Glacial history in the Khumbu region, Nepal, in relation to upheavals of the Great Himalayas.
INTRODUCTION
REGIONAL CHARACTERISTICS OF GLACIERS IN THE GREAT HIMALAYAS
HISTORICAL CHARACTERISTICS OF HIMALAYAN GLACIER GROUP
DISCUSSIONS
参考文献
Glacial history in the Khumbu region, Nepal Himalayas, in relation to upheavals of the Great Himalayas (1981) Symposium on Qinghai-Xizang (Tibet) Plateau (Beijing, China), 2, 1641-1648.
Glaciations in the Khumbu Himal Ⅱ (1978) Journal of the Japanese Society of Snow and Ice, Vol. 40, Special Issue,p.17-20.
The highest mountains in the world forms a vast glacial topography seen near Chukkun in Khumbu region, east Nepal.
It is the Late Cenozoic that the world-wide mountain building has begun and the Inland Asia, including the Great Himalayas, has risen. There was already formed the Antarctic Ice Sheet in the Late Cenozoic and, since then, the huge mass of the Antarctic Ice Sheet has been continuously existing upto the present(l). We must now think about the Late Cenozoic glacial ages instead of merely the Pleistocene glacial ages(2).
The waxing and waning ofa glacieris one of the natural phenomena caused by the changes of the topography and climate. When the topographic scale in a certain area is small, the climatic condition is controlled by the changes of the global climate. However, the local climate will be formed when the topographic characteristics become as large as the present conditions of the Inland Asia. When the Inland Asia with the Xizang (Tibet) plateau has been rising, the Tibetan High is formed and it gives a large influence on the monsoonal precipitation process in Asia as well as on the global air circulations.
The upheaval of the Great Himalayas started from the Miocene and it had the synchroneity to the warld-wide mountain building and to the Late Cenozoic glacial ages(3). The glacial phenomena in the Great Himalayas are fundamentally related to global changes of the topography-climate system as well as to local changes of the system during the upheaval history of the Great Himalayas situated in the southern margin of the Tibetan plateau. There are rising mountain ranges such as the Great Himalayas, Karakoram, Kunlun and Tien Shan, and subsiding basins such as the Tarim, Tsaidam and Dzungaria basins,so it can be said that the topographic change of the Inland Asia is not regionally and historically uniform.
Since the Himalavan reaions are thought to have been influenced bv the large changes of the topography-climate system, this regions are important fields to clarify the necessary conditions of the topography and climate for the formation of glaciers in relation to the upheavals of the Great Himalayas. Studies on glaciations were mainly carried out in the Khumbu region, east Nepal, from 1970 to 1978 as a part of activities of the Japanese Skiing Expedition to Mt. Qomolangma (the highest mountain in the world) and of the Glaciological Expedition to Nepal in order to study on the glacial history of the Nepal Himalaya as a typical glacial phenomenon in the central part of the Great Himalayas (Fig. 1).
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The area ratio of the existing glaciers to the maximum extent of the Pleistocene glaciers shows the large fluctuation especially in the Northern Hemisphere as compared to that in the Southern Hemisphere(4)(5). Present glaciers in Asia are seen in a part of the past Scandinavian and Siberian Ice Sheets and in parts of the high mountains and plateaus of the Inland Asia. The total area of the Asian glaciers is 120,000 km2 (4), and more than 90% of the Asian glaciers stay in the Great Himalayas, Tien Shan and other mountains in the Inland Asia (6)(7) (8) (9) (10).
There are semi-deserts in the Middle East and in the north-west of India, where the annual precipitation is less than 500 mm, and this dry zone stretches north-east ta the Takla Makan and Gobi deserts. In the further east, there is Ganges plain with the so-called monsoonal climate, and Assam and Burma regions with heavy annual precipitations more than 2,000 mm. The vegetation type shows that the steppe with winter precipitation lies in the Middle East and West Himalaya, and the tropical forest with summer precipitation in the East Himalaya and Tibetan marginal mountains. The Nepal Himalaya is situated in the Central Himalaya that is a transient region between both of the contrastive climatic zones.
The satellite image shows that there are, in the Karakoram and West Himalaya, large glaciers such as Biafo, Hispar and Baltoro Glaciers which lengths are more than 50 km and which terminal elevations are low to be 3,000-4,000 m. Fig. 2 is the air photograph of the Khumbu region, east Nepal, and shows that there are small glaciers such as Khumbu, Nuptse and Lhotse Glaciers which lengths are less than 20 km and which terminal elevations are high at about 5,000 m in the Central Himalaya, and many glaciers have rich supraglacial tills especially in the down-stream part. While, Fig. 3 shows an ice cap with gentle inclinations and without supraglacial tills in the Dolpo region, north-central Nepal, in the northern side of the Great Himalayas(11). In the Nyainqentanglha of the East Himalaya, the Kaqing Glacier is reported to have the length of 35 km and the lower terminus at 2,530 m(12), so, it is thought to be also large glaciers in the East Himalaya.
Therefore, the glaciers in the Himalayas can be divided into three groups ; West, Central and East Himalayan glacier groups, according to the regional characteristics of the glacier scales and distributions along the west-east direction. Glaciers of the Nepal Himalaya were divided into two types; the Tibetan glacier group as a cold type and the Nepal glacier group as a warm type(l3), and glaciers in the West China were classified into three types; the continental type glaciers with law precipitations in most parts of the Tibetan plateau
from the Great Himalayas to the qilian Shan, the maritime type glaciers with high monsoon precipitations in the south-eastern part of the plateau and the complex type glaciers with a large height-difference of over 3-4 km in the ~arekoram(l2). It is important to know the historical processes how the regional cheracteristics have been formed and it is fundamental to show the future processes of the glacial phenomena in the Great Himalayas by clarifying the regional and historical characteristics of each glacier group which is caused by the changes of the topography-climate system.
The distribution of precipitation in the Himalayan regions are strangly influenced by the topographic conditions. Fig. 4 shows the relationships between the distributions of onspicuous mountain ranges and the annual precipitation of 1970 in east Nepal reported by the Meteoralagical Office of Nepal (14). This clearly shows that the large amount of precipitation occurs along the south side of the Mahabharat ranges with 3,000-4,000 m/yr and the frontal zone of the Great Himalayas with 3,000 mm/yr. The area near the Great Himalayas has lower annual precipitation less than 1,000 mm.
In the southern part of this region, Shorong Himal, the present terminal elevations of glaciers are low to be about 5,200 m, while they are high ta be about 5.400 m in the northern part, Khumbu region(l0). There is reported to be higher glacier termini in the northern side of the Great ~imala~as(l5). So, it is considered that these present areal characteristics of the terminal elevations are basically caused by the local differences of climates with larger precipitation in the south and less in the north of the Great Himalayas and glaciers in the Nepal Himalaya can be divided into two groups; Nepal glacier group and Tibetan glacier group.
On the other hand, there are, during the winter monsoon season, heavy snow falls reported in the West Himalaya, Karakoram, Panjab, Kumaun regions and the western part of the Nepal Himalaya, but the winter precipitations hardly occur in the East Himalaya. Consequently, it is thought that the topographicconditions of the Himalayan regions cause effects of the rain shadow on the vapor transportation coming fram west in the winter monsoon seaaon as well as on that from south in the summer monsoon season, and the summer solid precipitation is the main source of accumulation for the formation of glaciers in the East Himalaya and the winter precipitation in the West Himalaya. The summer monsoon season is the accumulation as well as ablation periods far the glacier mass-balance in the East Himalaya.
The orographic snow line is formed by the topography-climate system and the existence of the snow line is a necessary condition’ for the formation of glaciers. The snowline distributions of the Inland Asia show the higher snowline in the Central Himalaya and the lower snowline in the West and the East Himalayas along the east-west direction, and it stays lower in the south side of the Great Himalayas and higher in the north along the south-north direction , and the highest snowline lies in the Gandise Shan (Trans Himalaya) ( 6 ) (l6).
Thus, the regional characteristics of each glacier group; the larger and lower glacier group in the West Himalaya having the heavy winter accumulation , the smaller and higher glacier group in the Central Himalaya caused by the less precipitations, and the larger and lawer glacier group in the Bast Himalaya with the heavy summer accumulation, can be explained by the topographyclimate system in the Great Himalayas. The similar regional characteristics of glacier distributions in the Central Himalaya and in the southern part of the Tibetan plateau will be also explained by the local differences in the present distributions of precipitations and topographic conditions.
The past large glacier expansion was stated to be caused by the lowering of the air temperature due to the upheavals of the north India and by the rich vapor transportation from the Palaeoganges Sea(17). The precise history of the glaciers in the Great Himalayas are still unknown, because the upheaval rates of the Great Himalayas, the related climatic changes and the ages of the glacier expansions have not been precisely studied yet.
Four stages of glaciations were reported by studiing on the lake deposits and terraces in the West Himalaya(18)(19), three stages of glaciations by the moraine systems and glacial topography in the Central Himalaya(20)(2l)and twothree stages of glaciations by analysing the terrace and moraine tap0 raphy in the areas from Nepal to Bhutan in the Central and East Himalayas(22) (~5). These results indicate the problem whether there were the synchraneity of the past glacier expansions in the whole regions of the Great Himalayas.
There are three different series of U-shaped valleys observed near Periche village in the Khumbu region, east Nepal. The oldest, U-l, is partly preserved and the two younger U-shaped valleys, U-2 and U-3, are well preserved and three series of moraine formations with these U-shaped valleys are observed These series are divided into several stages accordingto the moraine topography ; scales of moraine shapes, characteristics of surface soils and locations of springs. The youngest U-3 series is divided into three stages; A, B (Periche stage) and C (Thuklha stage) named after Muller(24) as seen in Fig. 2.
Fig. 5 shows the relative height between the present river bed and the upper part of each series of the U-shaped valleys; U-1 (680 m), U-2 (380 m) and U-3 (170 m). These prominent erosion actions of the past glaciers must be amplified by the intense upheavals of the Great Himalayas. The moraine of the U-1 series continues up to a rock slope which has a flat ridge (Argte). Fine surface soils are different in colour and thickness; U-1 (brown to dark brown) and U-2 (brown) with thicker surface soils, and U-3 (yellowish brown) with thinner surface soils. There are springs coming out of the boundaries of these moraine series. There is found the oldest U-shaped valley (U-1 series) near Lukla (2,500 m) about 50 km to the south of the present terminus of the Khumbu Glacier (5,000 m). There is, in Lukla, partly preserved an end moraine formed at the maximum glacier expansion which is sheared by the steep north-south fault.
Fig. 6 shows the areal changes of the glacier distributions at each stage. At the maximum expansion (U-1). mast of the mountains below 6,000 m were covered by glaciers. The total area of the preRent glaciers is 170 km2 and that of the oldest 650 km2 in the north-east part of the Khumbu region.
The changes of the scale of the U-shaped valleys and areas of the past glaciers show that the younger the age of the U-shaped valley is, the smaller the scale is. This suggests that the areal changes of the glacier expansions have been markedly decreading from the older to the newer stage and the topographic changes take an irreversible course which tends to preferentially form the type of the valley glacier by the strong erosion in response to intense upheavals of the Great Himalayas and to the intermittent glacier expansions.
There are found aeolian deposits and palaeosols covering the moraines in the Khumbu region, Since the 1 4 a~go of the plant remnants and palaeosols sampled from the surface of the moraine indicates the minimum (youngest) age of the moraine formation and that from che basement the maximum (oldest) age. Though the ages of the past glacier expansions were not known, the age of the Thuklha stage was correlated to the ‘1850’ moraine and that of the Periche stage to the ‘1600’ advance in Europe(24).
The plant remnants were sampled from the moraine basement of the Thuklha stage and the charcoals from the surface soils of the Periche stage. Since the 14 age of the former sample is 410±110 yr B.P. and that of the later 2640±170 yr B.P.(21), the age of the Thuklha stage is younger than 16th century and that of the Periche stage older than 700 B.C. It is reported to be in the 19th century that glaciers in Europe showed the maximum expansion during the Neoglaciation, however, glaciers in the Khumbu region showed the continuous retreat with intermittent smaller advances during the same period. The ages of the older moraine formations can not be determined since there are shown only theminimum ages;U-2 series alder than 3630 B.C. and U-1 series older than 6100 B.C.
The terminal fluctuations of 15 glaciers were measured in the Khumbu region(25) (26). According to the fluctuation rates, these glaciers were divided into four groups; retreating (8 glaciers), stationary (3 glaciers), advancing (3 glaciers) and irregular (L glacier). Therefore, it is considered that many glaciers have been retreating in the recent years and the recent trend toward glacier retreat in this region can be thought as a link in the long-term receding trend with intermittent advances since the 16th century.
Hong Nup glacier receding and its lake expanding are observed in the upper part of the Hong valley, east Nepal.
Though we do not know the exact history of the topography-climate system in the Great Himalayas, we have the facts that the Himalayan glaciers shared the large expansions in the past and there are now formed the regional characteristics of glacial phenomena in the Great Himalayas. Thus, there were the necessary topography-climate systems for the formations of the past glacier expansions. How was the topography-climate system for the past glaciers and
how has it been changing from the past to the present?
The average upheaval rate was reported to be several mm to cm/yr(27), the topographic conditions might be much different from the present ones if the age of the maximum glacial stage was several tens to hundreds thousands years ago. ~uo(20)stated that the average height of the Great Himalayas rose from 4,500 m to 6,100 m since the last ice age and the glaciers in the north side of the Great Himalayas retreated by decreasing vapor transportation from the south to the north of the world highest ranges. Guo’s idea as well as Strachey ‘s(17) are suggestive because they considered bath conditions of topographic and climatic changes to form the glacial history in relation to the precipitation process during the long period of the Himalayan mountain building. They took the summer precipitation from south as an important climatic factor, however, they did not taka the winter precipitation from west as an another important climatic factor for the formation of the Himalayan glaciers as it is seen in the West Himalaya.
It is mainly in the summer monsoon season that the present glaciers in east Nepal are accumulated, and the orographic snowline is situated at about 5,500 m, but the snow cover is appeared even ak 2,500 m during the winter monsoon season. It is considered that the past summer solid precipitations would not be formed without the prominent lowering of the air temperature when the average height of the Great Himalayas was lower than that of the present orographic snowline. When the average height of the Great Himalayas was lower, the topographic effects of the rain shadow were so weaker that the winter solid precipitations might reach to the further eastern part of the Himalayan regions than that seen today. The lower the average height of the Great Himalayas was in the older stage, the more the winter precipitations contributed for the formation of glaciers in the Central and East Himalayas.
As the Great Himalayas continued to rise and reached to the height of the present snowline, it is thought that the summer solid precipitation began to contribute the glacial phenomena in the Himalayan regions together with the winter precipitation. Since Strachey stated the topographic changes of the Palaeoganges Sea(17) and Grasswald reported the large changes of the Palaeo- Caspian Sea and other lakes in the Central ~sia(28), it will be necessary to study on the palaeogeography as an important vapor source for the Himalayan glaciers. Further more, the Great Himalayas and the Mahabharat ranges have risen, so the winter precipitation started to concentrate in the West Himalaya and the summer precipitation in the southern part of the East Himalaya. So, it is considered that the present regional characteristics of the Himalayan glaciers are formed by the historical changes of the topography-climate system in the Great Himalayas. When the average height of the Great Himalayas was lower than the present orographic snowline, there would he seen the synchronous glacial history in the Great Himalayas due to the only one climatic condition of the winter accumulation. However, when the Great Himalayas became higher than the height of the present snowline, it can not be always said that there was the synchroneity of the glacial history in between the West Himalaya and the East Himalaya due to the two different climatic factor; the winter and the summer accmulation.
When the Great Ximalayas, the Mahabharat and other ranges continue to rise as they have shown before, the oragraphic snowlines of the Central Himalaya and the south-western part of Tibet become so higher that the climate of these regions will be colder and dryer, an& the glaciers of the Central Himalaya will show the continuous retreat in contrast to the regional characteristics of the glacier groups in the West and the East Himalayas.
Mt. Manaslu has the Tulagi glacier rapidly retreating in recent years.
References
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(2) Turekian, K. K. ibid.
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(7) Vohra, C. P. Ibid.
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(9) Muller. F. ibid.
(10)Higuchi, K. et al. ibid.
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