Methodological Study On Measurement Of Exposure Age Of Tiankeng By ³⁶CL-AMS Technique

The south-west of China is one of the most widely distributed and types of karst in the world.Tiankeng is a special geomorphology in karst area,and the exposure age of Tiankeng is of great significance to the study of Quaternary.The formation condition,biological population and the development process of the Tiankeng have been studied however are still very different between the different data.The accumulation of in situ-produced cosmogenic nuclides in the surface rocks is related to the exposure time and the rate of erosion of rock samples on the surface,therefore,long-lived radionuclide can be used to determine the age of exposure and erosion rates over thousands to millions of years.The composition of CaCO₃ in rock in the karst area reaches more than 90%and Ca is the main target element of the long life radionuclide produced by cosmic rays,³⁶Cl can be rapidly produced in rock and accumulated as an ideal nuclide for determining the age of limestone exposure and erosion rate.In order to get the rate of erosion and the exposure age of the Tiankeng in the area around the Tiankeng,the samplings of the area around the doline and the crater wall are required.The long-term exposure rock samples used to determine the rate of erosion were choiced according its geological information and field surveys.In this paper,the erosion rate and exposure age of the doline are calculated by using the mathematical model.The work is shown as follows:1.Collection of Tiankeng samples.After the approval of the local authorities,samples were collected from the Leye Daishiwei Tiankeng Group in Guangxi province?include Daishiwei,Huangjing,Chuandong,Datuo,Dacao?and Xiaozhai Tiankeng in Fengjie,Chongqin.Three types of samples were collected:doline foundation sample,the Tiankeng rock wall sample and the doline erosion rate sample.The sample collection of the Tiankeng in Fengjie Chongqing was commissioned by professional staff of the Beijing Seismological Bureau.2.Separation of the doline samples and preparation of Chemical Samples.Samples were firstly performed ICP analysis for the Chemical Element in the rock samples.Then the rock samples were pulverized and sieved to of 2³ mm size for AMS preparation.AgNO₃ solution was added after the samples dissolved with HNO₃,to precipitate AgCl,and then the sample were added Ba?NO₃?₂ to remove Sulphur as BaSO₄.3.AMS measurements and exposure age calculations.The AMS measurement is performed basing on the HI-13 serial accelerator at the Beijing Tandem Accelerator Laboratory and the Beijing Q3D Spectrometer on the R20 pipeline.Considering the latitude and altitude of the area where the Tiankeng is located,the erosion rate in the area of Dashiwei Tiankeng group is determined to be 1.7±0.1*10^-5m/a,the erosion rate in the area of Xiaozhai Tiankeng is 1.1±0.1*10^-5m/a.The exposure age of Tiankeng was calculated by considering the yield of ³⁶Cl in the Tiankeng sample to be at least60³80 thousands years.The innovation of this work is shown as follow:The in situ climatic ³⁶Cl was used to study the exposure age of Guangxi Tiankeng.In order to make the age of the Tiankeng more accurate,fully consider the factors such as elevation,latitude and azimuth of the Tiankeng and use the existing model to correct the ³⁶Cl production rate of the Tiankeng rock wall sample,and then obtain the accurate ³⁶Cl production rate of the sample.Second,the chemical element concentrations of Tiankeng samples were measured by inductively coupled plasma?ICP?spectrometry in aliquots of the AMS sample solutions.Third,the?E-Q3D magnetic identification system was used to enable the improvent of the end voltage of the accelerator,the beam stability and efficiency of the ion source,the identification ability,and the measuring efficiency of ³⁶Cl-AMS measurement.