The Comparative Study Of Paleolakes Evolution Between The Eastern And Western Parts Of The Hexi Corridor On Holocene

The arid region of China with plenty of inland rivers and lakes is the key area of geologic and geographic studies on Holocene. The lithology, grain size, geochemical proxies and pollen proxies of lake sediments, as well as lake displacement, all represent abundant environmental information, so climate and geomorphology evolution can be analyzed based on them. In this paper, the Shiyang River drainage basin, which is in the eastern part of the Hexi Corridor, and the Shule River drainage basin, which is in the western part, are chosen to be studied. According to the studies of terraces from the upper reaches, the sections from the middle reaches, the sections from the lower reaches, and the surface sediments from the terminal lake, I rebuilt the climatic and geomorphic evolution of the Hexi Corridor on Holocene and studied the change of Asian summer monsoon boundary; besides, the Logistic regression model can be built on the basis of spatial analyses of the Shiyang River and the Shule River drainage basins.During the early and middle Holocene, the upper reaches from the Shiyang River drainage basin are eroded, but the middle and lower reaches are deposited; however, the upper and middle reaches are both eroded during the late Holocene, but the lower reaches are still deposited. The middle and lower reaches of the Shule River drainage basin are deposited during the early and middle Holocene but eroded during the late Holocene. The geomorphology of the Shiyang River drainage basin is relatively complicated with unconsolidated sediments and the climate changes drastically in this area, so the deposition rates of this region are relatively higher than those of other drainages. In the upper reaches of the Shiyang River drainage basin, TOC content has a negative relationship with deposition rates. In the middle reaches, the grain size has a negative relationship with deposition rates, but the TOC content has a positive relationship; besides, the CaCO3 content increases continuously from bottom to top. The shrub and herb pollen contents decrease along with aridification, but the needle leaf trees and Picea pollen contents increase. In the lower reaches, the relationship between grain size and deposition rates is not very clear; however, the TOC content, C/N, shrub and herb pollen contents all have positive relationship with deposition rates. The needle leaf and Picea pollen contents decrease continuously, and the δ13Corg content changes with no rules. The grain size of surface sediments from the bunding of Zhuye Lake is relatively coarse with low TOC content and C/N; nevertheless, in the middle of the lake basin, the sediments are thin with high TOC content and C/N. In the Shule River drainage basin, the grain size, TOC content and herb pollen content all have a negative relationship with deposition rates; besides, the other pollen contents have a positive relationship with deposition rates.On the basis of the spatial analyses of the Shiyang River drainage basin, the Zhuye Lake is a basin-shape region, and the Logstic regression model of lake formation in this area is P(1)=1[1+e-(1356.0930759+0.00004759D-1359.3707216R+1.32323064S)].In this equation, D means Distance, R is Roughness, S represents Slope, and the accuracy of it is 69.4%. The probability of lake formation has a positive relationship with distance, but has a negative relationship with earth surface roughness; besides, the area with high slope can form lakes more easily. The Gumingze of the Shule River drainage basin is also a basin-shape area, but the lower reaches of this drainage are flat. The Logistic regression model of lake formation in this area is P(1)=1/[1+e-(1266.439446+0.006063DE+0.00000752D-1275.58117R+1.0918188S)] .In this eauation. DE means DEM and accuracy of it is 68.5%. The probability of lake formation has a positive relationship with DEM in the Shule River drainage basin, and the relationships between distance, roughness and slope are familiar with those of the Shiyang River drainage basin. According to modern climatology, the Asian summer monsoon can only reachthe eastern part of the Hexi Corridor, but the Shule River drainage basin which is in the western part is controlled by the Westerly Winds. The geomorphic and paleolake evolution of the Shiyang River and the Shule River drainage basins is familiar with that of typical monsoon region during the early and middle Holocene; however, the climate of the Shule River drainage basin starts to be more humid than that of the Shiyang River drainage basin. Therefore, the Shiyang River and the Shule River drainage basins are both controlled by Asian summer monsoon during late glacial, early and middle Holocene, but the western part of the Hexi Corridor is affected by the Westerly Winds during the late Holocene. The northern boundary of Asian summer monsoon changes on Holocene.In this paper, the analyses of sediment proxies from the Shiyang River and the Shule River drainage basins are not clear enough, so I should improve calculation methods of erosion and deposition rates; besides, more geochemical proxies and pollen proxies should be considered. The boundary of Asian summer monsoon should be studied more accurately by analyzing the climate change of the Shiyang River and the Shule River drainage basins. Above all, the spatial analyses in this paper is only about modern geomorphology, so dynamical model should be considered in the future work.