Optical Luminescence Dating Of The Loess Profile In Zhanggou, Yangxian County, Hanzhong Basin And Its Significance

Quaternary chronology is a quaternary dating science based on quaternary environmental research.Its main purpose is to determine the occurrence time of geological historical events by dating quaternary sediments by various dating techniques.The study of quaternary chronology is helpful to understand the evolution mechanism of the earth's surface system,provide favorable conditions for accurate interpretation of the complex and changeable ancient environment of the earth,and indicate the future development trend of the earth's surface system.Optical dating technique(OSL)is an absolute dating technique developed in recent 20 years.Because of its mature theoretical basis and simple process,OSL has been widely used in the dating of quaternary sediments and has been widely recognized by geologists.OSL has become the most commonly used technique in quaternary dating.Hanjiang river basin is located in the south of Qinling mountains,which belongs to the marginal area of north subtropical monsoon climate.The natural and geographical environment of the two sides is very different,and the response to environmental change is very sensitive.Influenced by the uplift of Qinghai-Tibet plateau,through the evolution of long geological history,multi-stage river terraces are widely developed in the south of Qinling mountains,and a large number of thick aeolian loess are widely deposited.In recent years,with the comprehensive development of soil and water resources projects in China,many problems in the Hanjiang river basin have been paid more and more attention by many scholars,and the photoemission dating technology provides reliable date for these studies.In the study of the age of the Hanjiang river basin,there is no lack of study on the stratigraphic age of the loess section of the first-order overlying the Hanjiang river,but the study on the formation age of the first-order terrace itself is relatively small and there are great differences.The discussion of this problem is helpful for other scholars to understand the quaternary geomorphological and geological evolution of Hanjiang river basin,and provide theoretical data and scientific basis for this.Based on this,this paper uses OSL dating technique to date the yangxian Zhanggou(YXZG)profile in Hanzhong basin,which is the first terrace of Hanjiang river.The main conclusions are as follows:(1)According to the experiment,it is considered that the best experimental temperature of the light-release sample in Zhanggou section is 240?280?,and the equivalent dose value appears a stable plateau in this range.When the experimental temperature is between 200 and 280?,the cycle ratio is 0.96 to 1.01;The recovery coefficient is between 0.9 and 1.1 when the experimental temperature is between 200 and 260?;When the experimental temperature is 240?260?,the equivalent dose due to heat transfer is 0.87?1.04Gy.Accordingly,the optimum experimental temperature was finally set at 260 ?.(2)The experimental analysis shows that OSL samples in Zhanggou section have undergone good sun-retrogradation,which is suitable for photolysis study.The curve of drying leg,growth curve,dispersion of equivalent dose and histogram of frequency distribution of equivalent dose of OSL samples showed that all samples were quickly dried back to the background value within 0.8s,the standard variance RSDn-osl was very close to RSDR.OsL,and the frequency histogram of equivalent dose showed normal distribution.(3)OSL age distribution of Zhanggou profile corresponds well to stratigraphic sequence,and the age satisfies the new sedimentary law of lower-old and upper-old.OSL samples ranged in age from 2.23±0.11 to 54.40±3.86ka,and the experimental error ranged from 4.9%to 7.1%.After correction,the complete stratigraphic sequence of Zhanggou profile since the end of late Pleistocene is determined:the depth of profile 126-1300cm is Malan loess layer(L1),the age is 53?11.5ka B.P.,and the age of weak paleosol layer(L1-S1)sandwiched by 700-1200cm loess is 45?33 ka B.P.;The depth of profile 60?110cm is paleosol(So)and the age is 8.5?3ka B.P..(4)On the basis of the systematic optical dating data,the formation age of the first terrace of Hanjiang river is determined to be about 55ka by research and analysis.(5)Based on the element content index and OSL age of Zhanggou profile,the climate change process of the first terrace of Hanjiang river since its formation is revealed as follows:53?45ka B.P.(cold dry)?45?33ka B.P.(cold wet)?33?11.5ka B.P.(cold dry)11.5?8.5ka B.P.(cold dry)?8.5?3ka B.P.(warm wet)? 3?Oka B.P.(cold dry).