Pattern of Holocene glaciation in the monsoon-dominated Kosa Valley, central Himalaya, Uttarakhand, India

    Pattern of Holocene glaciation in the monsoon-dominated Kosa Valley, central Himalaya, Uttarakhand, India Pinkey Bisht, S. Nawaz Ali, Naresh Rana, Sunil Negi, Poonam, Y.P. Sundriyal, D.S. Bagri, Navin Juyal PII: DOI: Reference:

S0169-555X(16)31133-3 doi:10.1016/j.geomorph.2016.11.023 GEOMOR 5845

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

4 May 2016 28 November 2016 30 November 2016

Please cite this article as: Bisht, Pinkey, Nawaz Ali, S., Rana, Naresh, Negi, Sunil, Poonam, Sundriyal, Y.P., Bagri, D.S., Juyal, Navin, Pattern of Holocene glaciation in the monsoon-dominated Kosa Valley, central Himalaya, Uttarakhand, India, Geomorphology (2016), doi:10.1016/j.geomorph.2016.11.023

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ACCEPTED MANUSCRIPT Pattern of Holocene glaciation in the monsoon-dominated Kosa Valley, central Himalaya, Uttarakhand, India

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Pinkey Bisht1*, S. Nawaz Ali 2,3, Naresh Rana1,4, Sunil Negi 1, Poonam1, Y.P. Sundriyal 1,

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D. S. Bagri1 and Navin Juyal 3

Department of Geology, HNB Garhwal University, Srinagar, Uttarakhand – 246174, India

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Birbal Sahani Institute of Paleosciences, Lucknow – 226007, India

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Physical Research Laboratory, Navrangpura, Ahmadabad – 380009, India

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National Center for Seismology, New Delhi – 110003, India

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*Corresponding author. Email: [email protected] (P. Bisht)

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Abstract

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Reconstruction based on the geomorphology, lateral moraine stratigraphy, and limited

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optical chronology indicate that the monsoon-dominated Kosa Valley experienced four glacial advances during the late glacial to late Holocene. The oldest and most extensive glaciation, which is termed as Raj Bank Stage-1 (RBS-1), is represented by the degraded moraine ridge. This glaciation remains undated; however, the chronology of outwash terrace gravel dated to 12.7 ±1.3 ka indicates that the RBS-1 probably represents the Last Glacial Maximum (LGM). The second glacial advance (RBS-2) is preserved as a curvilinear lateral moraine and is dated to 6.1 ±0.4 ka. The third glacial advance viz. RBS-3 is bracketed between 5.0 ±0.5 and 4.0 ±0.4 ka. Following this, the glacier receded in pulses that are represented by two distinct recessional moraines (RBS-3a and b). The forth glacial stage (RBS-4), which is dated between 2.2 ±0.2 and 1.6 ±0.2 ka, shows a pulsating recession and is represented by a prominent recessional moraine (RBS-4a). Whereas, presence of unconsolidated, poorly defined moraine mounds proximal to the glacier snout are ascribed as neoglacial advance corresponding to the Little Ice Age (LIA).

ACCEPTED MANUSCRIPT With the limited chronometric data, we speculated that the glaciation was driven during the weak to moderate Indian Summer Monsoon (ISM) aided by lowered temperature. Presence

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of recessional moraines associated with mid-Holocene glacial phase indicate that the

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monsoon-dominated glaciers respond sensitively to minor (submillennial scale) changes in temperature and precipitation conditions. The observations are broadly in accordance with the

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studies carried out in other monsoon-dominated valleys in the central Himalaya, implying that in

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ISM-dominated regions, lowered temperature seems to be the major driver of glaciations during the late glacial to late Holocene.

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1. Introduction

The sensitivity of the Himalayan glaciers to the temporal changes in precipitation and

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temperature are used successfully to reconstruct the past climate variability (Ali and Juyal, 2013; Ali et al., 2013; Sati et al., 2014; Bisht et al., 2015; Sharma et al., 2016). Particularly, the

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reconstruction of Holocene glacier fluctuations has grown in significance owing to the anticipated global climate change (IPCC, 2007; WGMS, 2008). The study pertaining to the Holocene glacier fluctuation allows us to verify and eventually adjust the glacier models for predicting future glacier changes (Winkler and Matthews, 2010). In recent times, significant progress has been seen in the Holocene glacier chronologies from the Himalayan-Tibetan orogen (e.g., Ali et al., 2013; Owen and Dortch, 2014; Sati et al., 2014; Bisht et al., 2015). Considering that the Holocene period has experienced centennial to millennial scale climatic oscillations (Hong et al., 2003; Wang et al., 2005; Fleitmann et al., 2007), glacier response particularly from the ISM-dominated regions like the central Himalaya needs to be understood. Some studies from this region (e.g., Ali et al., 2013; Sati et al., 2014; Bisht et al., 2015) have demonstrated that the

ACCEPTED MANUSCRIPT moraines left behind by receding glaciers can be used to infer relative changes in past precipitation and temperature.

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In addition to this, the timing and amplitude of Holocene glaciation has become

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extremely important because studies suggest that despite an increase in the solar insolation, glaciers in the southern part of the Himalayan ranges advanced during the early Holocene

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(Rupper et al., 2009). Himalayan glaciers are nurtured by the mid-latitude westerlies and ISM

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(Finkel et al., 2003; Yang et al., 2008) and the influence of these two weather systems varies spatially and temporally (Benn and Owen, 1998). For example, in the central Himalaya, studies

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suggest that an increased landward moisture flux during the LGM (Dykoski et al., 2005; Sinha et al., 2005; Wang et al., 2005; Herzschuh, 2006) corresponds to the periods of widespread glacial

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advances (Phillips et al., 2000; Finkel et al., 2003; Barnard et al., 2004a). Existing studies from the central Himalaya suggest that the relict glaciated valleys have

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preserved moraines since late glacial to the LIA (Owen et al., 2005; Mehta et al., 2012; Bali et al., 2013; Murari et al., 2014; Sati et al., 2014; Bisht et al., 2015). Absence of older records of glaciation in these valleys is attributed to the erosion caused by heavy monsoon precipitation or the concealment by the late glacial advance (Sati et al., 2014). Processes responsible for driving the Holocene glaciation are contradictory e.g., the temporal changes in the ISM intensity is implicated for driving the glaciations in the Himalaya (Phillips et al., 2000; Finkel et al., 2003; Gayer et al., 2006; Ali and Juyal, 2013). On the contrary, a number of studies have also suggested that the glaciers in monsoon-dominated regions are more sensitive to summer temperature changes rather than the ISM (Denton et al., 2005; Oerlemans, 2005; Anderson and Mackintosh, 2006; Schaefer et al., 2008; Zech et al., 2009; Bisht et al., 2015). Rupper et al. (2009) suggested that the glacier advances during early to mid-Holocene were influenced by

ACCEPTED MANUSCRIPT changes in the ISM and by temperature. Considering above, we can argue that the processes responsible for driving the glaciations in the Himalaya still remain far from clear (Scherler et al.,

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2010; Murari et al., 2014; Owen and Dortch, 2014). The present study is therefore a contribution

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toward improving our understanding on the chronology and the processes responsible for driving

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glaciation in the monsoon-dominated Kosa Valley, central Himalaya. 2. Geomorphology and Climate

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Kosa Valley (Raj Bank glacier) is located in the Dhauliganga watershed of the central

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Himalaya, Uttarakhand, India (Fig. 1). The valley is occupied by Raj Bank and Kosa glaciers having an area of ~101 km², out of which ~30 km² is glaciated (Fig. 2). The relict glaciated

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valley morphology suggests that in the past, these two glaciers were coalesced into a single trunk

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glacier that together descended proximal to the Kosa village. The valley is surrounded by the peaks of Hathi Parbat (6727 m asl), Gauri Parbat (6601 m asl), Rataban (6126 m asl), and Ukhi

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Pahar (5756 m asl). The valley lies in the northern fringe of the Higher Himalayan Crystalline and the lithology is dominated by the Vaikrita group of rocks (Sinha, 1989), which consists of mica schist, granite, quartzite biotite schist, kyanite schist, and augen gneiss (Fig. 3). The ISM is the major source of precipitation (~80%). The distribution of the vegetation is governed by latitude. In the lower reaches (between 2000 and 2500 m asl), Pinus, Quercus, Carpinus, Ulmus, Juglans regia (walnut), and Alnus dominates; whereas above 2500 to 3000 m asl, mainly herbaceous species and scrubby vegetation like Juniperus, Rhododendron, Betula utelis, Cassiope, Salix, and Saxifraga are found. 3. Materials and methods 3.1. Mapping of moraine stratigraphy

ACCEPTED MANUSCRIPT Using geomorpho-stratigraphical methods, we mapped the moraine in the Kosa Valley. The moraine stratigraphy is built, based on field survey using a Thales mobile mapper (GPS),

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Survey of India topographic map (1:50,000 scale), supplemented with Google Earth images.

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Additionaly, the conventional technique such as the position of moraines, degree of weathering on moraine crests and extent of vegetation cover is also used to supplement field observations.

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Based on the above, four generations of glaciation are identified (Figs. 4A, B).

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3.1.1. Raj bank glacial stage-1 (RBS-1)

The RBS-1 is the oldest and most extensive glaciation in the valley and is represented by

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a discontinuous and degraded right lateral moraine ridge preserved preferentially on the southern flank of the valley. This RBS-1 moraine can be traced laterally untill ~3145 m asl to a distance

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of ~5.5 km from the glacier snout (Fig. 5A). The degraded patches of the moraine are often covered by the active debris flow fans. The vegetation on the moraine is dominated by betula

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utilis and conifers. Because of the discontinuous nature and concealment by the debris flow fans, the emergence point of the RBS-1 moraine is obscured. A tentative reconstruction based on the occurance of the moraine suggests that during the RBS-1 glaciation, Raj Bank and Kosa glaciers were coalesced and descended down to the elevation of 3145 m asl. This is further indicated by the occurance of a glaciofluvial outwash terrace around Kosa village that laterally connects with the RBS-1 deglaciation (Fig. 5B). The sediment textural attributes of the glaciofluvial outwash terrace

shows dominance of angular to subangular granitic boulder, cobbles, and gravels

embedded in a silty-sand-dominated matrix. A sample (OWT-1) for optical dating is collected from the middle part of the glaciofluvial outwash terrace (Fig. 5B). 3.1.2. Raj bank glacial stage-2 (RBS-2)

ACCEPTED MANUSCRIPT The RBS-2 glaciation is represented by a weathered latero-frontal moraine, which extends ~1.2 km downstream from the present glacier snout position. The moraine terminates at

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an altitude of ~3684 m asl and is preferentially preserved along the right flank of the valley. The

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moraine surface is covered with shrubs and grasses. At places, the moraine is either eroded or concealed by the active scree fan deposits.A sample for optical dating (RBS-2) is collected from

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fluviolacustrine deposits preserved behind the RBS-2 moraine (Figs. 6A, B).

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3.1.3. Raj bank glacial stage-3 (RBS-3)

The RBS-3 glaciation is represented by the well-preserved latero-frontal moraine ridge

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that runs parallel to the modern glacier and terminates at ~3713 m asl (30˚41'52.16" N, 79˚48'53.15" E). The moraine ridge is relatively stable, moderately vegetated with grasses and

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junipers. In the lower reaches, the main moraine ridge (RBS-3) bifurcates into two latero-frontal recessional moraines viz. RBS-3a at ~3800 m asl (30˚41'58.94" N, 79˚48'26.62" E) and RBS-3b

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at 3847 m asl (30˚42'05.30" N, 79˚48'18.26" E). Two samples, RBS-3 and RBS-3(1), are collected for optical dating from the RBS-3 moraine, one from the lateral moraine (RBS-3) and the other from the terminal moraine (Figs. 7A, B). 3.1.4. Raj bank glacial stage-4 (RBS-4) This is the youngest glacial stage which is preserved in the form of well-defined, sharpcrested linear moraine ridges 20−30 m high (RBS-4) and which runs parallel to the modern glacier terminating at ~3820 m asl (30˚42'07.69" N, 79˚48'22.12" E). A subsidiary crecentric moraine ridge (RBS-4a) emanates at ~3844 m asl (30˚42'06.63" N, 79˚48'17.90" E) from the main ridge (RBS-4) and terminates at ~3835 m asl (30˚42'08.58" N, 79˚48'18.48" E). Two samples for optical dating viz. RBS-4 and RBS-4(1) are collected from the cryogenically

ACCEPTED MANUSCRIPT deformed silty-sand layer exposed at the left lateral moraine (Fig. 8A) and the terminal moraine of the RBS-4 glaciation (Fig. 8B).

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3.2. Equilibrium line altitude (ELA) and ice volume estimation

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Equilibrium line altitude (ELA) being a climate-sensitive parameter defines the position on a glacier where the annual mass accumulation is equal to the annual mass loss and responds

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sensitively to the temperature and precipitation changes (Ramage et al., 2005). The precise

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location of moraines is required in order to demarcate the former extent of glacier and valley morphology to determine the ELA. In the present study, the former extent of glaciation is

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determined on the basis of moraine morphology. Toward this, we used the Google Earth image, topographic maps (Survey of India, at 1:50,000 scale), and a field survey supported by a Thales

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mobile mapper. The above information along with the ASTER DEM (30-m resolution) data is used to calculate the hypsometry of the study area (Benn et al., 2005), which is then used to

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reconstruct the former ELAs. The estimation of former ELA employed (i) area accumulation ratio (AAR), (ii) toe-to-headwall altitude ratio (THAR), and (iii) area weighted mean (AWM). For reliable estimation of former ELAs, averages of all three methods are used (Benn et al., 2005).

The modern ELA is mapped using multi-temporal Landsat images (2005-2015) at the end of the ablation season because snowline altitude at the end of the ablation season mimics/ coincides with the ELA (Duncan et al., 1998; Klein and Isacks, 1998; Kulkarni et al., 2005; Guo et al., 2014). The accurate demarcation of ELA on a glacier requires differentiation of the snow from ice. Toward this, we used the classified reflectance images that are then draped over SRTM (Shuttle Radar Topographic Mission) DEM, and the elevation points are determined along the

ACCEPTED MANUSCRIPT glacier. Finally, the ELA of the present glacier is calculated by averaging the elevations obtained.

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The paleo-ELAs are calculated using all three methods viz. AAR, THAR, and AWM. In

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the case of AAR, if the assumed ratio is <0.50, the glacier has a negative mass balance. Whereas, steady state condition of the glacier is reflected by the ratio between 0.50 and 0.80, and the ratio

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above 0.80 corresponds to positive mass balance regimes (Mier and Post, 1962; Andrews, 1975).

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The studies carried out so far in the monsoon-dominated, debris-covered Himalayan region (Mehta et al., 2012; Ali et al., 2013; Dobhal et al., 2013; Mehta et al., 2014; Sati et al., 2014)

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suggested that a glacier having an AAR of ~0.45 can be considered in a steady state condition. Considering that the Raj bank is a debris-covered glacier, we used the average AAR value of

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0.45 and 0.55 to calculate the paleo-ELAs.

The THAR method uses the ratio between the maximum and minimum altitude of a

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glacier and provides a rapid assessment of former ELA (Meierding, 1982; Porter, 2001). Recent studies from the adjoin valleys in the central Himalaya suggested that THAR of 0.5 and 0.6 give the relatively accurate estimate of paleo ELAs (Mehta et al., 2012; Ali et al., 2013; Dobhal et al., 2013; Sati et al., 2014; Bisht et al., 2015). The above values are used in the present study. ELA=At+ THAR (At-Ah)

(1)

where, At is terminus altitude, and Ah is headwall altitude. The AWM method was introduced by Sissons (1974) to reconstruct the paleo-ELA. This method is based on the assumption that the amount of ablation decreases linearly with the heights and is used by many workers to reconstruct past ELAs (e.g., Sissons, 1974; Sharma and Owen 1996; Porter, 2001). The expression given by Sissons (1974), to estimate the former ELA, is

ACCEPTED MANUSCRIPT ELA= Ai*Hi/Ai

(2)

where Ai is the glacier area in km,2 and Hi is the mid-point altitude in contour interval.

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The determination of ELA change (∆ELA) is carried out by subtracting the average

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values of former ELAs from that of the average values of modern ELA. Although ELA is modulated by the changes in temperature and precipitation, the temperature estimates, however,

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are rather simple as they follow an average environmental lapse rate in the atmosphere (Porter,

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2001). In the absence of direct measurement of the lapse rate from the study, we used the lapse rate of 0.60 °C/100 m obtained from the monsoon-dominated Dokriani glacier in the central

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Himalaya (Thayyen et al., 2005; Table 1).

Additionally, an attempt has also been made to calculate an approximate ice volume

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during the different glacial stages. This was done using the empirical relationship developed by Chaohai and Sharma (1988). This method has been employed by many workers to calculate the

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ice volume for the Himalayan glaciers (e.g., Rana et al., 2006; Kulkarni et al., 2007; Ashraf et al., 2012; Ali et al., 2013):

and

H= -11.32+53.21F0.3

(3)

V=H*F

(4)

where H is the mean glacier depth (m), F is the glacier area (km2), and V is the volume of the glacier (km3). Table 2 gives ice volume estimated for the different glacial stages. 4. Optical dating Samples for optical dating were collected in specially designed metal tubes (Chandel et al., 2006). Pure quartz is extracted through sequential pretreatment using 1N HCl and 30% H2O2 to remove carbonates and organic matter, respectively. Following this, 90−150 µm grain size

ACCEPTED MANUSCRIPT samples were separated using a Frantz magnetic separator and etched with 40% HF for 80 minutes followed by treatment with 12N HCL for 30 minutes with constant magnetic stirring to

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remove the outer α-irradiated skin (~20 µm). The etched quartz was then mounted on stainless

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steel disks using silicone spray. Luminescence measurements were made using an automated Risø TL-OSL reader (TL/OSL-DA-20). The samples were stimulated using blue diode (470±20

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nm) and detected with EMI 9835QA photomultiplier tube coupled with a 7.5-mm Hoya U-340

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filter (emission 330±35 nm). Beta irradiations were carried out using an on-plate 90Sr/90Y beta source with the dose rate of 4.59 Gy/min. The equivalent dose (De) was measured using single

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aliquot regeneration (SAR protocol; Murray and Wintle, 2000; Banerjee et al., 2001). Dose growth curve was constructed using five regeneration dose points, including one point to

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estimate the recuperation and another point to estimate the reliability of sensitivity correction (recycling ratio). On an average, 40 aliquots for each sample were analyzed, out of which 11-20

<10%.

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aliquots qualified the criteria of having a recycling ratio within 20% of unity and recuperation

The radioactivity (uranium, thorium, and potassium) for estimating environmental dose rate estimation was carried out using the high purity Germanium (HPGe) detector. To attain the radioactive equilibrium, the samples were sealed in a plastic box for ~15 days. The errors of measurement (systematic and statistical uncertainties) are <5% (Shukla et al., 2002). An average water content of 10 ±5% was used in which the average water content of the in situ sample was determined by weighing the sample before and after drying and is expressed as the weight of the water divided by the weight of the sample (Aitken, 1998; Fuchs et al., 2014). Cosmic ray contributions in dose rate were calculated using the method suggested by Prescott and Hutton (1994).

ACCEPTED MANUSCRIPT In the computation of equivalent dose (De), we used central age model (CAM) for the samples showing overdispersion values <40%, and the minimum age model (MAM) for the

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samples showing overdispersion >40% (Galbraith and Laslett, 1993; Van Der Touw et al., 1997;

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Galbraith et al., 1999; Jacobs et al., 2006, 2008; Arnold et al., 2009). The distribution of equivalent dose of all samples is represented graphically by radial plots (Fig. 9). The De values

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are plotted on the y-axis. The majority of De values lies within the gray band (2σ band),

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suggesting that samples are reasonably well bleached (Olley et al., 1999).

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Out of the total of six samples dated, four samples were collected from sand lenses within the moraines, one from the fluviolacustrine deposits, and one from the glaciofluvial

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outwash terrace. The outwash terrace (OWT-1) yielded an age of 12.7 ±1.3 ka, the RBS-2

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moraine is dated to 6.1 ±0.4 ka, whereas the RBS-3 moraine yielded an age of 4 ±0.4 and 5.0 ±0.5 ka. Two samples collected from the youngest glacial stage viz. the RBS-4 are dated to 2.2

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±0.2 and 1.6 ±0.2 ka, respectively. Table 3 summarizes the De, dose rates, and ages of the samples analyzed. 5. Discussion

The Holocene glacial records from the Himalaya can be used as a potential archive to reconstruct the high resolution (submillennial scale) paleoclimatic history in the region (Owen, 2009). Interestingly, the most extensive glaciation after the LGM was observed during the early Holocene in the Himalayan region (Clapperton, 1993; Porter, 2000; Grove, 2004; Sati et al., 2014). This has generated a debate as to whether the early Holocene glacier advances in the central Himalaya were a response to temperature (cooling) or increased moisture through ISM. The ISM is consider as one of the major drivers of glaciations in the Himalaya (Phillips et al., 2000; Finkel et al., 2003; Gayer et al., 2006). However, more recent studies show that

ACCEPTED MANUSCRIPT precipitation has rather limited influence compared to the temperature in driving the glaciations on timescales of several hundred years or more (Denton et al., 2005; Oerlemans, 2005; Anderson

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and Mackintosh, 2006; Schaefer et al., 2008; Zech et al., 2009; Murari et al., 2014). In view of

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the conflicting suggestions, which probably reflect the ecological complexity of the Himalayan

with secured stratigraphic and chronometric data.

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glaciers, calls are made for wider geographical coverage on paleoglaciation studies in the region

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The present study, though constrained by the limited optical chronology, revealed four events of glaciation during the Holocene. Based on the chronology of the glaciofluvial

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outwash terrace (OWT-1) dated to ~12.7 ka, the oldest and most extensive glaciation (RBS-1) is speculatively assigned LGM. More recently in the eastern Zanskar Himalaya, Sharma et al.

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(2016) demonstrated that valley glaciers indeed expanded during the global glacial maximum corresponding to the LGM. Evidence similar to this was obtained from the monsoon-

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dominated Gori Ganga Valley (Ali et al., 2013) and from the Dhauli Ganga Valley (Bisht et al., 2015). Considering that the ISM was weaker during the LGM (Bisht et al., 2015, and references therein), these studies implicated the enhanced mid-latitude westerlies for contributing the required moisture for driving the LGM glaciations in the central Himalaya. Because of the poor preservation of the moraines corresponding to RBS-1 glaciation, the ELA depression and associated ice volume is not attempted. Further, unlike the adjoining Dunagiri Valley (Sati et al., 2014) and the more northerly Dhauli Ganga Valley (Bisht et al., 2015), where the evidence of early Holocene glaciation is well represented, these moraines are not preserved in the Kosa Valley. We hypothesize that either the moraines are concealed or eroded away because of early Holocene intensified ISM. We tend to suggest that the latter seems to be the more plausible reason for their absence. However,

ACCEPTED MANUSCRIPT subsequent younger moraines are well preserved, for example, the moraines corresponding to the RBS-2 glaciation are dated to ~6 ka, covering an area of ~21 km2. During RBS-2 glaciation, the

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ELA was lowered by ~237 ±9 m, which corresponds to a temperature decline of ~1.42°C.

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Climatically, the event possibly can be equated with moderate ISM and cooler air temperature that seem to prevail during the early part of the mid-Holocene (Gasse et al., 1991; Thompson et

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al., 1997; Phillips et al., 2000; Fleitmann et al., 2003; Wang et al., 2005; Juyal et al., 2010). A

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correlatable evidence of marginal advance in valley glaciers is reported from northern China (Zhou et al., 1991), the northern part of the Tibetan plateau (Shijie, 1993), and the Dunagairi

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Valley in the central Himalaya (Sati et al., 2014) thus signifying that the event was regional in nature.

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The RBS-3 moraines are dated between ~5 and ~4 ka (mid-Holocene), during which the

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glacier covered an area of ~19 km2 and ELA lowered by ~77 ±3 m, which translates into a

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temperature decline of ~0.46°C. According to Owen (2009), during the mid-Holocene glaciers advanced a few hundred to several kilometers beyond their present positions, which according to (Rupper et al., 2009) was driven by the lowered temperature. The RBS-3 glaciation correlates reasonably well with the evidence acquired from the central Himalaya. For example, (Scherler et al.,2010) dated moraines in Harki Dun to ~5 ka, Shivling glacial stage in Bhagirathi Valley is dated to ~5 ka (Sharma and Owen, 1996; Barnard et al., 2004a), and Kedarnath glacier stage (KGS) dated to ~5 ka in the Mandakini Valley (Mehta et al., 2012). In the adjoining Dunagiri Valley, this event is dated between ~7.5 and ~4.5 ka (Sati et al., 2014). The response of valley glaciers seems to be synchronous to mid-Holocene climate variability (i.e., moderate ISM and lowered temperature). According to Rupper et al. (2009), during the mid-Holocene an increase in snow accumulation owing to high precipitation was much smaller than reductions in ablation

ACCEPTED MANUSCRIPT caused because of lower temperatures. Therefore, we suggest that although rainfall was low during the RBS-3 glaciation, the marginal glacier advance and its sustenance however, were

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dictated by lowered temperature.

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The moraine corresponding to RBS-4 glaciation is dated between ~2 and ~1.6 ka. This period broadly correlates with the late Holocene glaciation. The glaciated area decreased to ~18

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km2; however, ELA was lowered marginally (~56 ±2 m) corresponding to a temperature decline

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of ~0.33°C. The advancement of glaciers during the late Holocene is reported from other parts of the central Himalaya (Barnard et al., 2004a; Dorch et al., 2013; Sati et al., 2014). This phase

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of glaciation accords well with the regional environmental reconstruction based on peat bog data from the Pindar Valley, suggesting a moist climate after ~1572 cal YBP and before ~762 cal

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YBP, (Rühland et al., 2006). On the basis of relative decline of broad-leaved and meadow taxa and a prolific increase of coniferous taxa from Chandra Valley, (Rawat et al., 2015) suggested a

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cool and moist climate during ~2032 to ~1158 cal YBP. Similarly, Kar et al. (2002) suggested a cool-moist climate in Garhwal Himalaya, e.g., between ~2500 and ~1700 YBP in Bhujbasa, Gangotri. The limited extent of glaciation can be attributed to relatively lesser humidity in comparison to the early and midHolocene respectively, which is also indicated by marginal lowering of ELA and insignificant increase in ice volume. The unconsolidated, poorly defined moraine mounds between RBS-4 and the snout are assigned the recessional phase of the neoglacial advance corresponding to the LIA. Evidence of LIA glaciation are gradually emerging from the monsoon-dominated central Himalaya e.g. Bhagirathi Valley (Sharma and Owen, 1996), Milam glacier (Barnard et al., 2004b), Alaknanda Valley (Nainwal et al., 2007), Dhauliganga Valley (Sati et al., 2014), and Pindar valley (Bali et al., 2013) implying the sensitivity of central Himalayan glaciers to centennial-scale northern

ACCEPTED MANUSCRIPT latitude climatic perturbations. Figure 10 summarizes the chronology of Holocene glaciations from the central Himalaya along with the ages obtained from the Kosa Valley and their

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comparison with the other climatic proxy.

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6. Conclusion

Based on the morphostratigraphic mapping of the moraines supported by limited optical

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chronology, four events of glaciations have been identified in the Kosa Valley. Speculatively, the

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oldest and undated degraded moraine are assigned the LGM. Absence of an early Holocene glacial record is ascribed to the erosion caused during the early Holocene intensified ISM.

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Following this, the valley glacier advanced intermittently since the early part of the midHolocene; however, the overall trend shows a consistent decrease in glacier ice volume until the

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present.

We hypothesized that Holocene glaciation was driven by a combination of reduced

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precipitation (moderate to weak ISM) and lowered temperature. Our inferences accord well with regional studies carried out in the monsoon-dominated central Himalaya suggesting that temperature seems to be the major driver of glaciation in regions dominated by the ISM. Acknowledgements

P. Bisht and S. N. Ali are thankful to the Department of Science and Technology, Government of India,

New

Delhi,

for

providing

financial

support

through

grant

number

DST/INSPIRE/Fellowship/2011/123 and SB/DGH-89/2014, respectively. We thank Dr Anil Shukla for providing the radioactivity data. Navin Juyal thanks Dr S.P. Sati for accompanying the reconnaissance survey of the Kosa Valley. We also thank the anonymous reviewers and Dr. Pradeep Srivastava for their constructive critiques of the manuscript.

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influence of the Indian Summer Monsoon (ISM) and the mid-latitude westerlies.

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Fig. 2. Geomorphological map of the study area.

Fig. 3. Lithological and structural map (modified after Sinha, 1989). The study area is marked by

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Fig. 4. (A) Google Earth image showing the different stages of glaciation and recessional moraine and location of OSL samples sites. (B) Field photograph showing the latero-frontal

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moraine corresponding to RBS-2, RBS-3, and RBS-4 glaciations. Fig. 5. (A) Field photograph showing lateral extension of RBS-1 glaciation in the valley. (B)

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Field photograph showing the location and stratigraphy of the glaciofluvio outwash terrace sample (OWT-1) corresponding to the deglaciation of RBS-1 glaciation.

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Fig. 6. (A) Field photograph showing the moraine ridge corresponding to RBS-2 glaciation and the location of RBS-2 sample. (B) Close-up and stratigraphy of RBS-2 sample site. Fig. 7. (A) Field photograph showing close-up and stratigraphy of RBS-3 sample on the right lateral moraine of RBS-3. (B) Field photograph and stratigraphy of the sampling site of RBS-3(1) on the terminal moraine of RBS-3. Fig. 8. Field photograph with close-up and stratigraphy: (A) sampling site (RBS-4) on the left lateral moraine of RBS-4 glaciation; (B) sampling site RBS-4(1) located on the latero-frontal moraine corresponding to RBS-4 glaciation. Fig. 9. Radial plots showing the graphical representation of equivalent dose (De) distribution. Precision is indicated by the x-axis, and De is plotted in the y-axis.

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KL-74. Depleted oxygen isotopic ratios indicated phases of strengthened Indian Summer

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Monsoon (Sirocko et al., 1993). (C) Effective moisture mimicking the pattern of summer monsoon variability during the Holocene (Herzschuh, 2006). (D) Isotopic record of Guliya ice

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core Tibetan Plateau (Thompson et al., 1997). Enriched isotopic values indicate strengthened

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monsoon. An * indicates the age obtained from the glaciofluvial outwash terrace deposited

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Table 1

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Reconstruction of paleoequilibrium line altitude (ELA) using accumulation area ratio (AAR; 0.45

N/A 0.15 0.94 1.32

5710 5540 5535 5245

5300 5275 5272 5005

5218 5179 5141 5105

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AWM

5479 5448 5418 5389

5264 5251 5220 5041

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6525 6525 6525 6525

THAR 0.6 (m asl)

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3910 3832 3757 3684

THAR 0.5 (m asl)

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Head (m asl)

AAR 0.55 (m asl)

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Present RBS-4 RBS-3 RBS-2

Toe (m asl)

AAR 0.45 (m asl)

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Glacial stage

Length (km) from snout

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and 0.55), toe-to- headwall altitude ratio (THAR; 0.4 and 0.5), and area weighted mean (AWM)

Average ELA

ΔELA (m)

Mean temp. decline

5394±181 5338±134 5317±142 5157±142

N/A 56±2 77±3 237±9

N/A 0.33 0.46 1.42

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Volume (km³) 1.92 2.08 2.23 2.55

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Average H (m) 113.2 115.3 117.4 121.3

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Glacier area (km²) 17.05 17.76 18.74 20.64

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Glacier stage Present RBS-4 RBS-3 RBS-2

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Temporal changes in average thickness, area, and ice volume

Water equivalent (km³) 1.76 1.90 2.04 2.34

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Table 3

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Details of the radioactivity, dose rate, equivalent dose (De), and optical ages obtained

Sample type

U (ppm)

Th (ppm)

K%

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Samp le no.

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during the present study

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De (Gy)

Dose rate (Gy/k a)

Age (ka)

54.2± 4.0 22.1± 1.6 17.1± 1.3 16.3± 1.2 8.4±0 .6 5.3±0 .4

4.3±0. 3 3.6±0. 2 3.4±0. 2 4.1±0. 3 3.9±0. 3 3.4±0. 2

12.7± 1.3

RBS3(1)

Moraine

RBS4(1)

Moraine Moraine

2.73±0 .05 2.61±0 .05 3.41±0 .07 3.39±0 .07 2.99±0 .06

2.21±0 .04 2.02±0 .04 2.54±0 .05 2.33±0 .05 1.86±0 .04

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RBS-4

2.54±0 .04

12.97± 0.3 13.14± 0.3 14.13± 0.3 13.69± 0.3 15.11± 0.3

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RBS-3

FluvioLacustrine Moraine

3.42±0 17.1±0 .11 .6

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RBS-2

Outwash Terrace

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OWT1

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%

Water content 10±5%, Cosmic ray dose 250±50 µGy/a.

37 18 36 59 41 58

6.1±0 .4 5.0±0 .5 4.0 ±0.4 2.2±0 .2 1.6±0 .2

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 Four stages of glaciations identified since late glacial to late Holocene.  Glacial stages were preserved in the form of curvilinear moraine ridges.  Glacier responded to the minor changes in the temperature and moisture.  Evidence of Last Glacial Maximum and Little Ice Age has been observed in Kosa valley.