Late-Quaternary dynamics and palaeoclimatic implications of an alluvial fan-lake system on the southern Alxa Plateau, NW China

Alluvial fan-lake complexes are common in the piedmont zone of mountains on the southern Alxa Plateau, NW China. Interconnected by the erosional and depositional processes, they can be treated as a typical source-to-sink system. Yet the dynamics of these coupled sedimentary systems during the Late Quaternary is poorly understood. For example, radiocarbon dating of alluvial fans and elevated shoreline features of megalakes in this area usually yields numerical ages in radical contradiction to those obtained using the optically stimulated luminescence (OSL) method. Resolving this controversy of timing of the alluvial fan-lake systems is important for understanding the driving mechanisms of climatic and environmental changes in this ecological vulnerable area. The Yabulai alluvial fan-lake system is an exceptional example for tackling this issue. Here we provide numerical ages from three sites to better understand the dynamics of this sedimentary system during the last interglacial-glacial cycle. Our results, when compared with palaeoclimate proxy records, suggest that the fan-lake system was highly regulated by the East Asian summer monsoon and it has experienced four stages of remarkable changes. Debris flow processes were active and lake level was similar to 30 m higher than that of today during Marine Isotope Stage (MIS) 5. A gradual decrease in the aggradation rate of the alluvial fan along with lake-level fall occurred during MIS 4-3, which was followed by a period of intensive wind erosion corresponding to the Last Glacial Maximum. Postglacial growth of the alluvial fan slowed down substantially and small-scale lake-level fluctuations occurred in the foreland basin. The desiccation of megalakes during recent times suggests that the East Asian summer monsoon may have retreated from this area as semi-arid climate emerged. (C) 2018 Elsevier B.V. All rights reserved.