Nebkha formation: Implications for reconstructing environmental changes over the past several centuries in the Ala Shan Plateau, China

Sediments of Chinese tamarisk (Tamarix chinensis Lour.) nebkhas in Chinaʹs arid Ala Shan Plateau document both the formation processes of these vegetated dunes and the environmental changes and geomorphological evolution in this region. Dating of the nebkhas showed that after surface water disappeared, the wadis and playas evolved into gobi deserts (desert pavements) within two centuries. The δ13C trends in the litter deposited within the nebkhas revealed three stages of fluctuating moisture conditions during the past five centuries: before the 1580s, between the 1580s and the late 1990s, and from the early 2000s to the present. However, changes in the coarse fraction of the nebkha sediments show that this region experienced at least five phases of aeolian sediment transport during the same period. The early stages of nebkha formation coincided with the initial stage, when the wadis and playas evolved into gobi deserts and nebkha sizes increased rapidly due to the abundant sediment supply provided by the playas. However, after the playa surfaces stabilized, the nebkha sediments documented environmental changes in adjacent regions. Our results also show that the formation of the Tamarix nebkhas in this region was not controlled by the groundwater table, because in most periods, the water table depth remained above the depth that limits Tamarix survival. In the Ala Shan Plateau, nebkha formation was mainly controlled by aeolian processes, and revealed the integrated effects of environmental changes, thereby providing possibilities for climate and environment reconstructions with a relatively high resolution in extremely arid areas.