Elemental distribution in the topsoil of the Lake Qinghai catchment, NE Tibetan Plateau, and the implications for weathering in semi-arid areas

An understanding of topsoil geochemistry within a watershed helps to identify the influence of natural processes and anthropogenic activities in the environment. Fifteen elements were measured in 73 topsoil samples (top 30cm) from the Lake Qinghai catchment, NE Qinghai-Tibetan Plateau (QTP) in 2009, to investigate geochemical patterns, identify possible sources and explore the implications of chemical weathering. Compared with their content in the upper continental crust (UCC), a deficit of Cu and Zn and an enrichment of Pb were identified in the study area. Enriched Ca levels were observed in alpine soil and pedocal, solonchak and aeolian sand soils, whereas there was a loss in hydromorphic and semi-hydromorphic soils. High Na in marsh solonchak indicated the possible presence of evaporites. Cluster analysis (CA) and principal component analysis (PCA) indicated the dominance of carbonate and the accumulation of P, possibly from grassland litterfall in topsoil. Some human activities have exerted an influence on virgin topsoil, as revealed by GIS mapping. The implications for chemical weathering, based on a chemical index of alteration (CIA) and an Al₂O₃-(CaO*+Na₂O)-K₂O (A-CN-K) diagram were characterised by a major carbonate control of the primary weathering stage under cold-dry climatic conditions with Ca and/or Na removal.