CHARACTERISTICS AND FORMATION MECHANISM OF STALAGMITE LAMINAE IN E’MEI CAVE, JIANGXI PROVINCE, CHINA

The fast-growing stalagmite with annual laminae is one of the important geological carriers for the study of past seasonal-interannual climate changes. In this paper, the laminae analysis and high-resolution oxygen and carbon isotope analysis of a modern stalagmite EM1 (ca.60 mm length) in E’mei cave(29°33'18?N, 115°29'44?E; 53 m a.s.l.), Jiangxi Province were carried out, and the characteristics of the stalagmite laminae and its formation mechanism were studied. Uranium series dating and stable isotope analysis were carried out, and mineral and laminar features were observed and photographed under transmitted light and UV light using Zeiss polarizing microscope at Isotope laboratory, Xi’an Jiaotong University, respectively. Scanning and laminar counts of stalagmite EM1 were performed using confocal laser fluorescence microscopy(CLFM) at state key laboratory for manufacturing system engineering, Xi’an Jiaotong University. The 230Th dates, laminar counts and d18O circles of EM1 are consistent within the error, which confirms that the laminae is an annual layer, with a grow period of 1810~2009 A.D., a total of 200 years. Studies have shown that the E’mei cave stalagmite laminae has the typical characteristics of “southern China type” stalagmite laminae: a dark dense sublayer visible to the naked eye, corresponding to a transparent sublayer under transmitted light and a non-luminescent sublayer under fluorescence; a milky white loose sublayer visible to the naked eye, corresponding to an opaque sublayer under transmitted light and a luminescent sublayer under fluorescent light. The calcite δ¹⁸O value of the dark dense sublayer changed from the minimum to the maximum during the year, while the δ¹³C value was a positive peak value during the year. The calcite δ¹⁸O value in the milky white loose sublayer changed from the maximum to the minimum during the year, while the δ¹³C value was at a negative peak during the year. Monthly-seasonal resolution δ¹⁸O and δ¹³C records were inversely correlated, with δ¹⁸O lagging behind δ¹³C changes by about 3~4 months. The research suggests that the formation of the stalagmite laminae in E’mei cave may be mainly related to the configuration of more precipitation and organic matter leaching(forming a milky white loose sublayer) in spring and early summer, and high temperature and drought in midsummer(forming a dark dense sublayer). It was also found that some milky white loose sublayers also included several extremely thin fluorescent sublayers, which was confirmed to be not annual laminae, and its cause was probably related to several times of organic matter pulse deposition caused by heavy rainfall events in spring and early summer.