Paleoclimatic Changes During the Last 2.5 Ma Recorded in the Kathmandu Basin, Central Nepal Himalayas

The Kathmandu valley is an intermountain basin located at about 1300 and surounded by mountains around 2400m.Thick fluvio|lacustrine sediments from late Pliocene to Quaternary are accumulated there under monsoon climate.We undertook palynological and sedimentological studies on 284|m|long drilled core that reaches the basement of the kathmandu basin,in order to clarify the paleoclimatic changes and their tectonic link with uplifting of the Himalayas..The core is lithologically divided into three members:sand predominant lower member (51m),slit and clay dominant middle member (182m) and organic silty clay predominant upper member (45m).We analyzed fossil pollens contained in muddy samples which were taken from the middle and upper member in every one meter interval.The pollen assemblage is characterized by predominance of %Quercus,%30%～80% of the all arboreal pollen.We inferred paleoclimatic changes on the basis of comparison between present vegetation and climate of Kathmandu valley and surrounded mountains and pollen diagram which we obtained.For the purpose of inference of paleoclimate,we used the following genera as climatic index:%Pinus% for cold climate, %Quercus% and %Cyclobalanopsis% for warm climate, Alnus for wet climate and Gramineae for dry climate.As the reference.we also examined amount of another eight genera,two family and all spores.The pollen diagram shows two pattern of paleoclimatic changes:oscillation pattern with seven times fluctuation of warm and cold climate in younger period (ca.115m in depth),and older less fluctuated pattern of warm climate.During the older period (209～215m in depth),one cold stage was recognized,and is characterized by predominance of Pinus and occurrence of Tsuga .