Macha Khola Valley

Boulders from the oldest moraine in the Macha Khola Valley, MK2, yielded exposure ages between 34 and 97 ka (Tab. 4.1), allowing for possible deposition of the moraine during the MIS 3 through 5, or even earlier. A comparison with exposure ages from the Khumbu Himal (Finkel et al., 2003), however, shows a good agreement between the oldest exposure age of MK2 and the Thyangboche I stage, all pointing to 70-100 ka, which makes deposi-tion during the MIS 5 most likely (Fig. 4.4). All younger ages on MK2 seem to reflect mo-raine degradation, as the like are found on the Thyangboche I momo-raine as well, where the respective boulders seem to have been exhumed during the younger Thyangboche II ad-vance during the second half of the MIS 3 (Fig. 4.4, Finkel et al., 2003). The 70-100 ka age of the first, most extensive late Pleistocene glacier advance in the Nepal Himalaya is in agreement with an MIS 4-5 estimate of 70 ka for the Ronbushi I advance on the northern slope of the Himalaya given by Zheng and Rutter (1998). No remnants correlating to a Thyangboche II advance have been identified in the Macha Khola Valley.

The next younger advance dated in the Macha Khola Valley, MK5, yielded exposure ages between 11 and 26 ka (Tab. 4.1, Fig. 4.5). While the ages around 11 ka are clearly indica-tive of moraine degradation, the three oldest ages allow for glacial advance between 19 and 26 cal. ka B.P., which is in agreement with the 18 cal. ka basal age of lake Rukche Tal (Schluetz & Zech, 2004), and with other MIS 2 moraine ages determined in Nepal.

Tab. 4.1. Results of ¹⁰Be surface exposure dating in the Nepal Himalaya.

1 correction factor for topographic shielding of fast neutrons, corrected for the influence of surface inclination

2 correction factor for topographic shielding of muons, corrected for the influence of surface inclination

3 maximum slope angle of the sampled surface

4 thickness of the sample

5 ratio of ages calculated using the scaling systems of Lal (1991) as used in this study, and the scaling system of Dunai (2001) as modified by Schaller et al. (2002), for details see section 3.

exposure age [ka]

0 20 40 60 80 100

120 max age

min age

TH I

MK2 TH II

Fig. 4.4. Comparison of (recalculated) minimum (white dots) and conservative maximum (black dots) exposure ages from MIS 5-3 moraines in the Macha Khola Valley (MK2, this work), and the Khumbu Valley (Finkel et al., 2003, TH I, II: Thyangboche I, II stages). Glacial advances as inter-preted from the data in crosshatched boxes, phases of moraine degradation in white boxes. See text for explanation.

A similar, but more narrowly constrained ¹⁰Be age of 18-23 cal. ka B.P. has been found for the main MIS 2 advance in the Khumbu and Chhukung Valleys (Pheriche I stage, Finkel et al., 2003) (Fig. 4.5). In the Khumbu Valley, the Pheriche I moraine further yielded OSL ages of 18-25 ka (Richards et al., 2000) and another ¹⁰Be age of ~20 cal. ka B.P. (Aoki &

Imamura, 1997). These results are confirmed by OSL dates of 20-23 ka from the Kang-chenjunga Himal (Asahi et al., 2000, Tsukamoto et al., 2002). An MIS 2 advance ~20 cal.

ka B.P. is thus firmly established in the Nepal Himalaya by now. This, however, is not the case for any MIS 4 advance after 70 cal. ka B.P., and it is remarkable, that in the Nepal Himalaya, MIS 2 glacier advances seem to have occurred only after 23 cal. ka B.P., whereas farther northwest, MIS 2 advances occurred prior to 23 cal. ka B.P. as well (section 5). These two facts might indicate, that indeed only during the late MIS 2, the influence of the westerly circulation on glaciation extended over the whole Himalayan system. At that time, the jetstream was at the southernmost position it reached during the late Pleistocene (Benn & Owen, 1998, Ono et al., 2004). Before, and afterwards, glaciation in the southeastern part of the Himalaya as it seems was dominated by the influence of the

Indian monsoon, which was strong during the MIS 5, 3 and 1, but weak during the MIS 4 and 2 (Leuschner & Sirocko, 2000).

exposure age [ka]

0 5 10 15 20 25 30 35

max age min age

CH3 KH3 KH4 KH5 CH5 CH6 LT2 LT3 LT6 MK5 MK7 MK4 BH KE

Fig. 4.5. Comparison of (recalculated) minimum (white dots) and conservative maximum (black dots) exposure ages of MIS 2-1 moraines from the Chhukung (CH) and Khumbu (KH) valleys (Finkel et al., 2003, 3: Pheriche I stage, 4: Pheriche II stage, 5: Chhukung stage, 6: Thukhla stage), the Langtang Valley (LT, this work), the Macha Khola Valley (MK, this work) and the Garhwal Himalaya (BH, KE; Barnard et al., 2004). BH = Bhagirathi stage, KE = Kedar stage. Boulders interpreted to be affected by inheritance in boxes crosshatched in grey, glacial advances as inter-preted from the data in boxes crosshatched in black, phases of moraine degradation in white boxes.

See text for explanation.

The lateglacial advance represented by MK7 is dated by our exposure ages to between 11.1 and 12.3 cal. ka B.P., covering the Younger Dryas event, excepting one age of 20-21 ka obviously affected by inheritance (Tab. 4.1, Fig. 4.5). Younger Dryas ages have not been reported from the Khumbu Valley (Finkel et al., 2003) and have not been found in the Langtang Valley as well. The like, however, have been reported from the Hunza Valley (Batura stage, Owen et al., 2002) as well as from the Lahul Himalaya (Kulti stage, Owen et al., 2001), and the Younger Dryas period is covered by the age range found on the Bhagirathi moraine in the Garhwal (BH in Fig. 4.5) by Barnard et al. (2004). All these re-gions are situated west of the Macha Khola Valley. In the Khumbu and Kangchenjunga regions east of the Macha Khola Valley, instead, the Chhukung stage is defined by surface

exposure ages of 8-10 ka (Finkel et al., 2003, KH5 and CH5 in Fig. 4.5) and OSL ages of around 9 ka (Asahi et al., 2000). It is thus definitely younger. Therefore, it may be that the influence of the westerly circulation during the time of the Younger Dryas just reached the Manaslu massif and did not extend farther to the east, while the monsoon influence on glaciation during the early Holocene just reached west of the Langtang Valley. Further dating however, is needed to confirm such a suggestion.

The neoglacial moraine MK4 finally is dated to around 3 cal. ka B.P. (Tab. 4.1, Fig. 4.5).

Glacier advances during that time are already well documented by radiocarbon dating all over the Nepal Himalaya, e.g. in the Langtang Valley (Shiraiwa & Watanabe, 1991, Baeumler et al., 1996, Heuberger & Ibetsberger, 1998), or in the Annapurna Himal (Zech et al., 2001a, b), as well as from all over the Tibetan plateau (Zheng & Rutter, 1998). In the Khumbu area, glacial advances around 3 cal. ka B.P. have been defined as the Thukhla stage (Finkel et al., 2003, KH6 in Fig. 4.5). Our dating thus merely confirms the appropri-ateness of ¹⁰Be SED of moraines in this area and the unlikeliness of inheritance in more than a single boulder out of a selection of five.