Developmental scale and wind direction influence particle size characteristics of surface sediments over barchan dunes at southeastern edge of the Taklimakan Desert

Particle size characteristics of surface sediments over Barchan dunes are the result of interactions of dune morphology, regional airflow and sand flux．These are also key parameters to understand dune morphology and dynamic formation．Particle size parameters of surface sediments over dunes were analyzed on three developing scales at southeastern edge of the Taklimakan Desert．Their changes with two opposite wind directions were also investigated．Gravel and very coarse sand were found to be the main components of surface sediments over dunes, accounting for 79.09% of total．Probability cumulative curves of sands were mostly composed of creep component, to a lesser extent of saltation component．Dune morphology and surface microgeomorphology resulted in three modes for grain size characteristics of sediments in different longitudinal axes over dune．First, sediments at the windward slope toe were the coarsest and gradually became finer along windward slope through crest/brink to leeward slope toe, while these in crest/brink had the best sorting and the sorting gradually became worse from the crest/brink to windward slope toe and lee slope toe．Second, sediments at the upper or lower windward slope were the coarsest and gradually became finer along windward slope to the toe and through crest/brink to lee slope toe, while the sorting on the crest/brink was better than other locations and gradually became worse along windward/lee slope to the toe．Third, grain size distributions were similar to the second, while the windward slope toe was the location with best sorting and the sorting gradually became worse along windward slope to the lee slope toe．With the increase in dune height, the sorting of sediments over dune surface became better, while their particle sizes took on different characteristics．Their particle sizes initially became finer and then coarser along windward and leeward slopes, while became coarser at crest/brink．Difference in particle sizes at two horns increased, short horn with the smaller particle size and better sorting converted to long one．Compared with action of airflow with main wind direction (NE), sediments along the windward slope and at the toe of lee slope became finer under action of airflow with wind direction of SW, while the former sorting became worse and the latter had no obvious change．The sediments at the crest became coarser and their sorting became better．The content of fine sand, very fine sand and silty sand along the windward slope and at the toe of lee slope increased, while the content of gravel and very coarse sand decreased．Changes in sediment composition at the crest were opposite to windward slope．