10Be Dated Bouldersfromthe Third Terraceof Nujiang Riverat Bingzhongluo, Yunnan Province, China
详细信息
- 责任者：Lü Yanwu(Key Laboratory of Cenozoic Geology and Environment ; Institute of Geology and Geophysics ; Chinese Academy of Sciences ; Beijing 100029) ; Gu Zhaoyan
- 刊名：Quaternary Sciences
- 出版年：2012
- 卷期：32(3)
- 页码：p.403-408
- issnNo：ISSN1001-7410
- isbnNo：CN11-2708/P
摘要
Fluvial terraces are common in the Nujiang River valley,while the third level is widest and mostly distributed.There are five level terraces at Bingzhongluo(28°01’N,98°37’E),where located southeastern margin of the Tibetan Plateau,Yunnan,China.The third and the forth level terraces are covered with reddish-brown soil,indicating a long history of the terraces.Many granite boulders are spread out on the surface of the first,second and third level terraces,whose exposure ages could represent the terrace formation time.Three granite boulders were taken as samples at the posterior margin of the third terrace of the Nujiang River at Bingzhongluo(Fig.1a and 1c).The diameters of all boulders are large( >2m)and partly buried in soil layers.Sample NJ2-1 was collected from one granite boulder(about 2.3m)on floodplain,which is used to estimate the inheritance.Sample BZL43 was chiseled about 1.5cm from the surface of a quartz vein,which insets a sandy slate of the third terrace at downstream about 2km.
The quartz purification, 10Be extraction,and BeO preparation,were done in the Cosmogenic Nuclide Laboratory at the Institute of Geology and Geophysics,Chinese Academy of Sciences.The 10Be/9Be ratio was measured by AMS facility at the Tandem Laboratory of Uppsala University in Sweden.The 10Be concentrations(Table 1)show that the 10Be concentration of sample BZL43 is the highest((1.742±0.043)×106 atoms/g)of the four samples from the third terrace,the average content of other three samples(NJ2-8,NJ2-14 and NJ2-15)from granite boulder is about 1.54×106 atoms/g,and sample NJ2-1 from the floodplain is very low,only (1.6±0.7)×104 atoms/g.Weathering processes have a greater impact on the exposure ages of boulder samples according to the landscape,environmental features and 10Be concentration.Assuming the weathering rate of sample BZL43 from the quartz vein as 0.05cm/ka,the weathering rate of granite boulders is represented graphically as 0.3cm/ka by the relationship among effective exposure age,exposure time and weathering rate(Fig.2).The exposure ages of the three granite boulders on the third terrace were obtained after the correction for weathering and ignoring inheritance.Combined with the corrected exposure age of sample BZL43,it is considered that the third terrace of Nujiang River at Bingzhongluo was developed during 150~203ka.
The quartz purification, 10Be extraction,and BeO preparation,were done in the Cosmogenic Nuclide Laboratory at the Institute of Geology and Geophysics,Chinese Academy of Sciences.The 10Be/9Be ratio was measured by AMS facility at the Tandem Laboratory of Uppsala University in Sweden.The 10Be concentrations(Table 1)show that the 10Be concentration of sample BZL43 is the highest((1.742±0.043)×106 atoms/g)of the four samples from the third terrace,the average content of other three samples(NJ2-8,NJ2-14 and NJ2-15)from granite boulder is about 1.54×106 atoms/g,and sample NJ2-1 from the floodplain is very low,only (1.6±0.7)×104 atoms/g.Weathering processes have a greater impact on the exposure ages of boulder samples according to the landscape,environmental features and 10Be concentration.Assuming the weathering rate of sample BZL43 from the quartz vein as 0.05cm/ka,the weathering rate of granite boulders is represented graphically as 0.3cm/ka by the relationship among effective exposure age,exposure time and weathering rate(Fig.2).The exposure ages of the three granite boulders on the third terrace were obtained after the correction for weathering and ignoring inheritance.Combined with the corrected exposure age of sample BZL43,it is considered that the third terrace of Nujiang River at Bingzhongluo was developed during 150~203ka.