Grain-size Characteristics of Sediments Formed Since 8600 yr B.P. in Middle Reaches of Yarlung Zangbo River in Tibet and Their Paleoenvironmental Significance

Widespread aeolian sediments have been found in the middle reaches of the Yarlung Zangbo River, China. The grain-size characteristics of sediments from Cha’er Section in the area were analyzed. The results show that the section include one stratum of paleo-mobile dunes, four strata of paleo-semi-fixed dunes, two strata of paleo-fixed dunes, one stratum of sandy immature soils. The paleo-mobile and paleo-semi-fixed dune sand in this section are simi- lar to modern aeolian sand in either grain-size composition or Mz and σ distribution. Compared the above types of dunes each other, the content of sand substance decreases, while the content of silt and clay increases for paleo-fixed dunes and sandy immature soils. Combined with age data for each stratum, the analysis shows that these strata are the products of climate changes and the evolution of aeolian landforms. The evolutionary sequence of the paleoclimate and of aeolian activities in the valley since 8600 yr B.P. reveals four stages: 8600–5700 yr B.P., when the paleoclimate was cold and dry, with strong winds, thereby activating dunes; 5700–3600 yr B.P., when it was warm and wet, with weak winds, causing dunes to undergo soil-forming processes; 3600–1900 yr B.P., when climate shifted from cold-dry with strong winds to warm-wet with weak winds, and activated dunes were fixed again; and 1900 yr B.P. –present, when the climate became fine, with weak winds, fixing dunes again.