Quaternary profiles and river chemistry reveal regional-scale weathering mechanisms of conglomerate heritage sites in the Danghe watershed, NW China

Abstract Numerous studies on the weathering of rock heritages mainly focus on the surface weathering layer or localized weathering features (such as efflorescence and cracking) in individual heritages, without considering the temporal and spatial scale effects of mineral weathering itself. Therefore, existing studies have had difficulty explaining the weathering patterns of rock heritages at a large regional scale (such as at a watershed scale). To overcome previous limitations, this study analyzed water chemistry, mechanical, mineralogical, and geochemical properties of Quaternary conglomerate/sediment profiles along the flow path, elevation gradient (~1300–3500 m above sea level), and climate gradient (extremely arid, arid, and semi-arid regions) in the Danghe watershed (where four key conglomerate Buddhist grottoes are preserved) of northwestern China. Results demonstrated the following points: (1) From upstream to downstream with increasing evaporation and temperature, the hydrochemical type of river changes from HCO3-Ca·Mg type with low total dissolved solids to HCO3-Na·Ca·Mg and Cl·SO4-Na·Mg types with high salinity. (2) Rock weathering (silicate weathering and carbonate weathering) is the main controlling factor of water chemistry in the watershed, with evaporation-crystallization processes and ion exchange as secondary influences. (3) During weathering, mineral abundances of calcite and clay minerals in conglomerates, particularly within the cements, decrease in midstream and downstream areas where grottoes are preserved, leading to reduced mechanical strengths of cliffs. Dolomite weathers faster in the upstream than in the downstream, and a higher chemical depletion fraction was observed at upstream than at downstream, possibly due to a wetter climate and/or a higher uplift rate in the upstream. (4) Compared to chemical weathering, physical weathering processes such as salt weathering and wind erosion contribute more to cement loss and porosity generation in the midstream and downstream conglomerate grottoes, with the highest loss of cementing materials and salt weathering potential observed at the Mogao Grottoes. The study revealed possible links and discrepancies between site-specific weathering profiles and catchment-scale river chemistry in the arid areas that may have significance in predictive modeling of rock heritage deterioration and development of targeted conservation strategies under future environmental changes.