Quartz OSL Dating Of Loess Sequences From The Western Sichuan Plateau

Loess is widely distributed in the western Sichuan Plateau,which is one of the most direct and sensitive records of the climate changes and the environmental evolution of the Tibetan Plateau?TP?.Chronology,as the prerequisite for the analysis of geo-environmental changes,has important significance for understanding the sedimentary process of loess and exploring the environmental evolution of the region.Based on the thorough field investigation of the western Sichuan Plateau,three typical loess sections from Ganzi,Jinchuan and Xiaojin were selected.We analyzed the characteristics of quartz optically stimulated luminescence?OSL?dating of loess deposits in this region,including condition experiments,internal detection,growth curves,decay curves,distribution of equivalent dose and dispersion analysis of natural luminescence signal.Combined with AMS ¹⁴C dating,we builded the age-depth model for each section and reconstructed the histories of the dust accumulation rates and mass accumulation rates in the eastern TP.The main conclusions can be drawn as follows:?1?The quartz minerals in loess samples from the western Sichuan Plateau have obvious preheat plateau between 240?and 280?,and the dose recovery results are satisfactory.The luminescence signal and sensitivity of quartz are both good.The recycling ratios of all samples are between 0.9 and 1.1 and the recuperation of all samples are below 5%.According to our research,for some old samples,the growth curves are close saturated,showing the upper limit of the dating.The quartz luminescence signal of the loess in this region is close to saturation at about 180²60Gy,which is similar to that of loess in the Chinese Loess Plateau?CLP?.?2?The combination of single aliquot regenerative method and the standardised growth curve?SAR-SGC?method can be used to date effectively the ages of loess in the western Sichuan Plateau.The OSL ages of the XS section,the MC section and the XJ section are 14.8±1.1⁷0.2±5.2 ka,23.8±2.0⁷9.6±6.5 ka and 0.2±0.0⁶1.8±4.8 ka respectively.For some old samples,since the slope of the growth curves are larger at the low dose part and smaller at the high dose part,the value of Ln/Tn is slightly smaller?or larger?,which will lead to the abnormal decrease?or increase?of the equivalent dose value obtained by interpolation with large errors.Attentions should be paid when the SAR-SGC method is used for those old samples.?3?The reconstructed chronological sequences by quartz OSL dating show that,the loess in the western Sichuan Plateau records the environmental changes in the eastern TP since the Last Glacial.The soil bulk density in this area can be used as a good indicator for the westerly and the plateau winter monsoon.Dust accumulation rates and mass accumulation rates have gradually decreased,plus with some frequent smaller scale climate fluctuations since the Last Glacial.During the MIS4 stage,the current of the westerly and the plateau winter monsoon were strong,with low temperature,less precipitations,and more dust deposition.During the MIS3 stage,the dust deposition decreased and the Indian monsoon were strong,with warm-humid condition.During the MIS2 stage,the climate was cold-dry,with less dust deposition and the strengthened westerly and the plateau winter monsoon.The loess deposition processes between the western Sichuan Plateau and the CLP and Central Asia are different.Further detailed work on loess in the western Sichuan Plateau are needed for better understanding the history and mechanism of the environment changes in the eastern TP.