On Ancitent Ice-Sheet and Ice Age in the Tibetan Plateau
Xu Daoming and Shen Yongping（Lanzhou Institute of Glaciology and Geocryology, Chinese Academy of Sciences，730000）
Another distribution,scattered but undoubed, of glacial landforms on th Tibetan Pla-teau，that goes far beyond the ability of modern mountain glaciers， is presented．The authorshold that the Tibetan Plateau was inundated by ice and divide Pleistocene glacial landformsinto the early （100-30 ka BP） and the late（20-10 ka BP）stadials of the Last Glaciation. The ice-sheet over the whole Plateau probably happened in the early stadial， that representsthe Last Glacial Maximum （LGM）as the North Plateau had been ice－free since. The Maxi- mum descent of ancient snowline reached elevations of 3800－ 4200m a. s．l．on northpiedmonts of the Plateau and 4600-4800m a. s．l． in the middle－upper Yarlung-Zangbo val- ley， that is the most part of the Tibet Plateau lay above or touched on the ancient snowlineduring the LGM. The current division of moraines into the fourfold glaciation through the wholePleistocene lacks of reliable foundation of Quaternary stratigraphy and contradicts the ex- isting dating materials．Those moraines clearly linked to modern glaciers and the corre-sponding snowline estimations perhaps only represent the glacial expansions after LGM.The interstadial of the Last Glaciation is represented by the alluvio-lacustrine depositswith the oldest datings of 2 5-18 ka BP， during the period so named “High- stand of Laked”in north plateau. The south plateau probably succeeded iee-sheet in the late stadial；the ex-isting datings from subsequent deposits show a range after 10 ka BP. The lacuna of earlierPleistocene landforms in all the plateau reflects，in a large degree，the existence of iee-sheetafterwards.The datings from direct glacial deposits from different landforms and areas range from 130-30 ka BP to 21－11 ka BP. This is coincident with the result of ice－core drilling in Mt.W. Kunlun，that evidences the glacial ice layers lasting for more than 100 ka the and an interstadial environment at about 32 ka BP. The onset of ice age and the LGM in Tibet Pla-teau seem much earlier than that in terms of global during the Last Glaciation.