Holocene environmental changes in Bangong Co basin (Western Tibet). Part 2: The pollen record

A better knowledge of environmental variations in key regions such as the Tibetan plateau, which plays a fundamental role in the Indian monsoon dynamics and thus on the whole Northern Hemisphere atmospheric circulation, is of great importance. We present here a detailed paleoenvironmental study based on pollen analyses of a sediment core from Lake Bangong. Results on the modern pollen rain are also discussed as they give valuable information for the interpretation of the fossil pollen data. llen results of the Bangong sequence and their paleoclimatic interpretation are integrated into a chronological framework on the basis of palynostratigraphical methods. Because of dead carbon in the lake system, it has been difficult to establish a reliable chronology of the Bangong sequence. Using a geochemical model for correcting measured 14C ages, the pollen record from Lake Bangong compares well with the record from Lake Sumxi in Western Tibet. Lake Bangong core provides a continuous Holocene pollen record. Before 9600 yr B.P., the dominance of desertic elements in the pollen record reflects an arid environment. At about 9600 yr B.P., an abrupt shift to steppe vegetation suggests the onset of moister conditions, and from ≈9500 to 6200 yr B.P., proxy data indicate conditions warmer and wetter than today. Proxy data also indicate short-term changes during this interval. Climate instability is suggested by the development of a local palustral flora, which peaks at 7700 yr B.P. and is interpreted as a lake regression, in phase with increase in the percentage of littoral and saline water diatom species between 8600 and 7700 yr B.P. The first humid pulse culminating at 9000-8500 yr B.P. is thus followed by drier conditions around 8000-7700 yr B.P. The interval between 7200 and 6500 yr B.P., regarded as optimal for vegetation growth from the pollen record, corresponds to a second humid pulse. A trend towards aridity is observed during the following period with dry events centered around 5500, 3900-3200 and 700 yr B.P.