Abstract: 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 mesokurtic, leptokurtic, and platykurtic, respectively. Moreover, all the three parts exhibit a positively 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.

Key words: grain size, Mauritania, west Africa, sedimentary environment