Nebkhas: An essay in aeolian biogeomorphology

Nebkhas are a type of anchored dune that forms by sediment accumulation around grasses, shrubs or trees. This paper outlines their importance, their distribution at global and local scales, summarises what is known about their morphology, discusses those plants with which they are associated, draws attention to the materials of which they are composed, and then concludes with a discussion of their ages. Nebkhas are very widespread in dry and coastal environments. They have considerable importance because they can be ‘islands of fertility’ and have major hydrological and biological effects. They can, if eroded, be major sources of dust emission. Examples of coastal nebkhas are known from both temperate areas and from lower latitude drylands, as along the Caspian shoreline in Iran, Kuwait, northern Peru, Namibia, Western Sahara, the Makran, southwest Madagascar, and Mauritania. Dryland nebkhas have been described from numerous areas, with concentrations of studies in Kuwait, the southwest USA, northwest China, and Iran. Within an area with suitable climatic conditions, nebkhas do not occur everywhere. Certain conditions need to be met for their development and these are illustrated with examples from the Tarim and Qaidam basins of China, the Lut of Iran, northwest Argentina, and the Namib. For this, Google Earth was employed. The heights of these features range from a matter of centimeters to as much as 20 m. There is a considerable quantity of data on the plants that are associated with the development of nebkhas. They have to have certain qualities for this role: ability to withstand drought, tolerate salt, thrive on sand burial, have wide shallow rooting systems to obtain water from fog and occasional rain events, extract moisture from groundwater, withstand sand abrasion, and develop adventitious roots. Although nebkhas are normally thought of as a type of sand dune, and most nebkhas are indeed composed of sand-sized material, largely consisting of quartz, there are many nebkhas which contain appreciable amounts of silt and clay, as well as salts such as gypsum. Although some nebkhas are ephemeral features, various dating techniques have shown that they can be decades, centuries and even millennia old.