Unique Landscapes - Dunes & Sabkha
Dunes & Sabkha
The landscapes of Abu Dhabi today result from a complex sequence of interactions of numerous natural forces and formative processes – some subtle and slow, others sudden and even violent. These include the regional mega scale building blocks of plate tectonics and geology; the impact of an ever changing climate, and the interaction of physical, chemical and biological processes. Human intervention has emerged recently as another powerful formative force that can radically alter the landscape as well as the environment itself, with decidedly mixed results. Working together, these agents of change have created the unique landscapes and landforms that characterise the Emirate today. These features are testament to the restlessness of the Earth and to the interrelated and dynamic relationship with its environments. At a local level, they have led, and continue to lead, to inexorable change in the sub surface, surface and submarine structures and landforms of the Emirate.
Surprising Variety of Soils
The surface soils and sediments of the Emirate may appear similar but closer examination reveals a diversity of features and types. Soil characteristics are closely related to local geography and, especially to geological conditions and mineral composition, which dictate the soil formation process. Salts are a particularly important component of Abu Dhabi's desert soils because they influence their properties and diversity. A recent survey undertaken for EAD for the first time classified the Emirate's soils. Such surveys are essential to better understand and manage the diverse and often fragile and salt-affected soils of the region.
Star dunes are a highly characteristic and complex type of dune formed in areas where winds blow from different directions during different seasons; these dunes tend to have little overall movement.
Coastal and island sabkhas are one of the most important features of Abu Dhabi. These flat lying featureless plains are characterised by surface halite (salt) deposits at least for part of the year. Sabkhas first formed around 4,000 years ago following a period when coastal sediments were eroded, primarily by wind, down to the level of the water table, this process being closely associated with fluctuating sea levels. High evaporation rates resulted in near surface fluids becoming highly concentrated in salts. The halite forms a crust which, with continued evaporation, expands laterally to produce polygons with uplifted margins. Within the sediment, the fluids become increasingly saturated with calcium sulphate and the mineral gypsum starts to crystallise. Where ground temperatures exceed 40°C, the near surface gypsum loses its water and dehydrates to form anhydrite. The seaward edge of the sabkha is typically marked by a relatively narrow 200–800-metre-wide belt of dark microbial mat; this is an important source of organic matter in the harsh coastal eco system. The organisms that make up this mat community can only develop within a narrow range of conditions and if grazing molluscs colonise the mat, they will destroy it.
Dynamic Deserts and Dunes
In Abu Dhabi the prevailing shamal (northerly) wind transports loose sand inland from the coast. This is deposited as aeolian (wind blown) ripples, whose axes are at right angles to the wind direction. The orientation of ripples changes constantly with the varying strength and direction of the wind and the ripples merge to form sand dunes, with unique shapes and characteristics. Transverse dunes have reasonably straight crests with a long axis perpendicular to the dominant wind direction. Where sand supply is limited, isolated crescent shaped barchan dunes form, their lateral extremities or horns pointing down wind. Where sand is more abundant, barchans unite to form barchanoid dunes. If winds are too strong for transverse stability, sand strips parallel to the wind direction unite to form longitudinal dunes. Star dunes, irregular sand formations, occur where winds blow in a variety of directions during the year. Dune geometries are highly unstable. Even the footprints of a person or camel tracks along a crest can disrupt the subtle movement of sand and change the future shape and evolution of the dune and its neighbours. As a rule, the larger the dune, the older it is. Many of the larger dunes of Abu Dhabi, up to 60 metres high, record sand movements that stretch back beyond the last glacial maximum over 19,000 years ago. In the Liwa area, the dunes are even higher at 150 metres. Here, alternating periods of wind transportation and dune (sabkha) cementation have preserved ancient dune sands that were deposited up to 141,000 years ago.
Inland sabkhas form where the water table lies very close to the surface. Near surface evaporation concentrates salts until a halite (salt) crust develops. Lateral growth of this crust causes cracking and uplifting to form halite polygons with uplifted margins. Inland sabkhas can be found throughout much of Abu Dhabi and may be up to 5.5 square kilometres in area. South of Liwa, a shallow water table has promoted sabkha formation at ~80metres above sea level.
The shallow water table, coupled with poor drainage, results in rainfall pooling at the surface to produce ephemeral ponds. As these dry, microbial mats akin to those seen in the coastal sabkhas develop, sediments exposed through quarrying and wind erosion record cyclic changes in climate over thousands of years. The presence of dune sands indicates dry periods while sabkha formations indicate wetter periods or periods with higher water tables.
Gypsum plays a crucial role in soil formation since its quantity and location in the soil profile determines the type of the soil. The structural versatility of this semi soluble salt is demonstrated by the variety of ways its crystals can be arranged, sometimes producing intriguing and often beautiful structures. For example, Bottlebrush gypsum initially forms over plant materials in hyper saline conditions, the complex, delicate structure resembling sparse, wind swept vegetation. The distinctive gypsum desert rose forms in salty sands with crystals possessing reddish surfaces arranged like rose petals. Other gypsum morphologies or shapes include fibrous and tubular fans.