毛里塔尼亚努瓦克肖特沙丘粒度分布特征及其环境意义

Characteristics of Sand Granularity and Its Environmental Significance in Nouakchott, Mauritania

毛里塔尼亚努瓦克肖特地区位于撒哈拉沙漠西部,基本被沙漠包围且常年遭受沙漠侵蚀,进行该地区沙漠粒度特征的研究,将对判别沙漠物质来源和沉积过程等具有重要参考意义。对努瓦克肖特周边纵向沙丘不同部位采集的50个沙样粒度进行分析。结果表明:沙丘沉积物的粒度以细沙和极细沙为主,两者所占比例超过60%;总体粒度要细于国内典型沙漠线性沙丘。沙丘东南坡、丘顶和西北坡整体上以细沙为主,其次为极细沙,二者含量依次递减,粉沙含量依次增加。沙丘沙粒径最细者为西北坡(5.13Φ),最粗者为东南坡(3.07Φ),分选性随粒径增加而变好。3个部位峰态存在明显差异,东南坡以中等和尖窄峰态为主,丘顶以尖窄峰态为主,西北坡为以宽平峰态为主,但三者均以正偏偏态为主。丘间地粒级级配趋势一致性最好,均以细沙为主,平均粒径大于沙丘各部位,平均粒径为2.76Φ,分选性优于沙丘,全为近对称中等峰态。该地区地表沙粒沉积环境和成因复杂,超过70%的沙样为风成沉积造成,其余为沿岸沉积造成。

The Nouakchott, Mauritania is located in the west of the world＇s largest tropical Sahara Desert. The local ecology is seriously threated by desertification. Studying the grain size characteristics in this area is important because it plays a key role in distinguishing the sedimentary environment and the source of the dust. In present study, 50 sediment samples from different positions of longitudinal dunes were collected and analyzed by means of sieving. The longitudinal dunes can be divided into three parts including southeast slope, top, and northwest slope, which are mainly composed of fine sand and very fine sand which together account for more than 64 % in total sand. It is finer than sands in the longitudinal dunes in China. Sediment form southeast slope with highest content of fine sand and very fine sand, and with lowest content of silty sand, while sediment form northwest slope with lowest content of fine sand and very fine sand, but with highest content of silty sand. The grain size parameters show that the finest and the coarsest grains can be found on the northwest slope and southeast slope, respectively. In addition, kurtosis of southeast slope, top, and northwest slope represent leptokurtic or Moreover, all the three parts exhibit a positively mesokurtic, leptokurtic, and platykurtic, respectively. skewness distribution. The sediments from inter-dunes mainly composed of fine sand and have a mean grain size of 2.76 Φ, which are coarser and better sorted than dune sands and with an approximately symmetric mesokurtic distribution. The sedimentary environment and origins of the surface sand in the area are varied, more than 70 % of the sand samples are dominate by wind, while others are dominated by littoral deposits.