Nabka

from Wikipedia, the free encyclopedia

Nabkas are stationary sand dunes that are piled up by the wind around obstacles (vegetation or objects anchored in the ground such as tree stumps or stones). These relatively small sand drifts often occur in arid regions. They belong to the structures created in flow shadows.

etymology

The word Nabka , also Nebka ( Nabkha or Nebkha in English ), comes from Arabic and means hill , small sand hill . It is older than 1400 years and goes back to pre-Islamic times . In English, Nabkas are also usually referred to as coppice dunes (stick shoot dunes ) or phytogenic mounds . The Arabic term Rehoub is used for smaller forms . There are also the terms Rebdou ' , Nebbe and Takouit .

The term nabka was first used scientifically in 1909 by Gautier and Chudeau.

description

California nabkas that formed around Prosopis glandulosa (mesquite tree)

Nabkas can deal with a variety of herbal obstacles such as: B. pile up bushes, shrubs, tufts of grass, etc. around. It is crucial here that the plants keep pace with the accumulation of sand in their growth and are not spilled. Despite their anchoring, Nabkas can still manage relatively quick changes of position downstream.

Nabkas are mainly formed in sandy plains with an annual rainfall of 50 to 200 millimeters and a plant cover that does not exceed 20%.

Nabkas show quite different sizes and shapes. The sand hills can reach 1 to 5 meters in height and 2 to 10 meters long; its streamlined outline is usually oval to elliptical, with the longitudinal axis lying parallel to the prevailing wind direction. The smaller Rehoub are only between 0.1 and 1 meter high and 0.5 to 5 meters long. In the Chihuahua Desert, with a width of up to 40 meters, heights of up to 4.30 meters were measured. The extremely large mega-nabkas are up to 10 meters high with a length of up to 1000 meters.

Nabkas are very asymmetrical in elevation, with a steep side facing the wind and a flat side in the slipstream of the obstacle.

Several nabkas can touch each other sideways and even form broad, hilly dune fields; they are often lined up one behind the other or are irregularly scattered.

The sand composition of Nabkas can be very different. Nabkas in Kuwait lead quartz, gypsum and feldspar sands, among which fragments of volcanic rock, oolite remains, mussel shells as well as calcite and dolomite (often) can be found. Silte and clay pellets are generally present. Garnet , topaz , zircon and tourmaline occur as trace minerals .

Nabkas in the coastal area can be attacked or completely washed away by the sea, so that only the vegetable core remains.

If the sand accumulation occurs episodically, an alternating layer of dust and sand with plant material arises. In general, the organic content inside the dunes increases over time, nutrients accumulate and microorganisms as well as plant and animal inhabitants multiply.

Vegetable barriers

Nabkas can be anchored by very different plant taxas:

ecology

Nabkas are characteristic of regions that are under the influence of desertification . In North America, the spread of the mesquite tree (Prosopis spp.) Is a sure sign of desertification. This plant is very efficient in accumulating sand and anchoring nabkas.

In northern China, nabkas were already present in the grassland before it was used for arable farming. However, the transition to arable farming increased their presence, especially in fields that have meanwhile been abandoned, nabkas are very common. In northern China, they are a sign of soil erosion .

Nabkas form their own specific ecotope . They are home to a varied fauna, including many invertebrates such as nematodes and the sand flea Talochestria capensis . Nabkas are richer in organic material than their surroundings, as plant material collects in them due to the wind and their interior is also penetrated by the root system of the anchoring plant.

In oil-polluted areas of Kuwait, nabkas represent ecological oases for other plants because of their nutrient-rich soil. Nabkas, which have formed around nitrogen-fixing legumes , have higher levels of nitrogen and other nutrients than their surroundings. Since other plants benefit from this oversupply, nabkas become islands of increased biodiversity in an otherwise impoverished habitat.

Emergence

Obstacles induce a rather complex flow behavior . The two-part flow is forced to accelerate past the obstacle. In the leeward direction, however, a negative pressure arises due to the formation of eddies, which no longer enables the medium to keep the sand carried in suspension any longer and which then deposits it. The respective shape of the obstacle is therefore decisive for the design of the individual nabkas.

It is therefore understandable that different plant taxa produce very different forms of Nabka. For example, behind Gazania such as Gazania rigens high, conical or elongated dunes form, whereas behind Arctotheca such as Arctotheca populifolia only short, semicircular forms emerge.

In general, it can be said that in order to form nabkas, the obstacle must reach a minimum height of 10 to 15 centimeters.

Occurrence

Nabkas are quite common and have a widespread occurrence. Well-known Nabka fields can be found in the Great Arabian Desert in Kuwait , in the Hotan Basin of Xinjiang , People's Republic of China , in New Mexico in the United States and in neighboring Chihuahua in Mexico .

Occurrence in detail:

Individual evidence

  1. ↑ EF Gautier, R. Chudeau: Missions au Sahara, I: Sahara Algerian . Armand Colin, Paris 1909.
  2. ^ HN Le Houérou: The desert and arid zones of northern Africa . In: Evenari, M. ua (Ed.): Ecosystems of the World, Volume 12 B: hot deserts and arid shrublands . Elsevier, Amsterdam 1986, p. 101-147 .
  3. ^ A b C. F. Francis: Plants on desert hillslopes . In: AD Abrahams, AJ Parsons (eds.): Geomorphology of desert environments . Chapman & Hall, London 1994, ISBN 0-412-44480-1 .
  4. ^ A b F. I. Khalaf et al .: Sedimentological and morphological characteristics of some nabkha deposits in the northern coastal plain of Kuwait, Arabia . In: Journal of Arid Environments . tape 29 (3) , 1995, pp. 267-292 .
  5. ↑ L. Milich: Dunes . Arid Lands Resource Sciences. The University of Arizona, Tucson 1998.
  6. ↑ a b c R. P. Langford: Nabkha (coppice dune) fields of south-central New Mexico, USA . In: Journal of Arid Environments . tape 46 (1) , 2000, pp. 25-41 .
  7. ↑ a b X. Wang et al .: Nebkha development and its significance to wind erosion and land degradation in semi-arid northern China . In: Journal of Arid Environments . tape 65 , 2006, p. 129-141 .
  8. ↑ a b c M. I. El ‐ Bana et al .: Role of host identity in effects of phytogenic mounds on plant assemblages and species richness on coastal arid dunes . In: Journal of Vegetation Science . tape 18 (5) , 2007, pp. 635-644 .
  9. ^ RU Cooke et al .: Desert Geomorphology . Taylor & Francis, 1993, p. 357 .
  10. ^ A b P. Hesp, A. McLachlan: Morphology, dynamics, ecology and fauna of Arctotheca populifolia and Gazania rigens nabkha dunes . In: Journal of Arid Environments . tape 44 (2) , 2000, pp. 155-172 .
  11. ↑ a b M. A. El-Sheikh et al.: Vegetation ecology of phytogenic hillocks (nabkhas) in coastal habitats of Jal Az-Zor National Park, Kuwait: Role of patches and edaphic factors . In: Flora . tape 205 (12) , 2010, pp. 832-840 .
  12. ↑ MM Ahmed et al.: Chemical and morphological characteristics of phytogenic mounds (nabkhas) in Kuwait . In: Arab Gulf Journal of Scientific Research . tape 27 (3) , 2009, pp. 114-126 .
  13. ↑ S. Wu et al: The morphological characteristics and growth mode of nabkha in the basin of Hotan River, Xinjiang . In: Geographical Research . tape 27 (2) , 2008, p. 314-322 .
  14. ↑ F. Mahmoudi: Les nebkhas de Lut, Iran . In: Annales de Géographie . tape 86 , 1977, pp. 315-321 .
  15. ↑ MM Ashour: Sabkhas of Qatar Peninsula (in Arabic) . Qatar University, 1990, p. 514 .
  16. ^ S. Al Sayari, JG Zotl: Quaternary Period in Saudi Arabia, Volume 1 . Springer, Vienna 1978.
  17. ↑ D. Kosmowska-Suffcyuska: Dune forms in Sebkhet el-Moo in Palmyra region . In: Prac i studia Geograpficzne . tape 2 , 1980, p. 177-188 .
  18. ^ A. Warren et al .: Dust-raising in the dustiest place on earth . In: Geomorphology . 2007, doi : 10.1016 / j.geomorph.2007.02.007 .