Abu Dhabi Sabkha, UAE

The sabkha-dominated At Taf coastal strip is approximately 320 km (200 miles) long, and the coastal sabkha belt is up to 24 km (15 miles) wide; on average it has prograded 1.5 km (1 mile) every 1000 years. Seaward slopes on the sabkha are low and range from 1:3,000 to 1:4,000, this means whenever water a few centimetres deep arrives on the sabkha, the strandline on the brine sheet will have migrated kilometres.

Water temperatures on the open platform in the vicinity of Abu Dhabi Island range from 23° to 34°C, and from 15° to 40°C in the inner lagoon. Shallow waters in the lagoon can experience daily changes of 10°C. Air temperatures on the adjacent mudflat can be as low as 5°C in the winter and as high as 50°C in summer, with average temperatures ranging from 23° to 33°C. Sediment surface temperatures on the sabkha are even more variable, with reported values up to 60° to 80°C atop dark-coloured microbial mats, while temperatures a few centimetres below the sabkha surface range from 22° to 40°C. 

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In summer, the sabkha is exceptionally humid, especially at night, when the humidity reaches 100%. Salinities in the open Gulf are 3 to 4‰ above normal open marine salinities. Where coastal islands restrict exchange with the open Gulf, as in the vicinity of Abu Dhabi Island, lagoon salinities range between 54 and 67‰, near double that of the open ocean.

Prevailing onshore winds influence evaporation levels on mudflats along the southern coast of the Arabian Gulf. In winter the intensity of the onshore winds increases as the Shamal blows seawater and sediment from the lagoon onto the mudflat. At other times offshore winds blow continental dune sand into the back-side of the sabkha. Winter Shamals occur from November through March and tend to follow cold fronts. Shamals tend to set in with great abruptness and force, often with wind speeds of 40–50 km/h and gusts up to 100 km/hr. Both summer and winter Shamals are known, those in summer being less powerful. This dominant northwest to southeast airflow is a result of orography. Sharply rising mountains lie to the east and north of the Gulf, while gently rising mountains lie to the west and southwest effectively funnelling the low-level airflow. The Shamal is an equally crucial sedimentological driver in southern Gulf sedimentation as the Caribbean hurricanes to the Bahamian platform.

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Abu Dhabi sabkha. A) Sabkha facies located to the west of Abu Dhabi showing beach ridges encased in sabkha sediment in the vicinity of Bu Labyad - Al Du'yybaya (after Kirkham, 1997). B) Sabkha cross section based on facies present in the vicinity of the beach ridges south of Al Du'yybaya Peninsula and the Mussafah Channel wall (after Warren and Kendall, 1985).

The sabkha is furnished with matrix sediment from two sources, carbonates from the lagoon and siliciclastics from the hinterland. Many earlier sabkha models tended to ignore the eolian input, yet some areas of the mainland side of the Abu Dhabi sabkha (e.g. Al Du’yybaya sabkha) have quartz silt and sand as the primary matrix material, and it is the principal unconfined aquifer supplying meteoric water to the coastal plain. Rainfall along the sabkha coast averages 70 mm/year and is very irregular, in some years no rainfall is recorded. In the six years from 1958 to 1964, the highest annual reading on the sabkha was 6.73 cm, the lowest 0.33 cm. Although infrequent, desert rains can be torrential, especially in coastal sabkhas near the foot of the Oman Mountains, such as at Rams, where flooded wadis typically carry continental sediments as sheet floods many kilometres into the sabkha. Rain floodwaters can sometimes cover portions of the sabkha for one to three months and so dissolve and recycle surface and nearsurface halite/bittern crusts, as occurred in late 2003 and 2009.

Physiographically, the Abu Dhabi and other sabkhas in the Gulf are characterised by billiard table flatness, a direct reflection of control by capillary wicking, fed from the underlying watertable. The sabkha surface is a Stokes surface (see sabkha hydrology). In general, the watertable control to any sabkha gives them one of the most levelled forms of any arid landscape feature. Below this flat surface, the Abu Dhabi sabkha preserves an evaporitic mixed-carbonate-siliciclastic stratigraphy. Deposition began some 7,000 years ago when a rapid transgression flooded interdunal (eolianite) depressions with a veneer of marine-reworked, predominantly quartzose sediments. About 4,000 years ago the Arabian Gulf shoreline reached its maximum Holocene strandline and the edge of this flooding event is defined by a line of well preserved beach-ridge sediment sometimes called the “Evans line.” This was followed by pulsed progradation of the sabkha sequence to its present configuration. The Abu Dhabi sabkha and other sabkhas along the Trucial Coast lie in a very well documented depositional setting. Further information and a comprehensive literature summary on this and other sabkhat can be found in Warren, 2016.

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Flat surface of the sabkha is controlled by deflation down to the top of the capillary zone (this is the moist zone above water table)