Mineral composition and structure of the stalagmite laminae from Chulerasim cave, Indian Himalaya, and the significance for palaeoclimatic reconstruction

摘要

Many noteworthy properties of climate recorded by stalagmites can result from their mineralogy and fabric as well as their mode of occurrence. In this study, X-ray diffraction (XRD) and Scanning electron microscope (SEM) investigations were carried out for a well laminated stalagmite from Chulerasim cave, north India, to identify the mineral composition and structure of the laminae. As some early reported stalagmite laminae from Thailand and Southwestern China, the laminae of this stalagmite are composed of alternating compact and porous sub-layers. The XRD results confirm that the stalagmite is composed mainly of primary aragonite, which corrects the previous interpretation. The SEM results show that the compact sub-layer is composed of elongated columnar aragonites with a general longitudinal orientation (parallel to the vertical growth axis) and the coalescence of the aragonite crystals is well developed, leaving few inter-crystalline voids. The compact sub-layer may have formed in quasi-equilibrium conditions and provides the main carrier of climate proxies. The porous sub-layer is made up of needles, drusy and fibrous aragonites intersecting each other. Accordingly, the coalescence is low, with many inter-crystalline voids, which suggests a short hiatus between two adjacent compact sub-layers. Therefore, the growth of alternation of compact/porous sub-layers may not be successive, and they may have formed in different seasons. The results suggest that, for stalagmite/palaeoclimate research, cave monitoring should be performed to reveal when and how the compact sub-layers were formed.