposition. In glacierised areas, solute acquisition processes vary in space and time. Hence, long-term monitoring of hydrochemical study helps to quantify the relative contributions of natural and anthropogenic constituents in glacier ice melt runoff (Ramanathan 2011).
The morphology, bedrock topography, meteorological and hydrogeochemical parameters as well as dynamics of Chhota Shigri glacier have been studied by various authors [11-15]. Chandra river, one of the important sources of water for Himachal Pradesh is fed by several glaciers including Chhota Shigri glacier. Information on mass balance and chemistry of meltwater of Chhota Shigri glacier is inadequate, and needs to be substantiated by comprehensive studies. First time mass balance in Indian Himalayan glaciers were started by Geological Survey of India on Gara glacier (Himachal Pradesh) in 1974 (Raina et al 1977). In Himachal Pradesh, Gor-Garang glacier during the period 1976-1985 and Shaune Garang glacier during the period 1984-1989 showed negative mass balance of -0.572 m w.e.a-1 (Shankar 2001) and -0.407 m w.e.a-1 (Singh and Sangewar 1989), respectively. Whereas in Uttarakhand, Tipra Bamak glacier during the period 1981-1988, Dunagri glacier during the period 1984-1990 and Dokriani glacier during the period 1993-2000 also experienced negative mass balance of -0.241 m w.e.a-1 (Gautam and Mukherjee 1989), -1.038 m w.e.a-1 (Srivastava and Swaroop 1989) and -0.320 m w.e.a-1 (Dobhal et al 2008), respectively . Mass balance measurement and accumulation estimations of Chhota Shigri glacier were carried out during 1987-89 [13, 15, 16]. Keeping these facts in mind this paper aims to address the decadal variation in the hydrogeochemical processes and their possible effect on the long-term changes in the chemistry of meltwater of Chhota Shigri glacier.
Area of study
Geographically Chhota-Shigri glacier is located between 32á´¼ 11/-32á´¼ 17/ N and 77á´¼ 29/- 77á´¼ 33/ E and is a valley type glacier. It lies on the northern ridge of the Pir Panjal range in the Lahaul-Spiti valley of Himachal Pradesh, India, in the western Himalaya. This glacier is oriented roughly north-south in its ablation area, and has a variety of orientations in the accumulation area. Table 1 gives the geographical and topographical characteristics of Chhota Shigri glacier . The meltwater stream from Chhota Shigri glacier flows in a NW direction and meets Chandra river at right angles at about 2.5 km downstream of the snout.
Geology of area
Chhota Shigri glacier lies within the Central Crystallines of the Pir Panjal range of the Himachal Himalaya. Meso- to ketazonal metamorphites, migmatites and gneisses are found here . The main lithologic unit of the Chhota Shigri glacier catchment is Rohtang gneiss , while 3 km upstream of Chhota Dara, in the upper Chandra valley, older Palaeozoic granitic rocks are exposed. The Haimanta formation overlies these within a tectonic break, where black slates, phyllites and fine-grained biotite schists are exposed . The rocks found between Chhota Shigri and Bara Shigri glaciers are granite, granite gneiss, augen gneiss, porphyritic granite, schistose gneiss, milky-white muscovite-quartzite and muscovite-biotite schist. Late stage pegmatitic veins are quite common in which light grey-green feldspar crystals are found .
Climatic records of glacier basins are highly imperative to understand glacier-climate relationship. Climate of Chhota Shigri glacier is typical of monsoon-arid transition zone where both the winter mid-latitude westerlies (January-April) and summer Asian monsoon (July-September) influence the climatic condition of this glacier (Wagnon et al 2007). Very few meteorological studies are carried out on Chhota Shigri glacier, mostly of very short period of time. Highest maximum temperature observed on glacier surface is about 10.5°C, 11°C, 8.1°C, 7.5°C, 9.64 °C and 11.85°C and lowest minimum temperature of about -4.5 °C, -1.3°C, -5.2°C, -1.6°C, -6.22°C and -13.64°C in 18 August-8 September 1986, 18 July -17 August 1987, 2 August - 5 September 1988, 17 August-11 September 1989, 2-8 October 2003 and 18 August- 10 October 2009 respectively (Rizvi 1987, IMD 1987, Apte et al 1988, Kulandivelu et al 1988, Upadhyay et al 1989, Sharma 2007, JNU-IFCPAR 2009).Total rainfall during the melt season June, July, August and September 2010 is recorded as 25.5, 122.5, 22.0 and 161.0 mm respectively (Singh 2011). During the summer season in most of the days average environmental lapse rate on Chhota Shigri glacier remained pseudo-adiabatic and varied from 0.38 to 0.67°C/100m (Bhutiyani and Sharma 1989).
Materials and Methods
Meltwater samples (n=15) were collected from Chhota Shigri glacier during 1-15 August 2008. Electrical conductivity (EC) and pH were measured using portable multi-parameter meter (HACH-Sension156). Suspended sediments were separated from the water samples in the laboratory by using 0.45 micron millipore membrane filters of 47mm diameter. Vacuum pump was used to accelerate the filtration. Bicarbonate was determined by following potentiometric titration method . Chloride ion concentration was measured by the mercury (II) thiocynate method; Sulphate concentration was measured by turbidimetric method . Major cations (Na+, K+, Ca2+ and Mg2+) were determined by atomic absorption spectrophotometer (Shimadzu-AA-6800). Na+ and K+ were analysed in the emission mode and Ca2+ and Mg2+ in the absorbance mode. The analytical precision for the analysis of dissolved ions was better than Â±5%. Mili-Q water was used for all analyses. The meltwater sampling site was located at 32á´¼ 17/24.58// N and 77á´¼ 31/55.21// E on the Chhota Shigri glacier stream at about 3900 m a.s.l., 2.0 km downstream from the snout of Chhota Shigri glacier (Fig.1).
Results and Discussion
The major ion compositions of meltwater draining from Choota Shigri glacier during August 2008 is given in Table 2. The average pH and EC values of the Chhota Shigri glacier meltwater were found as 7.4 (6.7-7.8) and 15.3 Âµs/cm (10.0-30.0 Âµs/cm), respectively. HCO3- is the dominant anion followed by SO42- and Cl-. HCO3- concentration ranges from 4.48-24.0 mg/l with an average value of 10.9 mg/l. Whereas SO42- and Cl- concentration varies from 2.40-5.17 mg/l with an average value 3.43 mg/l and 0.27-0.77 mg/l with an average value of 0.51 mg/l, respectively. Mg2+ is the dominant cation and its concentration varies from 0.65-1.33 mg/l with an average value of 0.96 mg/l. Whereas the average concentration of Ca2+, K+ and Na+ are found as 0.72 mg/l (0.54-0.94 mg/l), 0.62 mg/l (0.53-0.73 mg/l) and 0.45 mg/l (0.16-0.59 mg/l), respectively. TDS value of Chhota Shigri glacier meltwater varies from 11.3-33.3 mg/l with an average value of 20.2 mg/l. Meltwaters draining through subglacial channels become chemically enriched by interacting with basal moranic materials (Bezinge et al. 1973). The (Ca+Mg) vs total cations scatter plot (Fig. 2) shows that all points fall above 1:1 line with an average equivalent ratio of 0.77 (0.73-0.83). This shows that carbonate weathering is the dominant mechanism controlling chemical composition of meltwater draining from Chhota Shigri glacier, as reported by earlier workers (Sharma 2007, Ramanathan et al. 2009, Singh 2011). The scatter plot of Na+K/TZ+ (Fig. 3) shows that low average equivalent ratio i.e. 0.23 (0.17-0.27), indicating a relatively small contribution of solute from silicate weathering.
Chemical characteristics of meltwater draining from the glacier are mainly controlled by weathering of freshly weathered rock material at the base of the glacier and solid or liquid precipitation (Ahmad and Hasnain 1999). Higher temperatures are responsible for increasing biological activities, but also the physical and chemical weathering of rock (Rogora et al. 2003). Surface water is particularly sensitive to changes in temperature and precipitation regime in mountain areas [20-22]. The partial or total disappearance of permanent snowfields and the reduction of the ice cover period could lead to a marked increase in the mineral dissolution owing to the greater proportion of precipitation flowing over exposed rocks and into the soil [23, 24]. Decadal variation in the major ion chemistry of meltwater draining from Chhota Shigri glacier was studied by comparing the meltwater chemistry of August 2008 with previously published data  in 1987 (Fig. 4 and 5) . Table 2 summarizes the hydrochemistry of the meltwater in 1987 and 2008. Most cations like Ca2+ increased nearly 6 times, Mg2+ 87 times, Na+ 10 times and K+ 19 times, while anions like Cl- increased nearly 12 times, HCO3- 85 times and SO42- 49 times between 1987 and 2008, which could have resulted from the warming that is reported in the Northwestern Himalaya . Higher air temperatures would result in reduction of snow and ice cover and greater exposure of rocks in glaciated watersheds thus enhancing the weathering process, which could be the major factor responsible for increasing solute concentrations in the meltwater of Chhota Shigri glacier. Mg2+ and HCO3- show a highly significant increase, which may be due to enhanced carbonate weathering in a warming environment. According to Rogora et al.  variations in base cations, alkalinity and sulphate can be explained in terms of an increase in solute export from the catchment due to intensified weathering in a warmer climate. In cool climates weathering can also accelerated by more exposure of minerals due to physical weathering of ice (Turner et al. 2010).
Very limited meteorological data from Chhota Shigri glacier catchment is available during the studied period, and so it is difficult to directly correlate the observed increase in ionic concentration in meltwater to climate change. However, mass balance, snout retreat, thinning and glacier shrinkage are important indicators of climate change. The available mass balance data of Chhota Shigri glacier revealed a negative trend (Table 3), and is discussed here in support of climatic impact playing a significant role in enhancing the ionic concentration in Chhota Shigri glacier meltwater. Shrinkage or thinning of the glaciers is linked with climate change that results in decrease in winter precipitation (solid) and increase in summer precipitation (liquid) and atmospheric temperature . Position of glacier snout is the simplest indicator of glacier retreat or advance over a period of the time which generally linked with climatic fluctuations (Ramanathan 2011). Snout retreat of Chhota Shigri glacier was estimated to be 53.3 myr-1 between 1988 and 2003 by field observation and remote sensing study (Kulkarni et al 2007). Whereas retreat of snout was estimated to be 25 myr-1 between 1972 and 2006 (Shruti 2008), but this study has its own limitation due to resolution of different images. Snout retreat of the glacier shows that glacier vacating part of the area occupied by it means a greater exposure of rocks, thus enhancing the weathering processes. Which may be responsible for increasing the dissolved ions concentration of meltwater draining from Chhota Shigri glacier.
Mass balance study on Chhota Shigri glacier was carried out by Glacier Research Group, Jawaharlal Nehru University, New Delhi during the period 2002-2010, during which Chhota Shigri glacier showed negative mass balance. Annual specific mass balance of Chhota Shigri glacier was -1.4, -1.2, +0.1 and -1.4 m w.e. in 2002-2003, 2003-2004, 2004-2005 & 2005-2006 respectively . Between 2002 and 2010, Chhota Shigri glacier experienced a negative glacier-wide mass balance of -0.67Â±0.40m w.e.a-1 , indicating the possible glacier recession may be caused either by global climate change or local warming effect in the region. Some recent remote sensing studies indicate that overall deglaciation in Himachal Pradesh was as high as 21% during the period 1962-2007 (Kulkarni et al. , 466 glaciers investigated), while during the period 1999-2004, a general thinning of glaciers in the Western Himalaya is observed (Berthier et al. , with an overall specific mass balance of -0.7 to -0.85 m w.e.a-1 obtained over a 915 km2 ice covered area including Chhota Shigri glacier). According to Wagnon et al.  this variation is due to increase in the pace of glacier wastage in the western Himalaya, probably related to global warming.
Mass balance is strongly dependent on incoming solar radiation, which is irregularly absorbed by the glacier according to surface albedo (snow or ice, presence or absence of debris) . Hence surface albedo is likely to play an important role in the melting of glacier i.e. less albedo resulting in increased melting and ultimately to negative mass balance. Negative glacier mass balance is also the result of glacier vacating part of the area occupied by it, thus enhancing the weathering processes and the ionic concentrations in the meltwater stream. Warming also gives rise to an increase in meltwater temperatures, which would accelerate chemical weathering, releasing more ions into the portal meltwater.
Change in ionic concentration of Chhota Shigri glacier meltwater during this period (1987-2008) can be partly explained as the impact of warming. Apart from climate change, chemistry of atmospheric deposition also can play an important role in determining chemical characteristics of surface waters in remote areas, if we consider the geo-lithology of the watersheds as a constant, a further variable strongly influencing water chemistry [31, 32]. Thus the atmospheric fallout over the Himalayan region could have played a role in enhancing the solute concentration observed in Chhota Shigri glacier meltwaters from the 1987 to 2008. However, quantification of atmospheric fallout to the hydrochemistry of Chhota Shigri glacier needs to be carried out to substantiate the above inference in relation to the role of climate change.
The present study was aimed at investigating the factors responsible for enhancing the major ion concentrations in Chhota Shigri glacier meltwater based on empirical evidence as well as time-series data and secondary information. A comparison between the meltwater chemistry of 1987 and 2008 shows that most cations and anions increased 6 to 87-fold in concentration. This trend can be related to higher air temperatures resulting in reduction of snow and ice cover and consequent greater exposure of rocks in this glaciated catchment, enhancing the weathering process. At the same time, the atmospheric fallout over the Himalayan region in recent times could have played a significant role in providing easily available source material for the enhanced solutes in glacier meltwaters.
The Authors are thankful to Department of Science and Technology (DST), Govt. of India for funding the research project on Chhota Shigri glacier. The authors are also grateful to Jawaharlal Nehru University for providing infrastructural facilities for accomplishing this work.