The Time Series Establishment And Paleoclimatic Period Evolution Of Xijin Loess,Lanzhou

Quaternary loess-paleosol sequences of Chinese Loess Plateau are natural archives for the paleoenvironment reconstruction,these continuous aeolian deposits provide clues for geological evolutions over tectonic scale,such as the uplift of the Tibetan Plateau and the aridification of Asian inland.And they are of great significance to explore the coupling relationship among the East Asian monsoon evolution,the solar radiation and the polar ice sheet on the(sub)orbital scale.We obtained a continuous 416.2 meters long loess-paleosol sequence(the average recovery rate as high as 96%)by drilling at Xijin Village in Western Lanzhou basin of Chinese Loess Plateau,the drilling was performed from August to September 2012.This is one of the thickest loess-paleosol sequence in Lanzhou area,also the highest deposition rate and resolution in China.On the base of previous researches,we firstly tried to establish the time series of Xijin loess over 2.2Ma using orbital tuning method and the age of the closest matching with target time sequence,then verify with magnetic stratigraphic sequence established by Zhang(2016)each other.Secondly,the paleoclimatic period evolution over 2.2Ma of the proxies recorded by Xijin loess has been studied through the analysis methods of power spectrum,wavelet spectrum and cross spectrum.Finally,we discussed the change characteristics of carbonate content over 700 ka combining the characteristics of lay minerals and analysed the time phase relationship between calcium carbonate and magnetic susceptibility at S1.From what has been discussed above,we got the preliminary conclusions,as following:(1)A relatively reliable time series of Xijin loess stratigraphy over 2.2Ma has been established,on this base,conducte the power spectrum analysis.The results show that the change of calcium carbonate content has orbital periods of 100 ka,41ka and 20 ka over 2.2Ma,it also has non-orbital periods of 45 ka,30ka,27 ka and 17 ka.Among the orbital periods,the obliquity period of 41 ka is stronger than the eccentricity period of 100 ka and the precession periods of 22 ka and 19 ka.(2)The earth orbital periods recorded by Xijin loess-paleosol sequence shows a significant stage evolution characteristic,indicating different degrees of influence by earth orbit changes in different period.The results of the segmented power spectrum of calcium carbonate content show that Xijin core has record two major shifts in climatic period modes over the past 2.2Ma,one occurring at about 1.6Ma and the other at about 0.5Ma.The 1.6Ma shift is characterized by a transition from various periodicities to relatively single periodicities,at the same time,the eccentricity periods recede.while the 0.5Ma shift event shows a relatively gradual transition from constant obliquity to eccentricity period.(3)The study of paleoclimate recorded by carbonate of Xijin core over the past 700 ka through the continuous wavelet analysis shows that the climate oscillation in Lanzhou region is more prominent during the last glacial period,which is likely to be controlled by the change of the global ice volume and the meridional overturning circulation of the Atlantic.Heinrich events and Younger Dryas event are recorded well by carbonate of Xijin loess,and the response of the 20 warm events in the Dansgaard-Oeschger cycle is also strong,especially in the 8-12 and 14-17 stages(early MIS3),which support that the climate oscillation is more prominent and unstable.(4)The characteristic of increased calcium carbonate content supports the view that the East Asian summer monsoon has been strengthened since the early Pleistocene.The calcium carbonate content increased gradually over 700 ka,indicate a trend of increased rainfall and the East Asian summer monsoon increased gradually.The calcium carbonate content in paleosol unit S3 is extremely low over the past 700 ka,the main reason may be that there was a high CO2 content in the atmosphere and sufficient precipitation during S3.In the last interglacial(S1),the calcium carbonate and the magnetic susceptibility in the time series showed a significant inverse phase relationship and in macroscope they showed opposite increment regular pattern.