QUATERNARY SPOROPOLLEN ASSEMBLAGES AND ENVIRONMENTAL EVOLUTION OF THE GONGHE BASIN ON THE NORTHEASTERN MARGIN OF THE QINGHAI-TIBET PLATEAU

Relatively thick Quaternary strata are deposited in the Gonghe basin on the northeastern margin of the Qinghai-Tibet Plateau.They record Quaternary environmental changes in the basin in details.On the basis of an analysis of the characteristics of Quaternary depositional sequences and sporopollen assemblages in the Gonghe basin, combined with the thermoluminescent dating, the authors have studied the environmental change in the basin since the Quaternary and divided the change into two major phases:the early phase was one of lacustrine sedimentation (from the late part of the middle Pleistocene to 0.079 Ma), when the climate was relatively warm, and the late phase was one of fluvial sedimentation and eolian sedimentation (from 97.2 ka to the present), when the climate was relatively dry.The early phase may be further divided into three climatic stages and five climatic substages.Generally, in the Gonghe basin the climate has turned gradually from slightly warm to dry and the paleo-vegentation has changed gradually from forest vegetation to forest-steppe vegetation since the early Pleistocene.The main cause for this change is possibly that strong uplift of the Qinghai-Tibet Plateau since the early Pleistocene may have changed the southwest monsoon climatic pattern.