Abstract: The timing of glaciation is an important parameter that helps in the understanding of past climate change andprovides valuable information for developing the predictive futuristic models. There are evidences to suggest that duringthe late Quaternary, Himalayan glaciers fluctuated considerably thus implying their sensitivity to changes in past climaticconditions. Although the Himalayan region is fed by two major weather systems viz. the southwest summer monsoonand the mid-latitude westerlies, however, the existing chronology (mostly exposure ages) indicates that irrespective ofthe geographical position, glaciers seem to grow during increased insolation and enhanced southwest summer monsoonincluding the mid-latitude westerly dominated north-western Himalayan glaciers (Ladakh and Karakoram). Consideringthe limited geographical coverage and the dating uncertainty, the above inferences should be treated as tentative.
Keywords: Glaciation, late Quaternary, Monsoon, Himalaya.
The Himalayan region is fed by two major weathersystems viz. the southwest summer monsoon and the mid-latitude westerlies (Finkel et al. 2003; Yang et al. 2008)(Fig.1). The influence of these two weather systems variesspatially. For example, most of the southern and eastern partof Himalaya experiences a pronounced summerprecipitation, reflecting moisture advected northwards fromthe Indian Ocean by the southwest monsoon. The summerprecipitation gradient is orographically controlled, hencethe region north of the higher Himalaya receives scantymonsoon precipitation compared to its southern counterpart.Similarly, the influence of mid-latitude westerlies decreasesfrom northwest to southeast Himalaya (Benn and Owen,1998). The Himalayan region has preserved evidence ofpast glaciations in the form of well developed moraines andvalley fills that exceed several tens of meters in thickness(Benn and Owen, 1998). Owen et al. (1998) have shownthat throughout the Himalaya, the extent of glaciation variedconsiderably during the late Quaternary. Although moststudies based on the relative chronologies have providedevidence for multiple glaciations, quantitative chronologiesare just making a beginning. Therefore, meaningful inferencetowards the regional and global correlation of past glacialchanges is still in an embryonic stage.
Reconstruction of the former extent of glaciers requiresdetailed geomorphic mapping of the glaciogenic landformsand sediments and even the most accurate methods also
INTRODUCTION
Timing and amplitude of palaeoglaciations representimportant cornerstones of terrestrial paleoclimatic research,because glaciers are arguably the most sensitive recordersof climate changes (Kääb et al. 2007; Schaefer et al. 2008)as they respond to the combined effect of snow fall andtemperature (Pratt-Sitaula et al. 2011). However,palaeoclimatic patterns in general and the timing of pastglaciations in particular remain controversial for tropical/monsoonal regions of Himalaya (Schäfer et al. 2002).Mountain glaciers are sensitive probes of the local climate,and, thus, they present an opportunity and a challenge tointerpret climates of the past as well as to predict futurechanges (Owen, 2009). The evidence of rapid climate changehas been mounting over the last few decades (Watson, 1997;Dyurgerov and Meier, 2000; Hughton, 2001). Many of theseconcerns spin around the ongoing rapid changes in thecryosphere (Corell, 2004), particularly, the melting ofglaciers (Arendt, 2002). It has been suggested that duringthe last century, anthropogenically induced rise in globaltemperature (Hughton, 2001) has resulted in ~100 melevation of equilibrium line altitude of the mountainglaciers with accompanying changes in the distribution ofplants and snow cover (Grove, 2008). Therefore, in orderto develop a futuristic scenario, the evidence of modernglacier retreats needs to be compared with that of the pastglacier variation.
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require sufficient geomorphic evidence, usually lateral-terminal moraines and trim lines, to allow the shape of theformer glacier to be reconstructed. However, glacial moraineevidence is by nature discontinuous; and relatively youngerand larger advances of a glacier will destroy morainesdeposited in older, less-extensive advances, leaving anincomplete geomorphic record (Benn et al. 2005). With theabove limitations reasonable efforts have been made in therecent times towards understanding the timing and patternof glaciations in parts of the Indian Himalaya (Owen et al.1996; Sharma and Owen, 1996; Taylor and Mitchell, 2000;Owen et al. 2001; Barnard et al. 2004; Owen et al. 2006;Pant et al. 2006; Nainwal et al. 2007; Dortch et al. 2010;Scherler et al. 2010; Mehta et al. 2012) (Table 1). In additionto this, attempts have also been made to estimate the palaeoequilibrium-line-altitude (ELA) which defines the boundarybetween the zone of accumulation from that of ablation.Chronologically constrained former ELAs are usefulparameter for quantifying the past climate and the regionalvariations of ELAs can be utilized to show formerprecipitation gradients and ascertaining the moisture sourcesand reconstruction of atmospheric circulation patterns(Holmes, 1993; Lehmkuhl et al. 1998).
Due to the ecological diversity and geographicalvividness, major part of the Indian Himalaya is largely un-investigated, hence a definite conclusion towards climatic
implication of late Quaternary glaciations is yet to beanswered satisfactorily (Taylor and Mitchell, 2000). Forexample questions like (i) what caused glaciers to fluctuateduring the late Quaternary, (ii) what was the frequency,(iii) how earth surface processes responded, and (iv) werethe glaciations in the Himalaya synchronous?
The present study is an attempt to analyse the existingchronometric data obtained on moraines from IndianHimalaya in order to assess the suggestion that Himalayanglaciers advanced and receded asynchronously with thoseof the Northern Hemisphere (Finkel et al. 2003; Phillips etal. 2000), thus emphasizing the role of Indian SummerMonsoon (ISM). Further, it is not evident whether themonsoon (moisture flux) is the sole driving force in glacierdynamics in Himalaya or has only a secondary role relativeto global temperature changes (Gayer et al. 2006). We firstpresent the chronology obtained from the north-western,western and central Himalaya, followed by the inferencesdrawn and finally the implications of the existingchronometric data towards understanding the role of regionaland or global climate.
NORTHWESTERN HIMALAYA (LADAKH ANDKARAKORAM)
In the recent years, attempts have been made to generate
Fig. 1. Map showing the locations of the areas discussed in the text along with the trajectories of two major weather system viz. thesummer monsoon and the mid-latitude westerly.
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numerical ages on the moraine successions. Taylor andMitchell (2000) carried out a systematic investigation in theZanskar valley and suggested three glacial stages along witha minor advance. According to them, a glaciatedpalaeosurface (>280 m above river bed) with associatederratic represents the oldest and most extensive glaciationwhich they named as the ‘Chandra Glacial Stage’. A changein valley form from broad glacial troughs to narrow V-shapedgorges along with large subdued moraine ridges delimits aseparate later extensive valley glaciations; the ‘Batal GlacialStage’ which was dated using the luminescence techniqueat ~78 ka (Between~40 to ~78 ka) and correlated with theMarine Isotopic Stage -3 (MIS-3) and MIS-4. The distinctset of moraine ridges representing a less extensive youngerglaciation; the ‘Kulti Glacial Stage’ was dated at ~10 to~16 ka. However, they speculated that the Kulti Glacial Stage
could be older corresponding to the Last Glacial Maximum(LGM).According to them LGM in Zanskar valley was lessextensive (<10 km). A minor advance, which they calledthe ‘Sonapani Glacial Stage’ is represented by sharp crestedmoraines and is confined within 2 km of the present day icebodies which probably belongs to the Little Ice Age (LIA).It has been estimated that during the Batal and Kulti glacialstages, ELA was depressed by ~500 m and ~300 mrespectively (Taylor and Mitchell, 2000).
The change in the magnitude of glaciation from ‘ChandraStage’ to the later glaciations was interpreted as theupliftment of southerly ranges blocking monsoonprecipitation and incision of the landscape such that icereached lower altitudes over shorter horizontal distances.More extensive glaciation during ~78 ka which correspondsto MIS-4 compared to the LGM (MIS-2) was attributed to
Table 1. Glacial chronologies from the north-western, western and central Himalaya, India
S. Area Established glacial chronology OSL/ Reference RegionNo. CRN
1 Zanskar Chandra gl. Batal gl. Kulti gl. Sonapani OSL Taylor and NW-Range Stage Stage Stage gl. Stage Mitchell Himalaya
(No age) (~40-78 ka) (~ 10-16 ka) (~ LIA) (2000)
2 Ladakh Indus valley Leh glacial Kar gl. Stage Bazgo gl. Stage Khalling gl. CRN Owen et al.Range glacial stage stage (early part of (Middle of last Stage (Early (2006)
(430 ka) (Penultimate last glacial glacial cycle) Holocene)or older) cycle)
3 Nubra and Deshkit 3 Deshkit 2 Deshkit 1 CRN Dortch et al.Shyok valley (~ 144 ka) (~ 81 ka) (~ 45 ka) (2010)
10 Chorabari Rambara gl. Ghindurpani Garuriya gl. Kedarnath OSL Mehta et al.glacier valley Stage gl. Stage Stage gl. Stage (2012)
(~13 ka) (~9 ka) (~7 ka) (~5 ka)
11 Tons Valley ~ 16 ka ~ 11-12 ka ~ 8-9 ka ~ 5 ka < 1 ka CRN Scherler et al.(2010)
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more favourable combination of precipitation and tempera-ture condition in the region (Taylor and Mitchell, 2000).
In Leh valley, Owen et al. (2006) suggested five glacialadvancements with progressive decrease. The timing ofthese advancements was estimated using the 10Be surfaceexposure ages. Boulders of the oldest ‘Indus Valley GlacialStage’ are dated between 130 ka to 385 ka. Since the bouldersare intensively weathered, hence, it has been suggestedthat the 10Be ages are the minimum ages. The ‘Leh GlacialStage’ ages range between 79 ka to 369 ka and clusterbetween 100 ka to 200 ka. Wide scatter in the ‘Kar GlacialStage’ prevent from assigning any specific time and insteadit was attributed to the last glacial cycle. The ‘Bazgo GlacialStage’ is bracketed between 41 ka and 74 ka (middle of thelast glacial cycle) and ‘Khalling Glacial Stage’ is assignedthe early Holocene age.
The five glacial advances of decreasing magnitude inthe Ladakh Range (Trans- Himalaya) since >430 ka (MIS-11C to MIS-1) is attributed to the reduction in moisture fluxin the accumulation zone caused due to the uplift of theHimalayan ranges to the south and the Karakoram ranges tothe west which reduced the influence of the summermonsoon and the mid-latitude westerlies in the region (Owenet al. 2006). According to Owen et al. (2006), although theages range from 79 ka to 369 ka, however, they are clusteredbetween ~100 ka to 200 ka and assigned to the MIS-6 glacialevent. Subsequent glacial events occurred during the lastglacial cycle during Holocene.
In the north of the Ladakh Range three glacial stagesviz. Deshkit-1, Deshkit-2 and Deshkit-3 have been identifiedin the Nubra-Shyok valleys in the Karakoram ranges (Dortchet al. 2010). 10Be exposure ages indicate that Deshkit-1advancement occurred ~45 ka, Deshkit-2 ~81 ka andDishkit-3 ~144 ka. Reconstruction of the former ELAassociated with Deshkit-1 (~45 ka) indicated that the ELAwas depressed by ~290 m in the Nubra valley. The studyfurther suggested that Deshkit-1 and 2 glacial stages seemto be synchronous with regions influenced by monsoonprecipitation during MIS-3 and MIS-5/6 transition. This isan interesting observation considering that the present dayprecipitation is dominated by the mid-latitude westerlies(Bhutiyani et al. 2010), it was likely to expect that Nubra-Shyok valley glaciers should have responded to periods ofenhanced westerlies associated with global cold periods.
WESTERN HIMALAYA (HIMACHAL PRADESH)
Owen et al. (1997) using optical and radiocarbon datingmade the very first attempt in the Lahul Himalaya to ascertainthe timing of glaciations. Since the Lahul valley lies at the
junction of the monsoon influenced southern flank of thePir Panjal (lesser Himalaya) and the semi-arid mountainsof the Trans-Himalaya, it is ideally suited for examining theinfluence of the southwest summer monsoon and the mid-latitude westerlie weather systems on the process ofglaciations (Owen et al. 1997; Owen et al. 2001). Accordingto them, Lahul valley witnessed three glaciations which theynamed as the ‘Chandra Glacial Stage’, the ‘Batal GlacialStage’ and the ‘Kulti Glacial Stage’ along with the two minorHolocene advances. Optically Simulated Luminescence(OSL) dating indicates that glaciers probably started toretreat between 43,400 ± 10,300 and 36,900 ± 8,400 yr ago(during the Batal Stage). The Kulti Glacial Stage was less-extensive and was limited to ~12 km from the present icemargins. On the basis of an OSL age obtained on deltaicsands and gravels that underlie tills of Kulti stage, it wassuggested that Kulti glaciation occurred after36,900 ± 8,400 yr ago. The development of peat bogs,having a basal age of 9,160 ± 70 14C yr BP represents a phaseof climatic amelioration which was coincident with post-Kulti deglaciation. The Kulti glaciation, therefore, isequivalent to all or parts of late MIS-3, MIS-2, and earlyMIS-1 respectively. In addition, Owen et al. 1997 couldidentify two minor Holocene advances (Sonapani GlacialStage I and II) which are preserved as small sharp-crestedmoraines within a few kilometres of glacier termini. On thebasis of relative weathering, the Sonapani-I advance wasassigned early mid-Holocene age, whereas, the Sonapani-IIadvance was attributed to historical period probablycorresponding to the Little Ice Age (LIA). In a later study,Owen et al. 2001 used 10Be and 26Al exposure age dating onmoraines, drumlins, and from glacially polished bedrocksurfaces in the Lahul valley because according to them theexposure ages provided more definite estimate on the timingof individual glacial advances compared to their earlier studywhere OSL dating of pre or post glacial sediments wereused (Owen et al. 1996). Though they could not date theoldest Chandra Glacial Stage, the exposure ages obtainedon the representative samples of the Batal and Kultiglacial cycles indicate that the valley witnessed extensiveglaciations (Batal Glacial Stage) at around 12-15.5 ka, andhas been suggested to be coeval with the NorthernHemisphere Late-glacial Interstadial (Bølling/Allerød).Interestingly, these ages are significantly younger comparedto the Owen et al. (1997) OSL ages of 43.4 ka and 36.9 ka.The overestimated OSL ages were attributed to the partialbleaching of OSL signal. Based on the exposure ages,deglaciation of the Batal Glacial Stage was completed by~12 ka and was followed by a small re-advancementcorresponding to the Kulti Glacial Stage during the early
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Holocene, at ~10-11.4 ka and probably lasted for fewhundreds to a few thousand years.
Exposure age dating show that the Batal and Kulti glacialstages are significantly younger (Owen et al. 2001) comparedto the earlier ages obtained using the OSL dating technique(Owen et al. 1997). The reinterpretation of the data basedon the revised chronology (Owen et al. 2001) suggests thatthe millennial scale glacier oscillations in Lahul valley wereassociated with the periods of positive mass-balance whichcoincide with times of increased insolation and strengthenedmonsoon. The strengthened monsoon extended its influencefurther north and west, thereby, enhancing high-altitudesummer snowfall. However, there is no evidence to suggestthat glacial advances in Lahul valley were synchronouswith changes in Northern Hemisphere ice-sheets.
CENTRAL HIMALAYA (UTTARAKHAND)
The very first numeric dating was carried (Sharma andOwen, 1996) in the Gangotri valley where they could identifya major glaciation called the Bhagirathi Glacial Stage (BGS)along with two glacial advances named as the Shivling andBhujbasa glacial advances. BGS was the most extensiveglaciation which is luminescence dated at ~63 ka and 5 ka.However, relatively small end moraine complex suggeststhat it is unlikely that the terminus of the Bhagirathi glacieroccupied the valley fully to the Jhala end moraine throughoutthe whole of the BGS glaciation (Owen et al. 2002). TheBGS does not coincide with the Last Glacial Maximum(LGM) of the northern hemisphere (Sharma and Owen,1996). The Shivling glacial advance occurred during theHolocene (<5 ka), and reached 1.5 km to 3 km beyond thesnout of the present glacier, whereas, the Bhujbasa glacialadvance was attributed to the Little Ice Age (ca. 300 to 200year) during which glacier extended 1 to 2 km from thepresent day snout position. It has been suggested thatsince the Bhujbasa Glacial Advance (<5 ka), there has beenprogressive retreat of glaciers in the basin. A recent studyshow presence of two more stages in the Bhagirathi valleywhich they named as Kedar and Gangotri stages which aredated using the cosmogenic radionuclide dating to ~7 kaand ~1 ka respectively (Barnard et al. 2004). In addition tothis, the new chronology brackets the BGS glaciationbetween 63 ka and 11 ka (Barnard et al. 2004). It has beenestimated that during the BGS, ELA was depressed byabout 640 m, whereas, during Shivling and Bhujbasaglacial advances the ELA depressed between 40-100 mand 20-60 m respectively (Sharma and Owen, 1996).
In the Goriganga valley there are three distinct depositsof lateral moraines corresponding to three glacial stages of
decreasing magnitude (Pant et al. 2006). The oldest Glacialstage-I extended 15 km downstream from the present snoutposition and terminated at ~3,100 m near Rilkot. Glacialstage-II descended down to ~5 km at ~3,500 m near theMartoli Gorge, whereas, the youngest Glacial stage-IIIterminated proximal to the present day snout (~3,700 m)forming curvilinear ridges. There is only one age of 16 kaobtained on the laminated sand above the Glacial stage-IImoraine at 3,500 m. Based on this age it was suggested thatthe Glacial stage-I probably pre-date the LGM, GlacialStage-II correspond to the LGM, whereas, the Glacial stage-III was assigned the LIA.
Maximum elevation of lateral moraines (MELM) wasused in the Goriganga valley to estimate the past ELAdepression which indicated that during Glacial stage-I, ELAwas depressed by ~600 m, whereas, during stage-II and IIIit was around 300 m and 200 m respectively (Pant et al.2006). These estimates should be considered tentativeunless multiple methodologies are employed for palaeo ELAreconstruction. Evidence similar to the Goriganga valley wasobtained from the upper Alaknanda valley where three majorglaciations during the late Quaternary period have beenidentified (Nainwal et al. 2007). These are named fromolder to younger as the Alaknanda Glacial Advance (Stage-I), Alkapuri Glacial Advance (Stage-II) and SatopanthGlacial Advance (Stage-III). Stage-I was the oldest and thelongest which extended south of Badrinath (~3,000 m),followed by the Stage-II which was terminated ~3,550 m,whereas, Stage-III was terminated proximal to the presentday snout (~3,700 m). The Stage-I glaciation was assignedpre LGM age, and the optical chronology of recessionalmoraine dated at ~12 ka suggested that Stage-II glaciationsoccurred during the LGM, whereas Stage-III is dated at4.5 ka.
Recently, Scherler et al. 2010, based on 10Be dating oflateral and terminal moraines in the upper Tons valley inwestern Garhwal (Uttarakhand) identified a total of fiveglacial episodes that occurred after the LGM. The oldestand longest glaciation was dated at 16 ka when glaciersdescended down to ~2,500 m which represent a drop of~1,400 m compared to the present-day glacial extent.Following this, three glacial episodes of decreasingmagnitude have been dated between ~11–12 ka, ~8–9 ka,~5 ka, and <1 ka. The study observed that although therewas a significant change in the glacier cover, compared tothis, the ELA was depressed marginally (Scherler et al.2010).
The most recent work on the chronology of lateQuaternary glaciation comes from the Chorabari glaciervalley (Kedarnath) where Mehta et al. (2012) based on
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moraine stratigraphy supported by OSL dating showed fourmajor events of glaciations with decreasing magnitude.These are named as the Rambara Glacial Stage (RGS) datedat 13 ± 2 ka, the Ghindurpani Glacial Stage (GhGS) datedat 9 ± 1 ka, Garuriya Glacial Stage (GGS) dated at 7 ± 1 kaand the Kedarnath Glacial Stage (KGS) dated at 5 ± 1 ka.The RGS was most extensive during this event; glacierdescended down to an altitude of ~2,800 m and covered~31 km2 area of upper Mandakini river valley. Comparedto this, the other three glaciations (viz., GhGS, GGS andKGS) were of lower magnitudes terminating around ~3,000,~3,300 and ~3,500 m, respectively. It was also observedthat the mean ELA during RGS (~13 ka) was depressed by~373 m which may translate into a lowering of ~2°C summertemperature.
According to Sharma and Owen (1996), the longest BGSglaciation (~63 ka) pre-date the northern hemisphere LGMand the glaciated surface area increased by ~41%. Pant etal. (2006) supported the observations of Sharma and Owen(1996) that the LGM glaciation was limited in extent inCentral Himalaya. Similar inferences were drawn byNainwal et al. (2007) from the Alaknanda valley along withthe evidence for temporary hiatus in deglaciation around12 ka which they attributed to the post LGM Younger Dryascooling. Scherler et al. (2010) have suggested that decreasein glacial extent is associated with the coeval changes intemperature and precipitation. According to them, unlikemany other regions in the northern hemisphere, the Tonsglaciers had considerable extent during the early Holocenewhich they attributed to the enhanced monsoon precipitation.In the Kedarnath valley, Mehta et al. (2012) suggested thatthe post LGM glaciation was driven by increased insolationand strengthened monsoon. The maximum valley glaciationaround 13 ka is attributed to cooler summer temperaturesand increase in monsoon precipitation. Considering that themonsoon dominated glaciers are temperature-sensitive, theyattribute continuous decrease in ice volume since 13 ka tothe increase in temperature.
DISCUSSION
Glacier expansion is generally a response of lowertemperature, but at high altitude it may be more sensitive tochanges in moisture transport (Brown et al. 2002). It hasbeen suggested that in humid regions, glaciers advanced dueto changes in precipitation, whereas, in arid regions theyare temperature driven (Owen et al. 2005). However, in arecent study, Zech et al. (2009) suggested that glacierssituated in orographically shielded areas are more sensitiveto changes in precipitation, whereas, the one which are in
high precipitation areas are more sensitive to temperature.Therefore, although there is a general agreement on thesignificance of monsoon strength and glacier response inHimalaya (Benn and Owen, 1998), the exact mechanisms,timing and geographical extent of monsoonal influence isstill debated (Scherler et al. 2010).
Towards this, the existing chronometric data providessome insight in understanding the role of orography,temperature and precipitation in glacier dynamics. Evidencefrom the northwestern Himalaya (Ladakh and Karakoram)which receive dominant precipitation from mid-latitudewesterlies (Osmaston, 1994; Taylor and Mitchell, 2000),glaciers in principle should have responded synchronouslywith that of the northern hemisphere glaciations (during coldperiods mid-latitude westerlies were enhanced). Instead, theexisting chronometric data suggests that glaciations inLadakh and Karakoram were monsoon dominated. It hasbeen attributed to the orographic shielding caused due tothe uplift of northwestern Karakoram thus preventingmoisture transported by the mid-latitude westerlies duringthe last 400 ka into the eastern Karakoram ranges (Owen etal. 2006). Although high Himalaya in the south also acts asa barrier for northward penetration of monsoon, however, itwas observed that during the periods of intensified summermonsoon, moisture penetrated the higher Himalayanbarrier (Bookhagen et al. 2005). Similar analogy can beextended to the periods of enhanced westerlies associatedwith cold periods; they could have also crossed over theKarakoram ranges and would have supplied the moisture tothe Ladakh and Karakoram glaciers. Therefore, based onthe existing chronometric data it would be premature toconclude that the north-western Himalayan glaciersresponded in accordance with the western and centralHimalayan glaciers (monsoon influenced). This requiresfurther investigation covering large areas using multipledating techniques.
So far nearly 107 ages from the north-western Himalayaand 92 ages from western and central Himalaya have beenobtained on moraines. These ages are plotted in Fig. 2 and3. In order to understand the relationship between the glacialchronology and climate variability, the ages are subjectedto probability distribution (Venkatesan and Ramesh, 1993).In the north-western Himalaya a broad hump is observedbetween 160 ka and 70 ka. Climatically this period coverspart of MIS-6 and the entire MIS-5 (Fig. 2B). A secondprominent peak is observed between 60 ka and 30 ka, whichcorrespond to MIS-3 and the third prominent peak occursbetween ~12 ka and 5 ka (late MIS-2 and MIS-1). Comparedto this, the existing chronology of moraines in western andcentral Himalaya is restricted to 70 ka. In fact there are only
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three numeric ages between 70 ka and 30 ka and majorityof the ages are <20 ka (Fig.3A). The probability densitydistribution of ages shows a prominent peak between 18 kaand 10 ka (MIS-2) followed by a broad hump till 4 ka andanother prominent peak after 2 ka.
The events thus identified based on the distribution/
clustering of ages, indicates that in the north-westernHimalaya glaciers responded to a cold MIS-6 and continueduntil the last interglacial (MIS-5) (Fig.2B). During the MIS-5 (130 ka to 70 ka), we speculate that glaciers responded toboth precipitation and temperature changes which in turnwas modulated by orographic shielding effect (Zech et al.
Fig. 2(A) Age distribution of the north western Himalaya. (1) Owen et al. 2006; (2) Dortch et al. 2010; (3) Taylor and Mitchell, 2000).Vertical dispersion of ages represents Rank (0-1), (B) Probability density plot of the published ages, (C) Oxygen isotopic recordof Guliya Ice core. Enriched isotopic values indicate increase insolation and strengthen monsoon.
Fig.3. (A) Age distribution of the western and central Himalaya. (1) Owen et al. 2001; (2) Scherler et al. 2010; (3) Barnard et al. 2004;(4) Sharma and Owen, 1996; (5) Mehta et al. 2012; (6) Owen et al. 1997; (7) Nainwal et al. 2007; (8) Pant et al. 2006). Verticaldispersion of ages represents Rank (0-1), (B) Probability density plot of the published ages, (C) Oxygen isotopic record of GuliyaIce core. Enriched isotopic values indicate increase insolation and strengthen monsoon.
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2009). Following this, two prominent peaks of glacialexpansion can be suggested during 60 ka and 30 ka (MIS-3) and <20 ka to 5 ka (later part of MIS-2 and MIS-1) in themoisture limited north-western Himalaya (Fig.2B).Coincidently the major periods of glaciations compares wellwith the period of intensified monsoon inferred based onthe enriched δ18O values in Guliya ice core (Fig. 2C). Theseaccords well with the suggestion, that the Himalayan glaciersadvanced during times of increased insolation when moisturedelivered by an active monsoon system provided snow tocreate positive glacier mass balances (Phillips et al. 2000;Owen et al. 2008). According to Anderson and Mackintosh(2006), advance and retreat of temperate glaciers is largelycontrolled by changes in both temperature and precipitation,but the relative importance of these drivers is debated. Theirmodelling study in a New Zealand glacier suggested coolingis the major driver of glacier advance, but do not preclude asubsidiary role of precipitation changes. In most cases inthe high and mid-latitudes, summer temperature controlsmass balance and, hence, glacier size variations (Oerlemans,2005; Steiner et al. 2008). Thus it can be suggested thatglaciers in the arid north-west Himalaya advanced undercooler summer temperature (Schäfer et al. 2002) andoptimum moisture condition during the entire MIS-5, MIS-3 and post LGM MIS-2 and MIS-1. This should be treatedas a tentative suggestion, a firmer inference should awaitlarger chronometric data base with wider geographicalcoverage.
In the western and central Himalaya, barring three agesthat lies between 70 ka to 30 ka majority of the ages arebelow <20 ka and show two distinct population of moraineages viz. one immediately after the LGM (<20 ka to 10 ka)and second after 2 ka (Fig. 3A). Between 10 ka to >2 kathere is a wide scatter of moraine ages (Fig. 3A). It has beenobserved that in monsoon dominated Tons and Kedarnathvalleys, moraines corresponding to the LGM and pre LGMare absent. Since monsoon dominated valleys are subjectedto intense erosion, absence of older moraines could be dueto the erosion or alternatively, younger glacial events weremore active that might have destroyed the evidence of earlyglaciations (Owen et al. 2006). This is an enigma that needsfurther investigation. Continental record of monsoonvariability obtained using the proglacial lake record fromcentral Himalaya indicate a progressive increase in monsoonstrength after 20 ka particularly after 17 ka till ~11 ka (Juyalet al. 2009; Beukema et al. 2011). This accords well withthe chronology of post LGM glaciations indicating role ofmonsoon in post LGM glacier expansion. The wide scatterin moraine ages during early to mid-Holocene is difficult toexplain, however, one of the reasons could be the
orographically controlled spatial and temporal variabilityin the temperature-precipitation. A close cluster of ages after2 ka encompasses relatively high insolation/precipitationevents such as pre-medieval warm phase and medievalwarm phase but also the cold LIA. Studies have shown thatglaciers from the mid- to high latitudes in the northernHemisphere advances after ca. 6,000 years BP and reachingtheir maximum extent in the LIA. Considering that therewas a steady decline in insolation during the Holocene(Sachs, 2007; Tiwari et al. 2010), glacier expansion duringthe last 2 ka in the western and central Himalaya can beattributed to a combination of lower temperature and optimalmoisture conditions. Peat bog study in the periglacialregion of central Himalaya showed that climate fluctuatedbetween warm and moist to cold and moist during the LIA(Rühland et al. 2006).
Taking into consideration the uncertainties associatedwith ages, there are few ages that correspond to the globalglacial maximum viz. the LGM (Fig. 2 & 3). However, theseare insignificant compared to others. Thus based on theexisting chronology, glaciations during the LGM in theHimalaya were less extensive compared to pre-LGM (MIS-3) and post-LGM (MIS-1) events (Benn and Owen, 1998;Owen et al. 2002; Juyal et al. 2004; Owen et al. 2006; Pantet al. 2006; Owen et al. 2008; Juyal et al. 2009). Even in aterrain like Karakoram and Ladakh which presently receiveappreciable contribution of moisture from mid-latitudewesterlies (Benn et al. 2005; Bhutiyani et al. 2010), andthe same was known to be enhanced during the LGM (Dinget al. 1995; Benn and Owen, 1998), absence of extensiveglaciation during the LGM is intriguing. Could it be becausewe have not been able to locate them or is it because of thelimitation of the exposure age dating as we have seen that inmany cases the exposure ages are significantly under-estimated due to post depositional erosion of the bouldersurface (Barnard et al. 2004). May be we need to have morechronometric data using multiple methods on welldocumented moraine stratigraphy from the north-westernHimalaya before we conclude that the LGM glaciation inthis part of the Himalaya was limited or non-existent.Nevertheless, based on the existing chronometric data thesuggestions that abundant precipitation is required to allowglaciers to grow (Owen et al. 2002) seems to be logical.Considering that during the LGM monsoon precipitationwas significantly reduced compared to the MIS-3 and MIS-1 (Prell and Kutzbach, 1987), this probably explains thelimited glacial extent in part of the Himalaya thatpredominantly receive moisture from the southwest summermonsoon (western and central Himalaya). However, theexisting chronometric data does not provide a satisfactory
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answer to the absence of LGM glaciations in westerlydominated northwestern Himalaya.
FUTURE PERSPECTIVE
The role of westerlies in modulating the late Quaternaryglaciation in the north-western Himalaya is yet to bedemonstrated. Additionally, was it the precipitation or thetemperature change that drives the glaciations in theHimalaya is still being debated. The existing data are farfrom adequate to arrive at any meaningful conclusion.It is high time that intensified efforts should be madetowards studying the history of palaeoglaciation in theHimalayan region. Towards this a systematic mapping ofthe moraines located in different climatic zones should becarried out in order to reconstruct the local and regionalmoraine stratigraphy and chronology should be attemptedon stratigraphically constrained glacial events using acombination of exposure age and OSL dating techniques.
Glacier histories are often characterized in terms ofequilibrium line altitudes (ELAs). There is a need toreconstruct chronological constrained ELAs from theHimalayan region. Because changes in the ELA through
time are considered as one of the most useful glaciologicalmeasures for reconstructing climate change (Porter, 1975;Paterson, 1999; Benn et al. 2005; Owen and Benn, 2005) ascompared to the other glacier properties such as length,which depends on ice dynamics, bed geometry etc. (Paterson,1999). The variation in ELAs if correlated across largerglaciated region, they can be interpreted in terms of thehistory of the regional climate prevailing during a particularperiod.
In the geological and historical period, Himalayanglaciers fluctuated in accordance with the changing climaticconditions (natural forcing). However, in recent times,there is a growing concern towards the impact of anthro-pogenically induced global warming on the Himalayanglaciers. This may be true that increase in temperature isprobably causing the worldwide retreat of many glaciersduring the past few decades, the number of systematic studiesof longer records is quite small (Oerlemans, 2005). In viewof this, before we arrive at any conclusion, it is essentialthat we must generate data covering wider geographical andecological domains taking into consideration multipleparameters that directly or indirectly influence the glaciermass balance.
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(Received: 12 September 2012; Revised form accepted: 31 December 2012)
