The growth mechanism of the aragonitic stalagmite laminae from Yunnan Xianren Cave, SW China revealed by cave monitoring


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Stalagmites from Xianren Cave in Yunnan Province, SW China have visible bands of alternating compact and porous sub-layers, which have recently been demonstrated to be annual: the compact sub-layer consists of elongated columnar aragonites with a general longitudinal orientation (parallel to the vertical growth axis), while the porous sub-layer is composed mainly of needle aragonites forming radiating masses. In order to understand the growth mechanism of this type of stalagmite laminae, factors that could determine the growth rate and crystal structures of modern carbonates, such as cave-air temperature, CO2 concentration, drip rate and the hydrochemical parameters of drip water, have been monitored systematically. In this study, we selected five active drip sites to monitor for two years at a monthly frequency. Based on the monitoring results and the analyses of stalagmite laminae, we conclude that the compact sub-layer forms in winter, when the drip rate is moderate and the degassing is slow, continuous and prolonged. The porous sub-layer forms in early spring, when the drip rate is lower and Mg/Ca is higher than that during the formation of the compact sub-layer. We also found exceptionally some calcite laminae. Furthermore, we discovered modern calcites precipitating at all study sites in the first hydrologic year when the rainfall was very abundant. Therefore, it is suggested that the occurrence of localized calcites in an aragonitic stalagmite indicates an interval of higher rainfall.

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