Study sites

4.2.1.1 Physical geography and climate of the Nepal Himalaya

The central Nepal Himalaya (28°N, 83-86°E) is the highest mountain chain of the Earth. It separates the Ganges plain in the south and the Tibetan plateau in the north. In the west, it is dominated by the Dhaulagiri and Annapurna, in the east by the Khumbu and Kangchen-junga massifs, all culminating above 8000 m a.s.l. The high Himalaya mainly consists of metamorphic rocks, in most cases gneisses and migmatites, of the so-called Higher Himal-aya Crystalline (HHC) or Tibetan Slab formation (Barbey et al., 1996, Harrison et al., 1997). The relief gradient between the high Himalaya and the mountain foreland is extre-mely large, and valleys are deeply incised. Earthquake activity is high, and landslides and rockfalls are numerous (Fort, 1986, 2000).

Climate in the area is dominated by the Indian monsoon in summer, and cool, dry westerly winds in winter (Miehe, 1990, Denniston et al., 2000). West of the Annapurna and Dhaula-giri massifs, moisture advection with the winterly westerlies increases (Fort, 2000). Mon-soon precipitation decreases from south to north and has two altitudinal maxima, in ~2000 m a.s.l. and in ~6000 m a.s.l., caused by the primary monsoon, and secondary condensation rain, respectively (Zheng et al., 1989b). The present equilibrium line altitude (ELA) of gla-ciation ranges from 5200 m a.s.l. in the south to 5800 m a.s.l. in the north (Williams, 1983, Heuberger et al., 1984, Miehe 1990). Glaciers in the Himalaya, however, are often avalanche-fed and heavily debris-covered, so that their tongues reach down to lower alti-tudes than it is implied by the ELA data (Roethlisberger, 1986, Fort, 2000, Benn & Owen, 2002). ELA depressions are therefore not always an adequate parameter for correlation of glacial stages in this region.

4.2.1.2 Macha Khola Valley, Gorkha Himal

The Macha Khola is a small first-order river originating at the southeastern end of the Ma-naslu massif and joining the Buri Gandaki Khola (Fig. 4.1). The present ELA in its valley is about 5100 m a.s.l. (Zech et al., 2003). This valley was selected for soil geographic investigation because of its well-preserved glacigenic deposits. The detailed results of the soil investigations are presented by Zech et al. (2003).

Moraine stages in the Macha Khola Valley are shown in Fig. 4.2. The recent glacier descends down to 4700 m a.s.l. The youngest set of moraines, probably deposited during the Little Ice Age (LIA) several hundred years ago, reaches down to 4270 m a.s.l. Three lateral moraines inferred to have been deposited during the Neoglacial are present down to 3600 m a.s.l. The most distinctive wall of these has been sampled in 3900 m a.s.l. for age confirmation (MK4). Further sampled moraines are two lateral moraines present in a bend in the middle part of the valley, the younger one situated in 3260 m a.s.l. (MK7), and inferred to be of lateglacial age, the older one situated in 3550 m a.s.l. (MK5), and inferred to have been deposited during the MIS 2 (Zech et al, 2003). Finally, boulders from a moraine reaching down to 2150 m a.s.l. have been sampled in 2364 m a.s.l. (MK2). This deposit is the oldest and most distal glacial remnant identified in the valley, and is correlated with the outermost lateral moraines in the above-mentioned valley bend. By analogy with the stratigraphy of the Khumbu Himal (Finkel et al., 2003), an MIS 5 age has been inferred for this stage. Between the MIS 5 and MIS 2 moraines, on the left valley side, lake Rukche Tal is situated, which has been drilled for palaeoclimatic investigation.

Radiocarbon dating of the base of this core (~18 cal. ka B.P., Schluetz & Zech, 2004) has confirmed the MIS 2 age of the inner moraine wall.

Fig. 4.2. Sketch of the Macha Khola catchment with glaciers (black), inferred moraine stages and sampling sites.

4.2.1.3 Langtang Valley, Langtang Himal

The Langtang Valley (Fig. 4.1, 4.3) is an east-west-trending valley between the massifs of the Langtang Lirung and the Xixabangma in the north and the massif of the Gosainkund in the south. It has a mean annual precipitation of 1200 mm, a mean annual temperature of 2.7°C and an ELA of 5300 m (Miehe, 1990). The investigation of the glacial deposits in this valley has a long tradition. Heuberger et al. (1984) recognized two moraine genera-tions older than the (LIA), which they contribute to two lateglacial advances. The younger advance is thought to be represented by separate moraines from the Langtang Lirung south glacier at Langtang village (Fig. 4.3, LT6), from the Langtang Lirung north glacier west of Kyangchen Gomba, and from the main valley glacier east of Kyangchen Gomba (Fig. 4.3,

LT3). The older advance, on the other hand is thought to be represented by a high lateral moraine opposite of Kyangchen Gomba (Fig. 4.3, LT2). Ono (1986), staying with the late-glacial interpretation of the above-mentioned moraines, recognized additional remnants of an even older end moraine west of Ghora Tabela in 3200 m a.s.l. (Fig. 4.3, LT1).

Fig. 4.3. Sketch of the Langtang catchment with glaciers (black), LIA moraines (continuous bold lines), and sampled deposits (LT1, 2, 3, 6).

Shiraiwa & Watanabe (1991) in turn proposed four glacial stages. The oldest Lama stage is thought to be represented by remnants of a trough profile reaching down to 2600 m a.s.l.

The second stage is defined by the Ghora Tabela moraine. For the Lama and Ghora Tabela stages, no age estimates are given. All moraines upvalley, however, are interpreted to be neoglacial moraines by Shiraiwa & Watanabe (1991), based on radiocarbon dates of 3.0-3.6 ka B.P. for an advance of the Langtang Lirung south glacier reaching downvalley to Langtang village. The Langtang stage thus defined also comprises the main valley glacier moraine east of Kyangchen Gomba (LT3). Younger neoglacial moraines of the Langtang Lirung south glacier are placed into a separate Lirung stage with a radiocarbon age <2.8 ka.

Baeumler et al. (1996, 1997) and Baeumler (2001a) have used soil development and soil chemical analyses, as well as new radiocarbon ages, to reevaluate these chronologies. They show that the LT3 moraine is significantly older than the other moraines of the Langtang stage of Shiraiwa & Watanabe (1991), indirectly providing a radiocarbon age of >6 ka B.P., and that the LT2 moraine is even older than the LT3 moraine.

We sampled boulders on the moraines LT2, 3, and 6 to definitely determine the chronology of these lateglacial or Holocene glacier advances. In addition, samples were taken from a rockfall deposit on the moraine surface at Ghora Tabela (Fig. 4.3, LT1) in the hope of finding a constraint on the timing of the glacier advance which has left this oldest moraine deposit preserved in the valley.

4.2.2 ¹⁰Be surface exposure dating

For ¹⁰Be surface exposure dating (SED), chunks of up to 8 cm thickness have been loosen-ed by hammer and chisel from the centre surfaces of the largest and tallest boulders positioned on the culminations of each sampled deposit. Boulders showing signs of spal-ling or recent dislocation were avoided. Position and altitude were read from a GPS and barometric altimeter combination. Topographic shielding and surface inclination were no-ted using a compass and inclinometer. Samples were analyzed for ¹⁰Be following the pro-cedure of Kohl & Nishiizumi (1992) as modified by Ivy-Ochs (1996). ¹⁰Be/⁹Be was mea-sured at the AMS facility of the Paul Scherrer Institute at the ETH Zurich and corrected to conform to ICN standards.

Calculation of the exposure ages was done using TEBESEA (section 2), employing the scaling system of Lal (1991) as modified by Stone (2000) with a standard ¹⁰Be production rate at sea level in high latitude (SLHL) of 5.35 ± 0.15 atoms g^-1 a^-1, a negative muon capture contribution to SLHL production of 1.2%, as well as corrections for 1) geomagne-tic variations, 2) sample thickness considering the depth profile of Heisinger et al. (2002a, b) as parametrized by Schaller et al. (2002), and 3) shielding by topography and surface in-clination. The influence of surface erosion, tectonic uplift and snow cover has been estima-ted by calculating a minimum exposure age, assuming no erosion, uplift and cover, as well as a maximum exposure age, assuming a conservative maximum erosion rate of 3 ± 2 mm ka^-1 (section 4), a tectonic uplift rate of 3 ± 2 mm a^-1, and a mean annual snow cover of 5 ± 3 g cm^-2. The tectonic uplift rate used is a good approximation of the published values for central Himalayan uplift, which range from 1-4 mm (Jain et al., 2000). Where stated, a

modified scaling system of Dunai (2001) has been used for comparison, employing a standard ¹⁰Be production rate at sea level in high latitude (SLHL) of 5.47 ± 0.31 atoms g^-1 a^-1, a negative muon capture contribution to SLHL production of 1.9%, and a fast muon reaction contribution of 1.7% (section 3). Muon contributions in this case were scaled as proposed by Schaller et al. (2002). All other corrections were applied as mentioned above.

All surface exposure ages taken from the literature for comparison have been recalculated following the calculation scheme applied here and therefore may be slightly different from values given in the original studies. Interpretation of the exposure age distributions follow-ed the scheme proposfollow-ed in section 4.

4.3 Results & Discussion