Glacial Fluctuations and Cryogenic Environments in the Langtang Valley, Nepal Himalaya
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Title Glacial fluctuations and cryogenic environments in the Langtang Valley, Nepal Himalaya Author(s) SHIRAIWA, Takayuki Citation Contributions from the Institute of Low Temperature Science, A38, 1-98 Issue Date 1994-03-15 Doc URL http://hdl.handle.net/2115/20256 Type bulletin (article) File Information A38_p1-98.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP 1 Glacial fluctuations and cryogenic environments in the Langtang Valley, Nepal Himalaya by Takayuki SHIRAIWA B :E- 4: 1T The Institute of Low Temperature Science Received October 1993 Abstract This study aims at reconstructing the glacial distribution and climatic conditions since late Quaternary in the Himalaya. For this purpose, an inventory work of present glaciers, observations of present meteorological as well as glaciological phenomena, descriptions of present cryogenic features, and geomorphological studies of the late Quaternary glacial fluctuations, were carried out in the Langtang Valley, central Nepal Himalaya. In the valley, glaciers cover an area of l37.5 km'. Due to the northward decrease of summer monsoon precipitation, equilibrium line altitude of glaciers increases from 5120 m at the southern end to 5560 m at the northern end. Physical properties of deposited snow profiles on the Yala Glacier revealed that the monsoonal snow deposits are distinguished from the non-monsoonal ones by the snow types and the existence of a thin dirt layer. By observing deeper snow cores recovered on the Yala Glacier, contribution of the non-monsoonal precipitation to the annual mass balance was found to be amounted to approximately 30 % on average during the last nine years. Measurements of the ground temperature indicate that the forefields of glaciers in the valley lack an alpine permafrost, regardless of sufficient coldness. This is also explained by the large amount of winter snow which covers the forefields enough to prevent the ground from deep-freezing in winter. There exists six glacial steges in the valley, the Lama of the oldest (the penultimate or the early stage of the last glacial glaciation), the Cora Tabela of last glacial maximum (LGM), the Langtang dated between 3650 and 2850 yr. BP, the Lirung dated between 2980 and 550 yr. BP, and the Yala I and II stages of the Little Ice Age. The glacier extent decreases from the older to the younger. Paleoclimate in the Langtang Valley was finally reconstructed by introducing a steady-state glacier mass balance model which considers both climatic parameters such as the summer mean air temperature (Ts), the summer total precipitation(Ps), the winter balance (Ew), and topographical parameters of the * Contribution No. 3697 from the Institute of Low Temperature Science ~t ifii:i]l::k."t'llf1lt"t11tmiii:Jc 2 Takayuki SHIRAIW A ground surface area (5). With the aid of some geological paleoclimatic date, the model reconstructed reduced summer total precipitation (200 mm), increased winter balance (400 mm), and air temperature decrease of 6 DC during LGM (the Cora Tabela Stage), and for the Holocene maximum glaciation (the Langtang Stage), air temperature decrease of 4 oC, slight increase in the winter balance (300 mm), and the same amount of summer precipitation (200 mm) as that of today. The simulated results suggest that the glacier extent of the Cara Tabela Stage is mainly supported by non-monsoonal precipitation. This can explain the dilemma how the Himalayan glaciers developed during the LGM when the summer monsoon, that maily supports the current Himarayan glaciers, was weakened. Contents I. Introduction·················································································································· ......... 4 I . 1. Introduction· .... · .... ·· .. ········ .. ··· .......... ············································································· 4 I . 2. Aims of the study ......................................................................................................... 4 II. Physical setting ..................................................................................................................... 5 II. 1. Landforms of the Nepal Himalaya ................................................................................. 5 II. 2. Climate of the Nepal Himalaya .................................................................................... 8 II. 3. Glaciers in the Nepal Himalaya .................................................................................... 9 III. Present glacial environments in the Langtang Valley ............................................................... 9 III. 1. Introduction··· .. ·· .. ········································································································· 9 III. 2. Previous work ........................................................................................................... ····10 III. 3. Distribution of the glaciers ............................................................................................ ·14 III. 3. 1. Inventory of the glaciers .......................................................................................... 14 III. 3. 2. Characteristics of the glaciers ................................................................................ ·15 III. 4. Recent fluctuations of the glaciers ................................................................................ ·16 III. 5. Meteorological condition of the glacial environment························································ ·17 III. 5. 1. Air temperature· .... · ............................................................................................... ·20 III. 5. 2. Precipitation ........................................................................................................ ·23 1II. 5. 3. Maximum snow depth ............................................................................................. 23 III. 5. 4. Short wave radiation ............................................................................................. 25 III. 5. 5. Regional characteristics of the climate in the Langtang Valley .................................... 26 III. 6. Seasonal variation of mass input on the glaciers and its significance ................................ ·26 III. 6. 1. Spatial variation of mass input and characteristics of deposited snow on the glaciers .. ·28 III. 6. 2. Seasonal and altitudinal variation in the mass balance of glaciers .................. · .......... ·31 III. 6. 3. Reconstruction of the rate of seasonal mass input in a deeper profile .......................... ·32 III. 7. Glacier-climate relationship in the Langtang Valley ........................................................ ·34 III. 7. 1. Climatic features at the equilibrium line .................................................................. 34 III. 7. 2. Relationship between the glacier mass balance and climatic factors .......................... ·36 IV. Present cryogenic environments ....................................................................................... · .... ·38 IV.1. Introduction ......................................................................................... · .... · ........ ·· .... · .... 38 Glacial fluctuations and cryogenic environments in the Langtang Valley, Nepal Himalaya 3 IV. 2. Previous work·········································· ...................................................... ···············38 IV. 3. Permafrost ., ....................................................................................................... ·········39 IV. 3.1. Estimation by freezing and thawing indices ...................................................... ·········40 IV. 3. 2. Measurements of ground temperature ...................................................... ··················42 IV. 3. 3. Reason for the lack of permafrost ..................................................................... ······45 IV. 4. Periglacial landforms ................................................................................................ ···46 IV. 4. 1. Polygons ............................................................................................................ "'46 IV. 4. 2. Other periglacial landforms ................................................................................ ····48 IV. 5. Frost shattering· .......................................................................................................... '49 IV. 5. 1. Measurement of rock breakdown ............................................................................. '49 IV. 5. 2 Measurement of air and rock surface temperatures ...................................................... 51 IV. 5. 3. Altitudinal distribution of the number of freeze-thaw cycles .......................... ·············55 IV. 6. Vertical zonation of cryogenic environments ............................................................... "'58 V. Glacial fluctuations in the Langtang Valley during the late Quaternary······ ............................. ····60 V. 1. Introduction················································································································· ·60 V.2. Previous work .............................................................................................................. ·61 V.3. Study methods ........................................................................................................... ·62 V.4. Glacial landforms .......................................................................................................