Investigation into rock moisture and salinity regimes: implications of sandstone weathering in Yungang Grottoes, China

摘要

Yungang Grottoes in north of China are the classical masterpieces of Chinese Buddhists, dating from the Northern Wei Dynasty 1,500 years ago. This rock art is threatened by weathering processes that has been accelerated due to coal mining. The dominant weathering processes occurring on the rock surface are collapse, flaking, and efflorescence. Salt crystal growth plays a major cause of these deteriorations, and it has a close relationship with water movement. The major salt on the rock surface are sulfates, with a primary composition of epsomite, thenardite, and gypsum. In addition, carbonate minerals are found, mostly calcite and nesquehonite. In this paper, three grottoes in the eastern part have been investigated via measuring the rock surface moisture and recording the surface temperature and humidity. These nondestructive techniques are used for the first time in China on cultural relics. The movement of moisture has been calculated and mapped, and then analyzed for its relationship with the weathering process. Three main sources of water transport in the sandstone are adopted to explain the moisture content patterns. The aim of this study is to help rock art conservation in Yungang Grottoes, thus to provide a better understanding regarding the driving processes of internal moisture transport, which results in significant weathering processes. The evidence shows that the natural character of sandstone, water transport mechanism, and continental semi-arid conditions are the major causes of rock deterioration. Meanwhile, air pollution due to coal mining is also an important factor in driving the weathering process.