2.5 ka BP以来额济纳绿洲沙丘的粒度特征及其反映的沉积过程

Grain-size Characteristics of Sand Dunes in the Ejin Oasis since 2. 5 ka BP and Related Sedimentary Process

额济纳绿洲位于我国极端干旱区,该地额济纳剖面(41°46.36'N,101°09.26'E)的地层含1层现代流动沙丘、8层古流动沙丘、18层古半固定沙丘、14层古固定沙丘、2层沙质弱成土和2层沙黄土-亚沙土。根据对该剖面11个14C年代样品测试和263个粒度样品的分析结果,本文确定了这套以沙丘沙沉积为主的沉积序列主要是2.5 ka BP以来气候一风沙地貌过程的产物,并详细讨论了自那时以来不同沉积类型的粒度及其Mz、σ和SC/D等分布规律和成因问题。这一研究结果表明:①额济纳剖面古流动沙丘沙、古半固定沙丘沙和古固定沙丘沙无论在粒度组成还是在Mz、σ分布上,都分别与现代流动沙丘沙、半固定沙丘沙和固定沙丘沙颇为相似,这为该剖面沙丘类型的划分提供了重要依据;相对这三类沙丘沙而言,沙黄土-亚沙土和沙质弱成土的沙物质含量显著降低,而粉沙和粘土含量明显增高,显示出沙丘嗣后的较强的成土作用和一定的土壤化过程。②该剖面古流动沙丘沙或古半固定沙丘沙与上覆的古固定沙丘或沙黄土-亚沙土或者是沙质弱成土可以构成11.5个沉积旋回,其每一个旋回又表现为颗粒由粗变细,Mz(Φ)、SC/D由小到大和σ显示的由分选好到变差的粒度旋回变化,代表了一个“沙漠化”时期被嗣后的“绿洲化”时期替代的正逆交替变化过程。③2.5 ka BP以来额济纳绿洲经历了11.5个旋回的“沙漠化”与“绿洲化”的生态环境变迁,在时间上具有若干百年时间尺度的环境变化,其驱动因子主要与自那时以来东亚冬夏季风交替演变有关。

The Ejin Oasis is located at an extremely arid area of China, where the Eiin section（41°46.36′N, 101°09. 26′E）named by us includes 1 layer of modern mobile sand dunes, 8 layers of paleo-mobile sand dunes, 18 layers of paleo-semi-fixed sand dunes, 14 layers of paleo-fixed sand dunes, 2 layers of sandy immature soils and 2 layers of sandy loess to loess-like sand. Based on 11 samples for ^14C dating and 263 samples for grain-size analysis, it has been confirmed in this paper that the sedimentary sequence composed mainly of dune deposition is a product of climate and geomorphological processes of wind-drifting sand. Grain-sizes of different sedimentary types and distribution regularities and geneses of Mz, a and SC/D are discussed in detail. The study shows that （1） the Paleo-mobile, paleo-fixed and paleo-semi-fixed sand dunes in the Ejin section are quite similar respectively to modern mobile, fixed and semi-fixed sand dunes in either grain-size composition or Mz and a distribution, thus it provides important evidence for the subdivision of dune types in the section. In comparison with the above three types of dunes, the content of sand substance of the sandy loess to loess-like sand and sandy immature soils markedly decreases, while the content of silt and clay obviously increases, indicating better pedogenesis and a certain soil-forming process. （2） The paleo-mobile sand dunes or paleo-semifixed sand dunes and the overlying paleo-fixed sand dunes and/or sandy loess to loess-like sand or sandy immature soils constitute 11.5 sedimentary cycles, each showing changes from coarser to finer particles, from small to large Mz（Φ）and SC/D values, and from good to poor sorting degree indicated by a, and representing a normal-reverse alternation process from “desertification” to subsequent “oasis formation”. （3） Since 2. 5 ka BP, the Eiin Oasis has undergone 11.5 cycles of ecological and environmental alteration between desertification and oasis formation, which are environmental changes with a time scale of several centuries. The driving force of such changes is chiefly related to the alternation between winter and summer monsoons in East Asia from that time on.