Unmixed loess grain size populations along the northern Qilian Shan (China): Relationships between geomorphologic, sedimentologic and climatic controls

Grain size analysis provides a useful tool in reconstructing aeolian transport processes and allows deductions about environmental evolution. However, the sedimentological mechanisms driving the investigated imprint are often underrepresented in the interpretation of loess sections and may therefore lead to erroneous conclusions. We present a study from the north-eastern margin of the Tibetan Plateau (Qilian Shan/Hexi Corridor) in which 11 mainly Holocene loess sections (up to 2.20 m depth) and a set of surface samples were analysed for their grain size distribution. A three end-member mixing model yields grain size populations with modes of 63 μm, 33 μm, and 9 μm, respectively, representing the major contributing transport processes from different sources. These processes were identified in short-term suspension and saltation from fluvial storages, low-level suspension driven by dust storms and high level suspension from distant sources. Sedimentology (supply, availability), site-specific geomorphology (distance to potential sources) and climate conditions, such as effective moisture (controlling dust-trapping vegetation covers), set up a complex system. Loess deposition and preservation depend on these steering factors. Large sediment supply from the last glacial cycle became available for deflation during early Holocene moisture optimum. This phase is represented by a high contribution of the coarsest grain size population in the late Glacial/early Holocene loess record when climate amelioration allowed the establishment of a dust-trapping vegetation cover (comparable to MIS-3 conditions). The mid-Holocene loess deposits formed under more stable landscape conditions, represented by much lower contributions from local sources. Accordingly, relative contribution of far-travelled dust was at its climax during the mid-Holocene. The late Holocene exhibits a relative increase in dust storm contribution which is in accordance to increasingly drier climatic conditions. Hence, this study suggests a thorough consideration of the different system components when interpreting grain size records obtained from loess deposits.