Climatic and Hydrological Variations during the Past 8000 Years in Northern Xinjiang of China and the Associated Mechanisms

Abstract:

This paper reviews the published literature regarding the climatic and hydrological variations of the past 8000 years in northern Xinjiang of China in hope that some of the controversies surrounding the Holocene climate history can be reconciled. Based on the well-tested premise that the overall relationship between the A/C ratios and the bioclimates in arid and semi-arid areas of China still holds, we compared the A/C ratio-based SMI (Standardized Moisture Index) of five selected lacustrine sequences. Our comparison shows that the regional average SMI had a general rising trend, and the rising trend was reasonably corroborated by other pollen proxies. The rising trend can be explained by invoking its climatic linkage with the North Atlantic Ocean, and a possible linkage with ENSO is an additional or an alternative mechanism for explaining the rising trend. The nearly parallel rising trends of seven δ¹⁸O-indicated SSI curves (Standardized Salinity index) mean that the lake-water salinity has been rising since ~8.0 cal. ka BP. The rising trend can be explained by invoking the Holocene variations in the areal extent of ice covers in the Tianshan Mountains and the Altay Mountains. That is, the persistent shrinking of the ice extent in the mountains generated less and less melting water supplies to the lakes and thus resulted in saltier and saltier waters in the lakes. The contradiction between the increasing trend of the basin-wide moisture and the increasing trend of the lake-water salinity can be reconciled as follows: the increasing trend of the precipitation accompanied by the lowering tend of the temperature might have increasingly enhanced the effective soil moisture in northern Xinjiang, but the increase in the basin-wide moisture was not enough to compensate for the decrease in the lake inflowing water resulted from the shrinkage of ice-covered areas in the mountains.