Stratigraphy of Zhangla Loess in Western Sichuan Plateau and its Paleoclimatic Change Record

Eolian loess is widely distributed on western Sichuan plateau with different thickness.A 9.5 m thick loess profile at Zhangla basin is measured for strata and palaeoclimate research.According to characteristics of strata,grain size and magnetic susceptibility and compared with Jinchuan and Ganzi loess profile,the loess sequence of Zhangla basin can be divided into four stratigraphic units: paleosol S0,loess L1,paleosol S1,loess L2 and paleosol S2,which were formed in the postglacial period,last glaciation,last interglaciation,last but one glaciation respectively.Among them,the compound paleosol S1 can be subdivided into five secondary stratigraphic units,i.e.,loess S1L1,S1L2,paleosol S1S1,S1S2 and S1S3;the compound loess L1 can be also subdivided into five secondary stratigraphic units,i.e.,loess L1L1,L1L2,paleosol L1S1,L1S2 and L1S3.Based on comparison the magnetic susceptibility curve with SPECMAP δ18O curve and integrated with characteristics of lithology and previous research result,the deposition of the Zhangla loess started at about 150 ka.Fluctuation of magnetic susceptibility value and median and coarse content of grain size of Zhangla loess were well consistent with the deposition of loess-paleosol sequence,and the climatic change can be indicated by the magnetic susceptibility of the Zhangla loess since 150 ka,in which six intensified summer monsoon circulations are shown by higher values of magnetic susceptibility,and six intensified winter monsoon circulations are indicated by the lower values.