The Process Of Climatic Aridification Since The Middle Pleistocene In The Songnen Plain ——Indications From The Harbin Loess Records

The Harbin loess is located in the easternmost part of the Eurasian loess belt.It is an ideal material for reconstructing the history of aridification of the Songnen Plain and determining the eastward advance of inland Asia.However,research on aridification of the climate in the Songnen Plain is quite weak.The relationships between aridification,tectonism,climate and drainage evolution in the area are currently poorly understood.In this study,these sedimentary characteristics,grain-size,magnetic susceptibility,total organic carbon and Sr-Nd isotopes are presented,combined with grain size end-member and wavelet analysis,to examine the history and driving mechanism of climate aridification in the Songnen Plain.The results showed that:(1)Compared with the typical loess deposits in the Loess Plateau and Chifeng,the Harbin loess has significant differences,especially the paleosol layer,which is thinner and gray-black,rich in argillaceous cements,lacking calcareous nodules,magnetized The rate also shows that the loess layer is higher than the paleosol layer.(2)EM1 of Harbin loess represents the dust material transported from a distant source under the control of high-altitude westerly wind;EM2 represents the distant source dust material transported by the winter monsoon;EM3 represents the coarse silt transported by the strong monsoon or dust storm components,which can indicate the degree of aridification of the Songnen Sandy Land.(3)In this paper,the interpolation method of granular data is used to establish the age frame of the Huangshan section,and optical luminescence(OSL)and electron spin resonance(ESR)are used to test the accuracy of the established age frame.According to the established stratigraphic age framework,the bottom age of the barren loess in Harbin is estimated to be 460 ka.(4)Under the combined influence of the high-latitude cold climate and the sedimentary environment of soil gilding,the sedimentological characteristics(such as color and thickness)of the Harbin loess-paleosol sequence are significantly different from other regions;the magnetic minerals are transformed into more stable weak magnetic minerals after deposition,which is the reason for the low value of the magnetic susceptibility in the paleosol layer.(5)Harbin loess accumulation(?460 ka)indicates the onset of significant aridification in the Songnen Plain and the formation of the Songnen Sandy Land.There were two occurrences of aridification intensification and sand expansion at 300 ka and380 ka,starting from 180 ka,the climate gradually tends to be warm and humid.(6)The increase in global ice volume is the main climatic driving factor that dominates the aridification of the Songnen Plain,and the Songnen Plain at high latitudes has a better response to the Siberian High than the neighboring areas.(7)By comparing with the records of global climate change in the Middle Pleistocene,the records of loess in adjacent areas,the tectonic movement of the Songliao Plain,and the Quaternary evolution history of the Songliao water system,combined with wavelet analysis,we believe that the formation of the Songnen Sandy Land and the drought The phenomenon of aggravated climate change is the result of the combined action of climate change,tectonic movement and water system evolution.