Evolution Of Summer Monsoon In Songnen Plain Since Middle Pleistocene Revealed By Harbin Loess

The East Asian summer monsoon(EASM)is an integral part of the global climate system.It is very important for the living environment maintenance and social sustainable development of populous areas in East Asia.Therefore,in order to improve our ability to predict the long-term trend of regional and global climate,it is very important to study the variability of the East Asian Summer Monsoon on different time scales and explore its potential driving mechanism.Harbin loess is located in the northeast of Songnen Plain and on the eastern edge of the arid semi-arid zone in the middle latitude of Eurasia.It is a transitional zone between monsoon and non monsoon areas and is very sensitive to the change of monsoon climate.Therefore,we have tested the magnetic susceptibility,total organic carbon(TOC)and geochemistry of Harbin loess,and combined with wavelet analysis to study the evolution and control mechanism of summer monsoon since the Middle Pleistocene in Songnen Plain.The results show that:(1)The variation characteristics of magnetic susceptibility of Harbin loess are opposite to that of the classic Luochuan Loess on the Loess Plateau,which is related to the unique reductive pedogenic environment of Songnen Plain;Rb/Sr ratio and(Ca O+Na2O+Mg O)/Ti O2 index can indicate the intensity change of East Asian summer monsoon in Harbin loess paleosol sequence;The special change of CIA value may be related to the cold,wet and reducing soil forming environment in Songnen Plain and the material source fluctuation in the formation of Harbin loess;The change of TOC content can roughly reflect the change of biological productivity of loess paleosol caused by paleoclimate change.In loess layer L1,TOC content showed a special and obvious high value,indicating that during the formation of L1,the biomass in the soil increased suddenly and the input of organic matter increased.(2)The evolution of East Asian summer monsoon in Songnen Plain mainly experienced two stages.1)460 ka ? 180 ka,which is the period of weak summer monsoon intensity,which is consistent with the overall drought trend of inland Asia.Harbin loess paleosol sediments show low chemical weathering degree and TOC value.The two summer monsoon intensity weakened rapidly at 280 ka and 360 ka,indicating the intensification of drought in Songnen Plain.Although the intensity of the summer monsoon increased slightly in the three periods of 400 ka ? 380 ka,340 ka ? 300 ka and250 ka ? 230 ka,it is still weak in general.2)From about 180 ka in this stage,the intensity of summer monsoon continues to increase,and the magnetic susceptibility,chemical weathering degree and TOC value of Harbin loess paleosol sequence show a gradual increasing trend.(3)We have made a regional comparison of the changes of summer monsoon indicated by loess in three typical areas in northern China(Harbin,Chifeng and Luochuan).It shows that Songnen Plain first responds to the climate change caused by the increase of temperature and precipitation dominated by the interglacial summer monsoon,but the duration of this response is relatively short;Harbin loess recorded two significant climate phenomena of weakening summer monsoon from 360 ka to 340 ka and 280 ka to 250 ka,which is the result of the comprehensive action of global climate and regional environmental change.(4)The wavelet analysis of Harbin loess magnetic susceptibility shows that the main oscillation periods of magnetic susceptibility are 29 ka,36 ka,48 ka,60 ka,75 ka,96 ka and 121 ka,the highest oscillation is 96 ka and the lowest oscillation is 29 ka.(5)The results show that the change of summer monsoon intensity in Songnen Plain is mainly driven by high latitude stress ice.The occurrence of 36 ka and 48 ka(about 40ka)earth axis dip cycles shows that the low latitude solar radiation driven by dip plays a weak regulatory role in the summer monsoon in Songnen Plain.The lowest oscillation is29 ka period instead of 20 ka precession period,which shows that the solar radiation change in low latitude driven by precession has little impact on the Songnen Plain in high latitude.