试论西藏全新世古地理的演变

本文从冰后期以来湖泊的退缩、泥炭沼泽的发育、冰川与冻土的变化、生物的演替以及人类活动范围的扩大与缩小等方面论述西藏全新世古地理环境特征和演变过程,并结合C¹⁴的年龄数据,认为全新世时期西藏自然环境的演变可分为早全新世（10,000-7,500年）环境好转阶段、中全新世（7,500-3,000年）环境最宜阶段以及晚全新世（3,000年至现在）环境恶化阶段.而且全新世不同时期古地理环境具有明显的地域差异.现时西藏地区仍在继续上升,高原内部仍将继续向干冷的自然环境变化.

ON THE PALEOGEOGRAPHICAL EVOLUTION OF XIZANG (TIBET) IN THE HOIOCENE

Xizang (Tibet) is the main body of the "World Roof" -the Qinghai-Xizang Plateau. Although the continuous uplift of the Plateau and the global climatic changes had no distinct influence on the entire geomorphological framework notable paleogeographical evolution has been witnessed. According to C¹⁴ dating, the process can be divided into three stages:1) early Holocene (10000-7500 years B.P.), when the environment became better, for instance, the retreat of glaciers, the increase of plants, the development of peat and marshes, and salt-lake precipitation in the interior of northern Xizang Plateau; 2) middle Holocene (7500-3000 years B.P.), when both Vegetation and peat-marshes were in the most favourable condition, the forest line occurred over the plateau surface in southern Xizang; glacier retreated to the present position; lakes were at a stage of relatively high water level; climate was warm and humid and human activities spread all over the plateau, 3) late Holocene (3000 years B.P.to the present), also known as neo-ice age. Three glacial advances are seen and natural environment deteriorates; vegetation on the plateau surface is becoming steppe and desert steppe while peat and marshes decline; lakes diminish, and most of the exterior lakes turn into interior ones on the Plateau. Furthermore, salt precipitation becomes stronger; ancient human beings migrate towards the lower parts due to the cold and dry climate, and the northern part of the N.Xizang Plateau became a depopulated area.Distinct paleogeographic regional diff erenciation is recognized in Xizang.The greatest environmental change has been witnessed in the high mountain and broad valley-lake basin region of Southern and central Xizang where the transformation of exterior lakes into interior ones, the development of peat-marshes and the advance and retreat of forest line mostly occur.