Physicochemical properties of surface sediments in the Taklimakan desert, northwestern China, and their relationship with oasis–desert evolution

Abstract

Oasis and desert are the dominant landscapes in arid and semi-arid regions, especially in northwestern China where the oasis–desert evolution mechanisms are important for understanding modern surface processes and future environmental evolution of the region. Nevertheless, the response of the physicochemical indicators of surface sediments to oasis-desert evolution is poorly studied and it is unclear how reliable these indicators are for paleoenvironmental reconstruction. A spatio-temporal approach consisting of measurements of surface-sediment grain-size, nutrient contents and elemental ratios was used to characterize a series of modern stages of oasis-desert evolution in the Taklimakan desert. The applicability of the indicators to two sequences of alternating fluviolacustrine-wetland and aeolian sand deposits in the same region was evaluated. The results show that the oasis sediments are characterized by high contents of fine-grained particle (End-Member 1, EM1), TOC (Total organic Carbon), TN (Total Nitrogen), and MgO/SiO2, and low values of the eluvial coefficient (SiO2/(MgO + CaO + K2O + Na2O)). By contrast, the desert sediments are characterized by high values of coarse-grained particle (End-Member 3, EM3), low soil nutrients and MgO/SiO2, and an increased eluvial coefficient. In the two sedimentary sections, the physicochemical properties of the clay-silt layers resemble those of modern oasis sediments, especially washland, and the properties of the interbedded aeolian sand resemble those of modern desert deposits. The observed co-variation of grain size, soil nutrients and geochemical ratios of the surface samples shows that our approach can be used to determine the trends of past oasis-desert evolution. The reconstruction suggests that oasis formation largely depended on the development of a fluviolacustrine-wetland environment, whereas oasis degradation was closely related to strong aeolian activity. The findings provide new insights into the selection of appropriate indicators for analyzing oasis-desert sediments and for reconstructing the evolution of oasis-desert landscapes.