Magnetic Characteristics And Its Quaternary Climatic Evolution Recorded Of Loess In Xijin,Lanzhou

The most significant characteristics of Quaternary climate evolution is the frequent fluctuation of cold and warm,and the alternation of glacial and interglacial periods.Previous studies believe that the magnetic characteristics of loess-paleosol sequence in the Loess Plateau,China can record climate changes in detail.The western Loess Plateau is close to the desert source area,located in the junction zone between the monsoon and non-monsoon areas,in which the relationship between the magnetic characteristics of loess sediments and climate change is more complicated.However,the previous research on the climate record of the loess in this area is less than that of the central loess plateau,so it is particularly important to study the magnetic characteristics of the loess and reconstruct the paleoclimatic record of the area.The Lanzhou Basin is located in the northeast edge of the Qinghai-Tibet Plateau,which is the junction zone of Qinghai-Tibet Alpine Area,Northwest Inland Arid Area and Eastern Monsoon Area.The loess of higher dust deposition rate and larger sediment thickness in the area recorded abundant information on climatic and environmental changes,and the special geographical location makes it an ideal area to study highresolution climate change and its driving mechanisms.In this paper,the loess of Xijin Village,located on the south of Yellow River in Lanzhou,is taken as the research object.Based on the existing stratigraphic age,the reconstruction of the magnetic mineral characteristics and detailed Quaternary paleoclimate evolution records of the Xijin Loess were carried out by using rock magnetic parameters and chromaticity proxy,and the following conclusions were preliminarily obtained:(1)The analysis of the magnetic minerals in Xijin loess shows that the main magnetic minerals of Xijin loess are the magnetite,which also contained magnetite,hematite and goethite,and the information of particle size exhibits PSD particle signal.In addition,?ARM??ARM/?lf and SIRM/?lf can better reflect the periodic changes of climate evolution in Lanzhou.(2)The comprehensive analysis of magnetic parameters and redness index of Xijin loess shows that an important climatic event occurred around 1.24 Ma in Lanzhou,afterwards which the amplitude and frequency of climate change increased synchronously and strongly.It can be inferred that the important climate event at 1.24 Ma in Lanzhou area may be related to the uplift of the Qinghai-Tibet Plateau.(3)The study of multiple loess magnetic parameters and redness index show that an abrupt climate change event occurred at about 0.7 Ma in Lanzhou area of western Loess Plateau,and then which was followed by a cooler and drier climate change,a significant increase in the change amplitude and period in the glacial cyclone.It was consistent with the end time of the global "Mid-Pleistocene Climate Transition"(MPT).It was preliminarily deduced that this event recorded by Xijin loess was a synchronous response to the global MPT.(4)The records of Xijin loess in Lanzhou area also clearly reveal the “Mid-Brunhes event”(MBE)occurred at about 0.43 Ma in Pleistocene,thereafter which the regional climate evolution shows a strong trend of long-term cold drying,which is similar to the previous records in the central Loess Plateau Region,but the trend of cold drying is more notable in Lanzhou area.