The formation and development of nebkhas based on chronology and sedimentology in the Ordos Plateau, northern China

The Ordos Plateau lies on the northwest margin of the East Asian monsoon region as well as the farming-pastoral ecotone in northern China, with a wide distribution of nebkhas. The formation and development of nebkhas in this region are closely related to natural envi ronmental conditions and human activities. However, the processes of nebkhas formation and development under natural and anthropogenic influence still remain unclear. In this study, four typical nebkhas in the Ordos Plateau were selected after detailed field investigations. Chronology and sedimentary features of the formation and development of nebkhas were studied based on optically stimulated luminescence (OSL) dating, lithology, grain size, etc. The results demonstrated that modern nebkhas had formed since at least ~0.59 ka in the southwest of the Ordos Plateau, followed by the middle region, at least ~0.34 ka, and later in the south and north regions, at least ~0.10 ka. There were thin layers of weakly-developed paleosols at ~0.32-0.25 ka, peaks in the silt and fine sand content and lower deposition rates, about 0.37-0.46 cm/a, indicating a relatively humid climate and weak aeolian activities. After ~0.10 ka, aeolian activities intensified and the nebkhas widely developed with a higher deposition rate, ~0.45-5.21 cm/a. Nebkhas in the study region developed primarily over paleo-channels or paleosol layers. Very fine sand and fine sand were dominant composition on grain size of nebkha sediments; saltation was a main means for the particle movements, indicating near-source accumulation for nebkha sediments. In recent decades, local farmers are used to adding nebkhas deposits to the soil of irrigation areas to improve the soil quality and alleviate soil salinization. Such agricultural activities, together with land reclamation, have accelerated the demise of the nebkhas in the Ordos Plateau.