Characteristics Of Grain Size And Magnetic Susceptibility Of Loess In Southeast Loess Plateau And Its Paleoclimate Implication

The thick dust accumulation sequence of China's Loess Plateau recorded the evolution of Asian inland drought processes and the East Asian monsoon since late Cenozoic,and was regarded as a natural archival of regional and global climate change.It is located in the second and third terraces of China and Keeps complete over the stacking sequence.The exposed thickness,integrity and the ancient climate information contained in the Loess-paleosol sequence are not inferior to the hinterland of the Loess Plateau.In this paper,we analyse grain size and magnetic susceptibility characteristics of the Mapocun and Koumacun located in southeast margin of the Loess Plateau area,and compared with the typical loess sedimentary section in Loess Plateau of Xining Dadunling section,Jiuzhoutai section,Jingyuan section,Duanxian section and Jingbian section,to explore the climatic evolution of the southeastern margin of the Loess Plateau,the climate change events in each stage and the mechanism.After the study,this paper gets the following conclusions:(1)The low frequency magnetic susceptibility of the MPC in the southeastern margin of the Loess Plateau ranges from 30.032-130.032×10-8m3/kg,the average is 74.06 × 10-8m3/kg,the high frequency magnetic susceptibility ranges from is 25.304-113.050 × 10-8m3/kg,the average value is 67.60 × 10-8m3/kg.The low frequency magnetic susceptibility of KMC is 26.974-229.639 × 10-8m3/kg,the average value is 92.29 × 10-8m3/kg,and the variation range of high frequency magnetic susceptibility is 26.361-199.152 × 10-8m3/kg,the average is 83.36 × 10-8m3/kg.Based on the average low frequency susceptibility of the two sections,and compared with the standard stratigraphy of the Loess Plateau,the 9 layers of the loess correspond to L0-L9,the 10 layers of the paleosol correspond to S0-S9.(2)The content of sand(<4? or>63?m)is 0.661-25.361%,the average is 7.061%.The average value of the loess layer is 8.396%and the average value of the paleosol layer is 6.490%.The average content of the silt is 50.733-91.450%,the average value is 77.06%,and the coarse silt is the main(the average content of coarse silt is 60.948%and the average content of fine silt is 16.158%).The average value of silt in the loess layer is 78.124%,and the average value of the paleosol layer is 76.275%.The content of clay(>8cp or<4?m)is 4.354-84.493%,the average value is 15.879%.The average value of loess layer is 13.630%,and the average value of paleosol layer is 17.287%.Obviously,the content of silt and clay in paleosol of the profile was significantly higher than that of the loess layer.(3)The magnetic susceptibility and particle size of the Mapocun loess sediments showed an excellent correlation.The magnetic susceptibility of the sediments was positively correlated with the fine components in the mechanical properties of the granules,and negatively correlated with the crude components.According to the stratigraphic correlation,9 layers of loess and 10 layers of pale soil were developed in Mapocun and Koumacun recorded in the dry period,the loess accumulation,the magnetic susceptibility is small and the grain size is slightly thick,which reflects that the climate of the loess is more arid and the dust storms are frequent and the soil formation is weaker.The soil development period obviously represents the climate is relatively warm and humid,precipitation increased,the role of bio-soil is strong,ferromagnetic mineral content increased,strong magnetic material content increased significantly.The characteristics of the composition of the sediments also indicate that sediments are mainly wind-driven,and are subjected to different depositional environment.There are near and distant source components.(4)Based on the analysis of the magnetic susceptibility of the Koumacun located on the southeast margin of the Loess Plateau area,it is found that the magnetic susceptibility and the frequency magnetic susceptibility show a downward trend from the bottom to the top.The magnetic susceptibility of the typical sedimentary section in Loess Plateau shows an increasing trend from the bottom to the top by the continuous uplift of Qinghai-Tibet Plateau.The north and south terrain of Koumacun is high,the middle is low.The wind direction is northeast wind and westerly The distance between Koumacun to Qinghai-Tibet Plateau is relatively far away.It is less affected by the tectonic uplift of the Tibetan Plateau.Magnetic susceptibility values from the bottom section to the top gradually weakened just record the background of Quaternary climate change and has the same trend with the deep sea oxygen isotope;On the other hand,it may be affected by the initial dust.The arid and semi arid trend in the source region of Central Asia is increasing,and the content of coarse magnetite particles is relatively high,soil forming is weak in the later.(5)According to the stratum and field observation that Mapocun area from 575cm to 1195cm belongs to the MIS3 stage,Koumacun area at 285cm?685cm belongs to MIS3 phase,the magnetic susceptibility,frequency susceptibility two summer wind sensitive indexes,indicating there are big changes in climate.The overall MIS3 stage can be divided into early,middle and late 3 times,the early is most warm and wet,the middle is slightly cold and dry,the late is warm and wet.(6)Koumacun section 50-85cm corresponds to the early Holocene,Mapocun profile 210-286cm is early Holocene.The climate shows slightly warm,but the overall is still a dry and cool.The climate is similar to the current northwest grassland.On the record of the 8.2kaB.P.event,the KMC and MPC profiles on the southeastern margin of the Loess Plateau are not found.On the contrary,the magnetic susceptibility of the KMC section increases at 50-60cm,and there are few reports on the related studies,so it is debatable.The KMC section corresponding to the Holocene is warm period between 15 and 60 cm.The MPC profile corresponding to the Holocene is warm period at 40-210 cm.The magnetic susceptibility is peak at this stage,showing the warmest and humid climate in the mid-Holocene.The magnetic susceptibility and frequency susceptibility of KMC at 25cm and 45cm showed a low value.The magnetic susceptibility and frequency susceptibility of MPC profiles at 60-80cm and 150-175cm abruptly decreased and the value of<5 u m increased,Indicating that even in the mid-Holocene warm and main features is humid,but climate fluctuations still exist,it may correspond to 5.5ka and 4ka events.The KMC profile at 0-15cm,MPC profile at the 0-40cm corresponds to the late Holocene,this stage of the susceptibility,frequency susceptibility,clay composition is fluctuation,>30 ?m content increased,showing temperature in late Holocene is descendant,the climate becomes cool and dry.