HOLOCENE ENVIRONMENTAL-CHANGES IN BANGONG CO BASIN (WESTERN TIBET) .3. BIOGENIC REMAINS

A 12.4 m core collected from the eastern part of Lake Bangong provided a continuous Holocene record. This core was analyzed for diatoms, ost racods, charophytes, content in total organic carbon and identificatio n of organic particles contained in the sediments. The record is remar kable for the abundance and diversity of its microorganisms. An oligo- to mesosaline marsh occupied the core site before 9.5 ka B.P. Bangong was most likely closed. A sudden filling of the lake basin by dilute, nutrient-poor water occurs around 9.4 ka B.P. Maximum lake extent see ms to have occurred around 9.4-8.7 ka B.P. Generally freshwater, relat ively deep conditions were maintained up to 6.3 ka B.P., although inte rrupted by short-term regressive events between 5.6 and 7.5 ka B.P. A trend towards palustral conditions begins after 6.3 ka B.P. and the in terval 3.9-3.2 ka B.P., rich in detrital organic material, coincides w ith an extremely low lake level. A last lacustrine episode of minor am plitude is recorded from about 3 to 2 ka B.P., before the closure of t he system leading again to slightly saline conditions. The resemblance s and synchronism between environmental changes recorded at Bangong an d in Lake Sumxi, situated 200 km northwards and 800 m higher in elevat ion, indicate that regional climatic changes have been the major drivi ng factor on ecological and hydrobiological changes in lakes of wester n Tibet.