Hydrological changes and Holocene climate variations in NW China, inferred from lake sediments of Juyanze palaeolake by factor analyses

The eastern Juyanze palaeolake in hyperarid north-western China (41.75-42 degrees N, 101.5-102 degrees E) provides evidence of a complex hydrological pattern during the Holocene. Multivariate statistics based on lithological, geochemical and mineral data from the reference site G36 in the centre of the lake were performed to reconstruct the interrelation between lake hydrology and climate variability of the last 11,000 yr. Principal component analyses (PCA) analyses enabled us to extract the three system variables runoff, lake level/water budget and salinity as the major factors which have influenced the lake history during a highly variable Holocene climate. We can also demonstrate that the independent variable total organic carbon (TOC) is not only a function of bio-productivity in lakes under warm-wet climate conditions but more frequently related to surface runoff processes, which can result in over-estimation of pollen types used for climate reconstructions. A combined consideration of all system variables with lithological parameters, catchment characteristics and the local groundwater situation show that lake formation and its persistence strongly depended on local rainfall/runoff and the status of the underlying aquifer. Our results indicate remarkable differences between the early Holocene and mid to late Holocene lake formation, owing to changes in the aquifer system beneath the lake. At about 10,700 cal BP freshwater lake formation starts with extreme runoff events under a wet climate. Repeated dry climate spells of reduced runoff. The overall climate during the early Holocene points to wet conditions with frequent phases of very high runoff and highest lake levels until about 8900 cal BP. Extremely low runoff occurred between 8900 and 8100 cal BP, indicating already dry climate conditions some hundred years prior to the worldwide recorded responses of the 8.2 ka cold-dry event. A second period of major dry climate conditions with generally low but fluctuating runoff, unstable water budget and deposition of aeolian sand lasted for about 2100yr between 7500 and 5400 cal BP. These climate conditions corroborate previous assumptions of a mid Holocene dry period in and north-western China. Increased runoff after 5400 cal BP under wet climate conditions led to a complete refill of the aquifer at about 5000 cal BP and favoured the development of a saline lake for about 3500 yr. In terms of hydrology the time between 5400 and 4000 cal BP appears to be the climate optimum. Compared with the early Holocene wet climate, the mid to late Holocene local rainfall was less pronounced and displays a general decreasing trend towards complete desiccation of the lake. Our results indicate an extremely variable moisture availability in and regions of north-western China during the entire Holocene. Owing to the interplay of the East Asian Summer Monsoon with the westerly waves at the boundary of the summer monsoon limit, variations from dry to wet climate conditions seem to have been more pronounced than in other monsoon regions of China. (C) 2007 Elsevier Ltd and INQUA. All rights reserved.