The Earliest LGP

Fig.3 shows the reconstruction of the maximum glaciation in Tibet, with an area of about 2.4 million km2. In the central part it formed a compact inland ice sheet with outflows that descended through the surrounding mountains and terminated at the steep edges of the high plateau.

To err on the safe side, the 50-70 m high, glaciofluvial gravel terraces east of 84°-85°E (Fig.1, Nos.10, 11), which lie there on a valley floor at 3800-3900 m asl, are classified as High Glacial deposits (Kuhle 1988c Fig.2, No.9 and Figs.7, 8). They indicate a glacier free Tsangpo section, which separates glacier complexes I3 and I2 as far west as 84°E (Fig.3a).

Further in the west the complexes I2 and I3 join again during the maximum of the LGP (Stadium 0: Table; Fig.3a). With an equilibrium line as low as 600 m below the average plateau altitude (Fig.3b) glaciation seems likely, even in the case of the more easterly Tsangpo section. This would place the gravel terraces and varved clays of the deepest parallel valleys in the Late LGP. Thus until now, at least for one time-interval during the LGP this valley section is regarded as free of ice. A precise dating of the terraces is to be done (Fig.3a, from north of Shisha Pangma to near Kola Kangri, Fig.1 from No.11 to 12, Kuhle 1991a Fig.43, Nos.37-45). The second ice free area is the Tsaidam Depression. In the block diagram of Fig.3a it shows up as narrow strip below I1 (Fig.1, No.50, Kuhle 1987b, p.302; 1998 Fig.15).

The third large ice-free area, documented by lake sediments, was observed in Mongolia. Dates of lake sediments in the area of the former ice sheet are all younger than 13,000 years old because they developed only after the glaciers had melted (Gasse et al. 1996, Avouac et al. 1996, Van Campo & Gasse 1993, Kashiwaya et al. 1991). By contrast, lakes in nearby, nonglaciated areas such as the Qaidam basin and the Gobi desert display continuous sediment records going back more than 40,000 years (Chen Kezao & Bowler 1986, Pachur & Wünnemann 1995, Wünnemann et al. 1998, Rhodes et al. 1996).

In a north-westerly direction from Mt. Everest to K2 (Fig.1, from No.14 to 15; cf. Fig.3b left), and from Dhaulagiri to the western Kuen Lun (Fig.1, from No. 19 to 16), the cross section shows that the LGP ELA runs parallel to the present ELA (Kuhle 1998 Figs.16, 17, 20, 21). Over southern Central Tibet the ELA attained an altitude of over 4700 m asl (see 2.3.2) (Kuhle 1988c Fig.2). Nonetheless an LGP ELA depression of at least 1200 m means that 83-86% of the plateau surface was above the ELA. The accumulating ice necessarily led to the filling of valleys that incised the Tibet plateau. The remainder accounts for the remaining 14-17%.

LGP glaciers attained an approximate thickness of 2700 m. Glacier thicknesses, ascertained by means od abrasions, polishings and erratics, reach 1600-2000 m in the Himalaya (Kuhle 1982a; 1991a), and 700-1200 m in Central and North Tibet. In the northern and western Karakorum thicknesses of more than 1750-2000 m have been observed (Kuhle 1994). However, these are probably minimum values. The ice may well have risen to 20003000 m in Central Tibet (Fig.3b) owing to a compact ground plan that extended over 1500 x 3000 km. The high viscosity of cold, continental glacier ice with annual temperatures of around -6 to -10°C at the ELA (Kuhle 1988e; 1994) supports the build-up of ice, provided that there is sufficient precipitation. An average thickness for all of the Tibet ice of approximately 1000 m implies that 2.2 million km3 of water was bound in the ice sheet of Tibet. This corresponds to a lowering of sea level of about 5.4 m (calculated on the basis of data provided by Flint 1971).

Earlier it has been shown (Kuhle 1998 Fig.22) how the glaciated area in Tibet and in the Karakorum relates to an ELA depression or an uplift of the plateau- and mountain relief of only 500 m. At the same time it permits an estimation of conditions if the equilibrium line drops by 1200 m, and makes the cupola-shaped build-up of the inland ice to a considerable thickness plausible. With the ice held back by mountain barriers, build-up was assisted by the low run-off, and initially by freezing to the subsurface. At a later stage, due to ice build-up, the pressure-induced melting point was reached. Run-off from the central inland ice sheet gradually increased until an equilibrium had been achieved. This was the end of ice build-up.

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