Abstract: The Zhaoxiayu loess section in Mangshan Yuan which is located at the transitional zone between the North China Plains and the Loess Plateau is typical of the loess stratigraphy in China Central Plains.The section is exposed from the top of Mangshan Yuan to the valley bottom of the Huanghe River,the loess-paleosol sequence from S₀ to S₁₀,with a total thickness of 172.1 m.It is known from the results of optically stimulated luminescene dating and thermoluminescene dating and the magnetostratigraphy that,the B/M boundary was recorded at the top of S₈.The sequence is characterized by a relatively thick paleoso S₁ (15.7m) and a thick loess L₁ (77.3m) of the Late Pleistocene.
Mangshan loess is a kind of near-source sandy dust deposit with the source area lying just at the head of an alluvial fan at the lower reaches of Huanghe River.There is great difference in the average rate of sedimentation of loess in different glaciations or stades and of paleosol in different interglaciations or interstades in Zhaoxiayu loess profile.The sedimentation rate of loess L₁LL₁,paleosol L₁SS 1,and loess L₁LL₂ in the Last Glaciation is as high as 3.45 mm/a,0.38 mm/a,1.51mm/a,respectively,and that of paleosol S₁ of the Last Interglaciation is 0.28mm/a,a rate not only higher than that of paleosol S₂(0.05 mm/a),but also higher than that of loess L₂ (0.18 mm/a).It has been shown that an abrupt change of the dust sedimentation rate happened in the end of penultimate glaciation,about 150 ka B P (equivalent to the top of L₂).The reason for this may be that the Huanghe River might have cut through the Sanmenxia Gorge to flow eastwards transporting abundant materials to the source area since that time.
The magnetic susceptibility of loess and paleosol is related to the strength of pedogenisis,and can be served as a proxy index for variations of summer monsoon.Magnetic susceptibility values of loess in Zhaoxiayu section,30×10^-5 SI or so,approximates to the background values of dust deposit that has undergone no little or no pedogenisis.And that of paleosol,70-170×10^-5 SI,and that of immature paleosol,50-60×10^-5 SI,can not be considered as a simple reflection of the strength of summer monsoon in interglaciation or interstades.They should also be affected by sedimentation rate.