A Unique Sd in the Coasta Sabkha of Abu DhabiEmirate

Shabbir Ahmad Shahid, Mahmoud Ali Abdelfattah, and Michael A. Wilson

An anhydrite (CaSO4)—rich developed over a 70-cm-deep water table is mapped in the coastal sabkha (salt flat) of Abu Dhabi Emirate. An anhydrite is unique in occurrence in a wet zone over a water table, although anhydrite in this setting has been reported before. The X-ray diffraction analysis clearly presents anhydrite spectra to provide evidence to ratio- nalize anhydrite dominance over . Anhydrite is observed as massive clayey material, soft nodules, and seams. Anhydrite soil type as a soil family was confirmed in a number of auger holes and a map unit in the coastal sabkha. The anhydrite mineralogy family class is absent in the current version of U.S. soil taxonomy, and we call for its inclusion in the future version to provide the opportunity for better placement.

he coastal sabkha of Abu Dhabi Emirate has developed over the The Study Area Tpast 7000 years, where the arid conditions produced large amounts As part of the Arabian Peninsula (Fig. 1), the United Arab Emir- of gypsum (CaSO4 2H20) and anhydrite (CaSO 4) and lesser amounts of ates (UAE) is one of the hottest countries in the world and experiences dolomite, magnesite, celestite, and (Evans et al., 1969). "Sabkha" extremely high temperatures during summers and short mild winters. is an Arabic word meaning "salt flat." The sabkha of Abu Dhabi Emir- Humidity is highest along the coastal fringes and decreases inland, ate (coastal and inland) has been an attraction of research in the past and annual rainfall is very low. The present study was completed on the (McKenzie et al., 1980; Sanford and Wood, 2001; Sadooni et al., 2005). coastline of Abu Dhabi Emirate that stretches over approximately 400 The sabkha is developed when hard rock lies below the km, extending between 24° 54 35" and 23° 48 09" N and between 540 table, allowing water to remain close to the surface and susceptible to 5339" and 51° 35 59" E and excluding offshore islands. Abu Dhabi evaporation (de Matos, 1989). Topographically, these sabkhas are on Emirate (Fig. 2) is the largest Emirate in the average about 2 m above sea level and have a gradient of only about 1 occupying about 77,000 km 2, of which 377,044 ha (5.4% of the Emirate) m over a distance 0110 km. The mineralogical characteristics include composes the studied coastline. mainly algal and dolomitic crusts underlain by a layer of secondary anhydrite (Sanford and Wood, 2001). Anhydrite found under the sabkha precipitates from groundwater like concretions in and . There are Materials and Methods two kinds of anhydrite, one with a cottage cheese texture, considered to The study was completed using Landsat-7 ETM imagery originate from secondary replacement of gypsum, and the second type (Enhanced Thematic Mapper) acquired in 2002, with a working field soil is of primary origin (de Matos, 1989), occurs more inland, and appears map developed through visual interpretation. Geographic Information as nodules and contorted layers. Systems (GIS) were used for the compilation and production of final at a 1:50,000 scale. The main physical and chemical characteris- The soils composing the coastline of the Abu Dhabi Emirate have tics of each mapping unit were determined and stored as attributes in recently been mapped in detail (Shahid et al., 2004). This study recog- its geographical database. A total of 775 soil observations to a depth nizes 13 soil families and this paper is focused on one of the families of 200 cm were described using the USDA field guide (USDA-NRCS, featuring anhydrite enrichment. The reader is referred to Shahid et al. 2002), and soils were classified to the family level using U.S. soil tax- (2004) and Abdelfattah and Shahid (2007) for more information on the onomy (USDA-NRCS, 1999, 2003). In this article we focus on one of coastline survey. the thirteen soil families mapped that is rich in anhydrite mineralogy. After delineation of the map unit with a dominant anhydrite rich soil com- S.A. Shahid, International Center for Biosaline Agriculture (ICBA), P.O. ponent, a typical profile was exposed, described, and photographed. Box 14660, , United Arab Emirates (email: s.shahid@biosaline. Samples were collected for routine physical and chemical character- org.ae); M.A. Abdelfattah, Soil Resources Department, Environment istics (Burt, 2004) and mineralogical composition by X-ray diffraction. Agency—Abu Dhabi (EAD), P.O. Box 45553, Abu Dhabi, United Arab Infiltration and bulk density were also determined. Emirates (email: mabdelfattah @ ead.ae); M .A. Wilson, USDA-NRCS, National Center, 100 Centennial Mall N., Room 152, MS 41, X-ray patterns were recorded using Cu-Ko radiation, using vari- Lincoln, NE 68508, USA (email: [email protected]). able divergent receiving and scattering slits. The step size was 10 Published in Soil Surv. Horiz. 48:75-79 (2007).

WINTER 200775 20/mm. The procedure is similar to Fig. 1. Geographical location of United Arab Emirates (Abdelfattah and Shahid, 2007; reprinted with permission of Taylor & Francis Ltd.). method 7A2i (USDA-NRCS, 1996). We used a Philips (Eindhoven, the Netherlands) X-ray diffraction model IRAQ PW/1840, with Ni filter, Cu-Kc IRAN radiation (X = 0.154 nm) at 40 kV, KUWAIT - 55 mA and scanning speed 0.02°/s. I Diffraction peaks between 2 and

60° 20 were recorded. The corre- - sponding d-spacing and the relative intensities (I/Pa) were calculated and compared with standard data. SAUDI ,RIIl-

Results and Discussion Soil Map The soil map of Abu Dhabi coastline published (Shahid et al., 2004) at a scale 1:50,000 is composed of 23 soil map units YEMEN comprising of 129 soil polygons and

covers a total area of 3770 km2. 0 103 200 431) 0C 1)00 Only one polygon was dominant in anhydrite material, occupying 800 Fig. 2. Abu Dhabi Emirate in relation to other Emirates. Coastal survey area is shown. ha. Anhydrite was also recorded in other map units as minor soils. The landscape and typical profile show- ing anhydrite accumulation is shown North Eastern in Fig. 3. Emirates

Physical and Chemical Characteristics Selected physical and chemi- cal characteristics (Table 1) clearly illustrate that the entire profile is strongly saline and sodic due to seawater (Na rich) intrusion and calcareous due to sea shells. Anhy- drite is dominant at the depth of 12 to 70 cm, as measured by the Emirate of acetone precipitation method. It is Abu Dhabi . ,.J impossible to separate gypsum from anhydrite by this analytical method, but X-ray diffraction confirmed the presence of this latter . 0 25 50 100 The pH is in the neutral range and water chemistry is dominated Mineralogical Composition byNa°>Mg 2°>Ca2°>K and Cl > X-ray diffraction is by far the most powerful technique to provide a SO4 > HCO3 . The profile is calcareous throughout. The surface hori- fingerprint of anhydrite. The mineralogical composition of the anhydritic zon is sandy and is pale brown (10YR 6/3) when dry and yellowish layer from 40 to 70 cm (Fig. 4) revealed anhydrite as the dominant min- brown (10YR 5/4) when moist. The soil below this horizon (12-70 cm) is eral. It may be speculated that X-ray patterns of anhydrite are artifacts very distinctly white (5Y 8/1) (dry and moist), representing anhydrite. The of sample preparation, but gypsum loses water at or above 70°C, and in anhydrite layer has a soft fine-clayey feeling. The anhydrite layer transi- the present study, air-dried (30°C) samples were used for XRD. Analysis tions to a light yellowish brown under a water table at 70 cm, where indicated that most major peaks can be associated with anhydrite and the anhydrite deposit ends. that minor amounts of halite, calcite, and quartz exist. There were no peaks that could be associated with the presence of gypsum.

76 SOIL SURVEY HORIZONS Table 1. Laboratory characteristics of fine-clayey, anhydritic, hyperthermic Gypsic Aquisalids.t

Physical data Total Sand TPL Horizon Depth Silt Sand Fine Coarse Very tine Fine Medium Coarse Very coarse <2 mm <0.002 0.002-0.05 0.05-2 0.002-0.02 0.02-0.05 0.05-0.1 0.1-0.25 0.25-0.5 0.5-1 1-2 cm %of.<2mm % A, 0-12 3.5 45.0 51.5 32.0 13.0 14.0 19.0 4.5 6.5 7.5 59.5 B, 12-40 Anhydritic layer ------22.5 B ,2 40-70 Anhydritic layer ------24.7 C 70+ 2.0 11.0 87.0 8.0 3.0 30.0 41.5 11.5 4.0 0.0 69.8 Analytical data Horizon Depth ESP CaCO3 eq. Anhydrite + gypsum H20 content (1500 kPa) Bulk density Porosity Olsen P NO3 cm % <2 mm % <2mm 9/cm, % mg/kg 0-12 53.1 24.8 17.2 2.8 1.389 47 0.30 290 12-40 59.2 8.0 56.2 - - - 0.25 27.3 B 2 40-70 59.0 7.9 58.7 - - - 0.25 10.9 C 70+ 63.6 52.8 0.8 3.2 1.527 42 0.21 8.6 Analytical data Water-extractable from saturated soil paste Horizon Depth ECe pH Ca2 Mg2 Na K HCO3 SO2 Cl SAR cm dS rn meq L 1 (mmol L 0-12 229.0 6.84 18.0 310.0 805.0 1831.0 159.0 0.36 16.7 4700.0 77.5 B 7 12-40 190.0 7.28 44.0 221.0 515.0 1901.0 113.0 0.54 46.3 3100.0 99.1 B 32 40-70 187.0 7.18 43.8 220.0 512.0 1889.0 110.0 0.50 44.2 3150.0 98.7 CkZW 70+ 183.0 7.15 20.8 162.0 367.0 1939.0 82.9 1.08 28.5 2600.0 119.2 Mineralogical data: Composition of whole soil (<2mm) Horizon Depth Quartz Plagioclase Dolomite Aragonite Calcite Gypsum Anhydrite Halite Mean surface infiltration rate cm mm/h 40-70 Mi Nd Nd Nd Mi Nd D Mi 112

t ID = dominant, Mi = minor. Nd = not detected, SP = saturation percentage, ESP = exchangeable sodium percentage determined from SAR. CEO = Cation Exchange Capacity; TPL = Total pretreatment loss, ECe = electrical conductivity of the saturated paste extract, pH = pH of the saturated paste, SAR = sodium adsorption ratio.

Genesis of Anhydrite equilibrium, and this is because the reaction is endothermic. In addition, The formation of anhydrite in the coastline is presumably due to it is suggested that this reaction must be facilitated by the oxidizing envi- both direct crystallization from marine water and dehydration of gypsum. ronment of the medium (Perfhuisot, 1977; West, 2006). Feuerbacher (2006) described fine-grained anhydrite in a sabkha The formation of anhydrite mineral in the coastal sabkha of Abu formed from groundwater precipitation growing in a similar mechanism Dhabi raises an important question: How does anhydrite remain stable as carbonate nodules in and soils, and with time coalescing to form an under an environment where the water table reaches 70 cm and the anhydrite layer. In the present study, anhydrite was found as massive site experiences seasonal water flooding? Why is anhydrite not con- material and in the form of soft white nodules or seams. The anhydrite verted to gypsum? seams form contorted (enterolithic) layers. They may have formed merely by the dehydration of gypsum layers within prograding sediment; Currently, evidence suggests that the anhydrite remains stable however, Hardie (1986) and Kirkham (1998) considered that anhydrite in this environment and does not convert to gypsum upon exposure to seams possibly represent dehydrated gypsum layers formed within pre- water. High salinity coupled with high temperature are likely behind the vious marine salinas. In other samples from the coastline, gypsum and mechanism that creates and/or preserves this mineral when gypsum anhydrite were recorded as a mineral assemblage as a consequence would seem to be the more stable component. Since this coastal area of climatic affect on the reversible alteration of anhydrite-gypsum. The has very high salinity due to sea water intrusion, the anhydrite forms in crystallization process forms a white surface. When dry, large areas this marine environment and remains pedogenically stable. glisten due to on the surface. Slowly with time, these areas turn brown due to admixture of wind-transported ferruginous dust, Conditions of Gypsum to Anhydrite Transformation and crusts form (Fig. 3 and 5). The transformation of gypsum to anhydrite takes place in the pres- ence of water; the crystals of gypsum attacked by anhydritization always The diagenetic history of the Arab and Hith formations (Hith occur in the humid capillary zone of the profile. The composition of this formation is located in the southern and southwestern Arabian Gulf) capillary water is very similar to that of the water table, but likely has a suggests that the anhydrite and much of the dolomitization are a result higher salt concentration. Groundwater temperature was recorded as of penecontemporaneous sabkha . The character and timing 24°C in December and would be higher in the summer. For example, of the paragenetic events are responsible for the excellent porosity of meteorological instruments registered a summer soil temperature of the Arab formation and the lack of porosity in the massive of 40°C at the surface and 30°C at 50 cm depth (Perthuisot, 1977; cited the Hith, which together result in the prolific hydrocarbon sequences of from West, 2006). Butler (1969) reported temperatures of more than these formations (Alsharhan and Whittle, 1995). Another recent study by 50°C at the surface of the sabkha in the Trucial Coast (UAE). Figure 6 Alsharhan and Kendall (1994) indicated that the Hith Formation is com- plots temperature versus chlorinity for the gypsum-anhydrite transition posed mainly of anhydrite. The transformation of gypsum to anhydrite in NaCl solutions (McDonald, 1953; An Zen, 1965). This figure suggests occurs at the Dukhan sabkha of western Qatar. It begins with tempera- conditions of temperature and salinity at the start of the reaction are tures and salinity conditions clearly beyond the theoretical conditions at

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Fig. 3. Typical soil profile showing white anhydrite material, water table, and associated landscape . (Abdelfattah and Shahid 2007 photo reprinted with permission of Taylor & Francis Ltd.).

2 31

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clearly above equilibrium conditions at the gypsum–anhydrite phase boundary. Thus, anhydrite could form due to either direct precipitation from solution or conversion of gypsum to anhydrite. W.

However, this formation of anhydrite stops at certain times because of the lowering of the temperature and the dilution of the brine by rainfall. Undoubtedly the lowering of temperature and the winter rains contribute equally to this cessation. There still remains a question—why the exclu- sive localization, at least at present, of the anhydritisation of gypsum - .- - at the surface in this region of the Arabian Gulf? In other regions of the globe there exist some conditions that are similar in temperature and salinity. For example, in the sabkhas of North Africa there are traces of anhydrite in a coastal salt lake of Libya. However, there is one marked difference between sabkhas of the two regions: most of the sabkha of North Africa have an environment that is extremely reducing, rich in organic matter, and producing significant quantities of hydrogen sul- Abu Dhabi region explain the localization of the gypsum–anhydrite tran- phide. The sabkhas of the Arabian Gulf are by comparison much more sition in the present day environment of the Arabian Gulf. oxygenated, lacking in odor, and generally have light-colored sediments. The intuitive hypothesis is that the transition from gypsum to anhydrite is Call for Anhydritic Mineralogy Class in U.S. Soil Taxonomy improbable or at least more difficult in a reducing medium in which sulfur Soil Taxonomy (USDA-N RCS, 1999) does not have an anhydritic is in its stable form. Thus, the particular geochemical conditions of the soil mineralogy class. The anhydritic features are very unique in the Abu

A 4000 I t,rl

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10 PA Fig. 4. X-ray diffraction pattern of the < 2-mm soil in the horizon at 40 to 70 cm for the profile shown in Fig. 3. Mineral codes: A, anhy- Fig. 5. The inner supratidal sabkha of Abu Dhabi showing anhydrite drite; H, halite; 0, quartz: and C, calcite. growing beneath the surface (ETHZ, 2007).

78 SOIL SURVEY HORIZONS

of the United Arab Emirates and Lake MacLeod of Western Australia. AAPG Bull. 78(7):1075-1096. oc Alsharhan, A.S., and G.L. Whittle. 1995. Carbonate-evaporite sequences of the Late , southern and southwestern Arabian Gulf. AAPG Bull. 79(11):1608-1630. Available at http://strata.geol.sc.edu/PDF-Files/Arabian- anhydrite field eejnQ Gulf/CarEvapSeqLJurassicSharhanWhittle95.pdf (accessed 9 Sept. 2007, verified 14 Nov. 2007). V E.P,thnott An Zen, E. 1965. Solubility measurements in the system CaSO 4, NaCl, H20 at 35°C, 50°C and 70°C and one atmosphere pressure. J. Sediment. Petrol. 6:124-164. gypsum field Burt, R. (ed.) 2004. Soil survey laboratory methods manual. SSIR No. 42. USDA- NRCS, Washington, DC. Butler, G. p. 1969. Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf. J. Sediment. Petrol. 39:70-89. de Matos, J.E. 1989: The coastal sabkha of Abu Dhabi. Bull. 37. Emirates Natural History Group. Available at http://www.enhg.org/bulletin/b37/37_16.htm 40 80 120 160 (accessed 9 Sept. 2007, verified 14 Nov. 2007). CHLORINITY Evans, G., V. Schmidt, P. Bush, and H. Nelson. 1969. Stratigraphy and geologic his- Th..pst. fr tb yp-.S.tit hñn m N.CI hflM, dthg A aS IL, flIIthk -g D.&h.. S. i,..,,.,dL.g I, tory of the sabkha, Abu Dhabi, . J. Sediment. 12(1/2):145-159. P.flMda ETHZ (Eidgendssische Technische Hochschule Zurich) Swiss Federal Institute of Technology Zurich). 2007. Testing the microbial dolomite model. Available at Fig. 6. Temperature (°C) against chlorinity (mM) for the gypsum-anhydrite http://www.geomicro.ethz.ch/research/index (accessed 9 Sept. 2007, verified transition in NaCl solutions (McDonald, 1953) and (An Zen, 1965) 14 Nov. 2007). Feuerbacher, A. 2006. The Mediterranean was a desert. In Research on the Watch- tower. Available at http://corior.blogspot.com/2006/02/part-10-mediterranean- Dhabi coastal sabkha, suggesting an additional mineralogy class in the was-desert.html (accessed 9 Sept. 2007, verified 14 Nov. 2007). Hardie, L.A. 1986. Ancient carbonate tidal-flat deposits. Cob. Sch. Mines 0 U.S. soil taxonomy is warranted. The question is: why should anhydrite 81:37-57. be considered as a new mineralogy class? We believe that those materi- Kirkham, A. 1998. A quaternary proximal forelandramp and its continental fringe, als enriched with anhydrite should be treated as a separate mineralogy Arabian Gulf, UAE. p. 15-41. In V.P. Wright and T.P. Burchette (ed.) Carbon- ate ramps. Spec. PubI. 149. Geological Society London. class for the following reasons: McDonald, G.J.F. 1953. Anhydrite-gypsum equilibrium relations. Am. J. Sd. 1. Anhydrite is unique in the sense that it is present as a dominant min- 251:884-898. eral in the soil matrix and not as traces. McKenzie, J.A., K.J. Hsu, and J.F. Schneider. 1980. Movement of subsurface waters under the sabkha, Abu Dhabi, UAE, and its relation to evaporite dolo- 2. This soil type is not reported in the literature, and Soil Taxonomy mite genesis. p. 11-30. In D.H. Zenger (ed.) Concepts and models of dolomi- does not adequately classify the mapping unit. The present study tization. Spec. PubI. 28. Society of Economic Palaeontology and Mineralogy, soil is classified as a Gypsic Aquisalid due to limitations in the U.S. Calgary, Canada. soil taxonomy. Perthuisot, J.P. 1977. La sebkha de Doukhane (Qatar) et la transformation: Gypse- 3. One major difference between gypsum and anhydrite is the solubility. anhydrite plus water. [The sabkha of Dukhan (Qatar) and the transformation: Gypsum to anhydrite plus water]. Bull. Soc. Geol. France 19(5):1145-1149. Anhydrite can increase the electrolyte concentration in soil solution Available at http://www.soton.ac.uk/-imw/Sabkhas-Bibliography.htm#West more than gypsum. The result is an increase in the osmotic potential, (accessed 9 Sept. 2007, verified 14 Nov. 2007). causing salinity hazards to plants. Sadooni, F.N., F. Howari, W. Haniza, and E. Abd El-Gawad. 2005. Modern and 4. In U.S. soil taxonomy, kaolinitic, halloysitic, gibbsitic, carbonatic, and coastal sabkhas of the UAE: Spatial and temporal evolution, sedimentology gypsic mineralogy and other classes are considered, but not anhy- and geochemistry. p. SCI 1-14. In Proceeding of the 5th Annual Research drite. Conference, United Arab Emirates University. Available at http://sra.uaeu. ac.ae/Conterence_6/ProceedingsfPDF/Science/SCL1 .pdf (accessed 9 Sept. 5. Anhydritic soils occur in other coastal areas that experience similar 2007, verified 14 Nov. 2007). conditions such as Dukhan sabkha, Qatar (Perthuisot, 1977), Hith Sanford, W.E., and W.W. Wood. 2001. Hydrology of the coastal sabkhas of Abu Formation, Arabian Gulf (Alsharhan and Kendall, 1994) and Coastal Dhabi, United Arab Emirates. Hydrogeol. J. 9:358-366. salt lake, Lybia (Perthuisot, 1977). Shahid, S.A., M.A. Abdelfattah, and K.R. Arshad. 2004. Soil survey for the coastline On the basis of the above justifications and also considering the litera- of Abu Dhabi Emirate. Volume I: Reconnaissance survey. Volume II Soil maps. Environment Agency-Abu Dhabi, UAE. ture cited earlier that rationalize the existence of anhydrite in the coastal USDA-NRCS. 1996. Soil survey laboratory methods manual. Soil Survey Invest. sabkha of Abu Dhabi Emirate, it is recommended that the anhydrite min- Rep. 42. USDA-NRCS, National Soil Survey Center, Lincoln, NE. eralogy class be considered in future versions of Soil Taxonomy. USDA-NRCS. 1999. Soil taxonomy: A basic system of for making and interpreting soil surveys. Agric. Handb. 436. 2nd ed. U.S. Gov. Print Office, Washington, DC. Acknowledgments USDA-NRCS. 2002. Field book for describing and sampling soils. Version 2.0. U.S. The authors highly acknowledge His Excellency Majid Al Mansouri, Sec- Gov. Print. Office, Washington, DC. retary General of Environment Agency-Abu Dhabi, for his continuous support USDA-NRCS. 2003. Keys to soil taxonomy. 9th ed. U.S. Gov. Print. Office, Wash- and encouragement to complete the coastline soil survey of Abu Dhabi Emirate. ington, DC. The assistance provided by the project staff, especially Environment Laboratory West, I. 2006. Selected bibliography on sabkha, salt lakes and . A supple- Department staff, Khaliq-ur-Rehman Arshad, Yasser Othman, Anil Kumar, and ment to Geology of the Wessex Coast of Southern England. Available at URL: Mohamed Al Meharibi is highly appreciated. Thanks also to Dr. Ayman K. El-Saiy http://www.soton.ac.ukl-imw/Sabkhas-Bibliography.htm#West (accessed UAE University for performing XRD analyses. 9 Sept. 2007, verified 14 Nov. 2007). School of Ocean and Earth Science, National Oceanography Centre, Southampton, Southampton University, UK.

References Abdelfattah, MA., and S.A. Shahid. 2007. A comparative characterization and clas- sification of soils in Abu Dhabi Coastal Area in relation to and and semi-arid conditions using USDA and FAO soil classification systems. Arid Land Res Manage, 21(3):245- 271. Available at http://www.informaworld.com/smpp content-content=a779704000--db=all--order=page (accessed 9 Sept. 2007 verified 14 Nov. 2007). Alsharhan, AS., and C.G. St. C. Kendall. 1994. Depositional setting of the Uppei Jurassic Hith Anhydrite of the Arabian Gulf: An analog to Holocene Evapori

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