Garze Loess Strata And Its Implication To Paleoclimatic Change In The Western Sichuan

The loess in Garze region is so far one of the relative old and successively deposited aeolian successions in the Western Sichuan.It is of important significance to probe into the evolutional mode of atmospheric circulation over east Western Sichuan Plateau and its relationship with climate change of surrounding regions.The dissertation takes Lama Temple loess section near Garze County as an object to discuss the age,the genesis and paleoclimatic implication of the Western Sichuan loess.The loess section is in unconformable contact with base rock below.We can divide the loess section with 15.3m thick into 13 loess layers and 11 paleosol layers according to the characteristics of loess lithology, structure,color,contact relationship etc.Result of Palaeomagnetic stratigraphy research indicates that B/M boundary is at the top of eighth paleosol layer and Jaramillo normal event of Matuyama reversed epoch occurs in between 12.7～14m of the section.By integration of OSL dating and palaeomagnetic method,the age of Garze loess is identified as 1.15Ma,which is so far the oldest loess section in Western Sichuan Plateau.The characteristics of grain size parameters of Garze loess-paleosol section suggest that it is aeolian genesis.According to the climatic indices characteristics of its grain size,magnetic susceptibility and geochemistry,the paleoclimate change process in this region can be divided into four stages as follow:(Ⅰ) 1.15～0.91MaB.P.:in which medium grain size fluctuate little,magnetic susceptibility values are totally low with some degrees of fluctuation,and ratio values of oxides indicate a relative weak weathering intensity,suggesting a comparatively constant arid-cold climatic environment.(Ⅱ) 0.91～0.5MaB.P:grain size fluctuates little with increasing tendency and relatively intensive fluctuation of magnetic susceptibility,and a relative weak weathering intensity.These suggest a period of relatively strong winter monsoon accompanied by increasing summer monsoon.(Ⅲ) 0.5～0.25Ma.B.P:characterized by totally fine grain,increasing fluctuation of grain size and tendency of magnetic susceptibility concentrated in this period and weathering intensity indicated by ratio values of oxides,the winter monsoon in this stage is comparatively weak and summer monsoon is increasing,which is relatively mild and moist.(Ⅳ) 0.25MaB.P.to present:the loess grain size rapidly become coarse with intensively fluctuation,the magnetic susceptibility totally presents as low values and ratio values of oxides indicate a increased weathering intensity.These suggest a relative arid-cold climate and a rapidly intensified winter monsoon.