Grain size characteristics of aeolian sands and their implications for the aeolian dynamics of dunefields within a river valley on the southern Tibet Plateau: A case study from the Yarlung Zangbo river valley

Typical aeolian landforms have developed in China's Yarlung Zangbo River valley, which is on the Qinghai-Tibet Plateau. Fully understanding the grain size characteristics of these dunefields is the first step to reveal the region's aeolian dynamics. We selected three distinctive dunefields for grain size analysis: the Maquan, Shannan, and Mainling valley dunefields. At the scale of individual dunes, we found a trend for sands on the dune surface, with the sediments gradually becoming finer towards the crest, gradually becoming coarser, or exhibiting an asymmetric M-shaped distribution. In Maquan valley, these patterns resulted from interactions between the local airflow and local sands with a diameter of 2.0-2.5 Phi (177-250 mu m). The asymmetric M-shaped distribution in the Shannan valley was mainly controlled by the positions on a dune and valley winds. The slightly coarser crests in Mainling valley were controlled by sands that are easily entrained by the wind (2.00-3.00 Phi or 125-250 mu m) and that undergo saltation. At a dunefield scale, strong winds that entrained medium sands cause these sands to accumulate downwind in Maquan valley. Multi-directional winds combined with a limited sand supply (1.74-2.30 Phi or 203-299 mu m) favored a stable aeolian environment in Shannan valley. The saltating sands produced by these winds build dunes under a constantly erosive wind regime in Mainling valley. Wind transported interdune sediments is an important local source for building dunes. The gradual variation of grain sizes moving downstream and the distinct evolution history of dunefield in Maquan valley as compared to Shannan and Mainling valleys may suggest the existence of a spatial connection among dune sands along the main stream of the Yarlung Zangbo River.