Methodological study on exposure date of Tiankeng by AMS measurement of in situ produced cosmogenic 36Cl

Tiankeng is a typical Karst relief of the late Quaternary Period. Studies on the exposure ages of Tiankeng are very important in geographical research to elucidate the formation condition, the developing process, and the features of biological species. 36Cl on the surface layer of the rupture cross-section of Tiankeng is largely produced by cosmogenic high-energy neutron induced reactions 40Ca(n, αp) and 39K(n, α), and has accumulated since the formation of the Tiankeng. Low-energy neutron reaction 35Cl(n, γ) contributes a small portion of 36Cl. In this work, the concentration of the cosmogenic 36Cl in rock samples taken from Dashiwei Tiankeng, Leye County, Guangxi Zhuang Autonomous Region, China, was measured jointly by Accelerator Mass Spectrometry (AMS) laboratories of CIAE and University of Tsukuba in an effort to estimate the formation time (or exposure age) of the Tiankeng. The results show that the exposure time of Da Shiwei Tiankeng is about 26±9.6ka (without erosion correction). The sampling strategy and procedures, experimental set-up, and preliminary results will be presented in detail.