The past valley glacier network in the Himalayas and the Tibetan ice sheet during the last glacial period and its glacial-isostatic, eustatic and climatic consequences

Since 1973 new data were obtained on the maximum extent of glaciation in High Asia. Evidence for an ice sheet covering Tibet during the Last Glacial Period means a radical rethinking about glaciation in the Northern Hemisphere. The ice sheet's subtropical latitude, vast size (2.4 million km²) and high elevation (6000 m asl) are supposed to have resulted in a substantial, albedo-induced cooling of the Earth's atmosphere and the disruption of summer monsoon circulation. Moraines were found to reach down to 460 m asl on the southern flank of the Himalayas and to 2300 m asl on the northern slope of the Tibetan Plateau, in the Qilian Shan region. On the northern slopes of the Karakoram, Aghil and Kuen-Lun mountains, moraines occur as far down as 1900 m asl. In southern Tibet radiographic analyses of erratics suggest a former ice thickness of at least 1200 m. Glacial polish and roches moutonnées in the Himalayas and Karakoram suggest former glaciers as thick as 1200–2700 m. On the basis of this evidence, a 1100–1600 m lower equilibrium line (ELA) has been reconstructed, resulting in an ice sheet of 2.4 million km², covering almost all of Tibet. Radiometric ages, obtained by different methods, classify this glaciation as isotope stage 3–2 in age (Würmian = last glacial period). With the help of 13 climate measuring stations, radiation- and radiation balance measurements have been carried out between 3800 and 6650 m asl in Tibet. They indicate that the subtropical global radiation reaches its highest energies on the High Plateau, thus making Tibet today's most important heating surface of the atmosphere. At glacial times 70% of those energies were reflected into space by the snow and firn of the 2.4 million km² extended glacier area covering the upland. As a result, 32% of the entire global cooling during the ice ages, determined by the albedo, were brought about by this area — now the most significant cooling surface. The uplift of Tibet to a high altitude about 2.75 Ma ago, coincides with the commencement of the Quaternary Ice Ages. When the Plateau was lifted above the snowline (= ELA) and glaciated, this cooling effect gave rise to the global depression of the snowline and to the first Ice Age. The interglacial periods are explained by the glacial-isostatic lowering of Tibet by 650 m, having the effect that the initial Tibet ice – which had evoked the build-up of the much more extended lowland ices – could completely melt away in a period of positive radiation anomalies. The next ice age begins, when – because of the glacial-isostatic reverse uplift – the surface of the Plateau has again reached the snowline. This explains, why the orbital variations (Milankovic-theory) could only have a modifying effect on the Quaternary climate dynamic, but were not primarily time-giving: as long as Tibet does not glaciate automatically by rising above the snowline, the depression in temperature is not sufficient for initiating a worldwide ice age; if Tibet is glaciated, but not yet lowered isostatically, a warming-up by 4 °C might be able to cause an important loss in surface but no deglaciation, so that its cooling effect remains in a maximum intensity. Only a glaciation of the Plateau lowered by isostasy, can be removed through a sufficiently strong warming phase, so that interglacial climate conditions are prevailing until a renewed uplift of Tibet sets in up to the altitude of glaciation.An average ice thickness for all of Tibet of approximately 1000 m would imply that 2.2 million km³ of water were stored in the Tibetan ice sheet. This would correspond to a lowering in sea level of about 5.4 m.     

The pleistocene glaciation of Tibet and the onset of ice ages — An autocycle hypothesis

During seven expeditions new data were obtained on the maximum extent of glaciation in Tibet and the surrounding mountains. Evidence was found of moraines at altitudes as low as 980 m on the S flank of the Himalayas and 2300 m on the N slope of the Tibetan Plateau, in the Qilian Shan. On the N slopes of the Karakoram, Aghil and Kuen Lun moraines occur as far down as 1900 m. In S Tibet radiographic analyses of erratics document former ice thicknesses of at least 1200 m. Glacial polishing and knobs in the Himalayas, Karakoram etc. are proof of glaciers as thick as 1200–2000 m. On the basis of this evidence, a 1100–1600 m lower equilibrium line altitude (ELA) was reconstructed for the Ice Age, which would mean 2.4 million km² of ice covering almost all of Tibet, since the ELA was far below the average altitude of Tibet. On Mt. Everest and K2 radiation was measured up to 6650 m, yielding values of 1200–1300 W/m². Because of the subtropical latitude and the high altitude solar radiation in Tibet is 4 times greater than the energy intercepted between 60 and 70° N or S. With an area of 2.4 million km² and an albedo of 90% the Tibetan ice sheet caused the same heat loss to the earth as a 9.6 million km² sized ice sheet at 60–70° N. Because of its proximity to the present-day ELA, Tibet must have undergone large-scale glaciation earlier than other areas. Being subject to intensive radiation, the Tibetan ice must have performed an amplifying function during the onset of the Ice Age. At the maximum stage of the last ice age the cooling effect of the newly formed, about 26 million km² sized ice sheets of the higher latitudes was about 3 times that of the Tibetan ice. Nevertheless, without the initial impulse of the Tibetan ice such an extensive glaciation would never have occurred. The end of the Ice Age was triggered by the return to preglacial radiation conditions of the Nordic lowland ice. Whilst the rise of the ELA by several hundred metres can only have reduced the steep marginal outlet glaciers, it diminished the area of the lowland ice considerably.     

The problem of historicity in physical geography

Historicity forms for logic the direct antithesis of regularity. In recognition of this dialectical premise physical geography has attempted to remove the historical contingent element of its phenomena in order to emphasise the regularity, and by so doing legimitise its status as a scientific discipline. This has resulted in a schism between empirical knowledge and the accepted theoretical structures. The regularity of geodynamic processes is apparent only on the basis of contingent clusters that in their essential characteristics are subject to historical change. Analysis becomes therefore a question of attribution, reconstructing individual clusters of causal determinants, each cluster being described as an individual outcome rather than the embodiment of necessary laws. The concept of the historical within geography must be clearly separated from the pseudo-historical development concept, as employed in relation to the theory of deterministic chaos. The scientific methodological problem, present in geography because of the incongruence between the logical assessment and the empirical subject matter, is here considered as the basis for the application of the evolutionary epistomological theory which not only recognises this incongruence but also clarifies and makes explicable its origin.The authors gratefully acknowledge the translation of this paper rendered by Prof. Dr. E. M. Yates, King's College, London.     

The Pleistocene Glaciation in the KarakoramMountains： Reconstruction of Past Glacier Extensions and Ice Thicknesses

Geomorphological and Quaternarygeological field- and laboratory data (Fig.1) are introduced and interpreted with regard to the maximum Ice Age (LGM) glaciation of the Central and South Karakoram in the Braldu-, Basna-, Shigar and Indus valley system as well as on the Deosai plateau between the Skardu Basin and the Astor valley (Fig.2). These data result from two research expeditions in the years 1997 and 2000. They show that between c. 6o and 2o Ka the Central Karakorum and its south slope were covered by a continuous c. 125000 km^2 sized ice stream network. This ice stream network flowed together to a joint parent glacier, the Indus glacier. The tongue end of the Indus glacier reached down to 850 ～ 800m a.s.l. In its centre the surface of this Indus ice stream network reached a height of a good 6ooo m. Its most important ice thicknesses amounted to c. 2400 ～ 2900 m.     

Würm glaciation of Lake Issyk-Kul area,Tian Shan Mts.: A case study in glacial history of Central Asia

Recent field research and modeling experiments by the authors suggest that Würm glaciation of Tian Shan Mountains had much larger extent than it was previously believed. Our reconstruction is based upon the following evidence: 1. a till blanket with buried glacier ice occurring on mountain plateaus at altitudes of 3700 to 4000 m asl; 2. trough valleys with U-shaped profiles breaching the border ridges and thus attesting to former outlet glaciers spreading outwards from the plateaus; 3. morphologically young moraines and ice-marginal ramps which mark termini of the outlet glaciers at 1600–1700 m asl (near Lake Issyk-Kul shores) and farther down to 1200 m asl (in Chu River valley); 4. clear evidence of impounding the Chu River by former glaciers and turning Lake Issyk-Kul into an ice-dammed and iceberg-infested basin; 5. radiocarbon dates attesting to the Late Pleistocene age of the whole set of glacial phenomena observed in the area.Our data on past glaciation provide a solution for the so called paleogeographical puzzle of Lake Issyk-Kul, in particular they account for the lake-level oscillations (by ice dam formations and destructions), for the origin of Boam Canyon (by impact of lake outbursts), and the deflection of Chu River from Lake Issyk-Kul (by incision of the canyon and build-up of an ice-raft delta near the lake outflow).The Würm depression of regional snowline was found to be in the range of 1150–1400 m. While today's snowline goes above the plateaus of Tian Shan touching only the higher ridges, the Würmian snowline dropped well below plateau surfaces making their glacierization inevitable. The same change in snowline/bedrock relationship was characteristic of the interglacial-to-glacial climate switches on the Tibetan Plateau resulting in similar changes of glaciation. The whole history of central Asian glaciations seems to be recorded in the Chinese loess sequences.A finite-element model was used to test two climate scenarios — one with a gradual and another with an abrupt change in snow-line elevation. The model predicted that an equilibrium ice cover would form in 19,000 (first scenario) or 15,000 (second scenario) years of growth. It also yielded ice thicknesses and ice-marginal positions which agreed well with the data of field observations.