1 23

Hydrogeology JournalOfficial Journal of theInternational Association ofHydrogeologists ISSN 1431-2174Volume 19Number 1 Hydrogeol J (2010) 19:155-161DOI 10.1007/s10040-010-0626-9

Source of paleo-groundwater in theEmirate of Abu Dhabi, United ArabEmirates: evidence from unusual oxygenand deuterium isotope data

1 23

Your article is protected by copyright and all

rights are held exclusively by US Government.

This e-offprint is for personal use only

and shall not be self-archived in electronic

repositories. If you wish to self-archive your

work, please use the accepted author’s

version for posting to your own website or

your institution’s repository. You may further

deposit the accepted author’s version on

a funder’s repository at a funder’s request,

provided it is not made publicly available until

12 months after publication.

Source of paleo-groundwater in the Emirate of Abu Dhabi, United ArabEmirates: evidence from unusual oxygen and deuterium isotope data

Warren W. Wood

Abstract Paleo-groundwaters of 6000years BP from theLiwa water-table sand dune aquifer in the Rub al Khaliand the Gachsaran artesian carbonate aquifer, on thecoast of the Emirate of Abu Dhabi (UAE), exhibitnormal δ2H/δ18O slopes, modest δ18O increases, andlarge negative deuterium excess “d” (Liwa aquifer:2.19‰ VSMOW and d=–15; Gachsaran aquifer: 3.16‰VSMOW and d=–28) compared to local Shamal precip-itation (0.05‰ VSMOW and d=7). This unusual isotopicsignature is hypothesized to result from re-evaporation ofcontinental runoff to the Red Sea catchment basin. It ispostulated that this continental water flowed onto thesurface of the Indian Ocean providing a moisture sourcefor the monsoon that dominated precipitation during this,the last wet period in the area from 5000 to 9000 BP.Carbonate precipitation, forming speleothems, travertines,tufas, lacustrine, and capillary-zone deposits, subsequentlyrecord this δ18O isotopic signature. This rock record is thusdominated by the water source, rather than environmentalconditions of deposition normally assumed to control therock δ18O isotopic signature. As a consequence, re-evaluation of paleo-climateδ18O data from the rock recordmay be necessary for this time period in southern Arabia.

Keywords Paleohydrology . Stable isotopes .Groundwater age . United Arab Emirates

Introduction

Isotopic ratios of hydrogen and oxygen in the hydrologiccycle change relative to one another in known ways andcan thus be utilized to determine the source of water.Carbonate minerals precipitating from waters capture the

oxygen isotope signal and, if dated, can be used as a time-dependent proxy (Clark and Fritz 1997). If the source ofwater is known or assumed, this rock record can then beused as a proxy to infer the paleo-environmental conditions(elevation, latitude, evaporation, temperature, rainfall fluxand other factors) at the time of carbonate precipitation. Thepresence of 6000-years-BP paleo-groundwaters in theEmirate of Abu Dhabi, United Arab Emirates (UAE), withan isotopically heavy δ18O, large negative value of“deuterium excess” and normal δ2H/δ18O slopes, sug-gests that these waters were not derived from typicalmarine water nor have they experienced significant postprecipitation evaporation; thus, care must be taken ininterpreting environmental conditions from the rockrecord in this area. This paper evaluates the source ofthis unusual isotopic signature.

Hydrogeology

The Liwa Crescent Quaternary-age water-table aquifer islocated on the eastern edge of the Rub Al Khali (EmptyQuarter) approximately 150 km southwest of the city ofAbu Dhabi, UAE (Fig. 1). Sand dunes with relief up to100 m dominate the topography of the area and form theskeletal framework of the Pleistocene-through-Holocene-age aquifer (Stokes and Bray 2005). The Liwa aquifer iscomposed largely of quartz plus minor amounts ofparticulate calcite, dolomite, anhydrite, feldspar, andopaque and heavy minerals (Ahmed et al. 1998; Hadleyet al. 1998). The aquifer, as far as is known, is composedlargely of uncemented or poorly cemented aeolian-sizedmaterial. Nearly flat valley floors (typically less than 1 mof relief), controlled by groundwater elevation, separatethe dunes over much of the area. Aquifer hydraulicconductivity is approximately 1.1 m/day, based on thearithmetic average of seven regionally distributed aquifertests in the Liwa area (Wood and Imes 1995). Porosity ofthree dune samples collected and analyzed by the authoraveraged 38% with little variance. The sand is depositedon essentially flat-lying middle Miocene-age carbonates,the top of which is near sea level (±10 m). The carbonatehas several orders of magnitude lower hydraulic conductiv-ity than the overlying sand dune Liwa aquifer and forms thebase of the dune aquifer system. Based on topography, theaquifer has a maximum saturated thickness of approximately

Received: 12 October 2009 /Accepted: 14 June 2010Published online: 14 July 2010

© US Government 2010

W. W. Wood ())Department of Geological Sciences,Michigan State University,206 Natural Sciences Building, East Lansing, MI 48824, USAe-mail: wwwood@msu.eduTel.: +1-517-3554629Fax: +1-517-3538787

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

110 m that thins in all directions away from the center of thedune complex. Natural discharge from the dune aquifer hassupported the traditional date oasis culture in the LiwaCrescent area. Carbon-14 dating suggests that water in theaquifer was last recharged between 5000 and 9000 years BP.Continuous water level measurement in remote areassuggests that it is not currently receiving recharge (Woodand Imes 1995; Wood and Imes 2003). Aeolian transport ofevaporated groundwater from between dunes and accu-mulation of solutes from rainfall over thousands of yearsin this hyperarid area generate a large mass of solublesalts stored on or near the surface (Wood et al. 2003;Wood et al. 2010). Wood et al. (2003) have shown thatthe solutes in the groundwater in the Liwa aquifer arelikely derived by infrequent recharge events that mobilizethese stored salts. Thus, one sees significant mass of solutesin the groundwater but little evidence of evaporation on thewater isotopes.

The middle Miocene-aged Gachsaran aquifer (and itsage-equivalent Lower Fars and Dam Formation)(Whybrowand Hill 1999) is a carbonate artesian aquifer that crops outnear the Gulf coast in the Emirate of Abu Dubai and isbelieved to subcrop below the Quaternary surfical depositsover much of the Emirate. (The Gulf has been referred to asboth “Arabian Gulf” and “Persian Gulf” in literature of thearea. To avoid confusion it is referred to as the “Gulf” in thispaper.) Because of the high mean specific conductance(125,700 μS: GWP piezometers, Table 1) in the coastal areait is not used for potable water supply. Solutes are derivedlargely from upward leakage of underlying aquifers. Thepiezometric surface suggests that recharge occurs in thehighlands associated with the Oman (Hajar) Mountains andfrom upward leaking of underlying formations. Dischargeoccurs by upward leakage to the overlying Quaternary agesediments and directly to the Gulf (Wood et al. 2002). Thehydraulic head is greater than the surface elevation of the

coastal sabkhas averaging approximately 12 m above sealevel in a 100-m deep piezometer (Wood et al. 2002). Theage of the water in the Gachsaran aquifer is assumed to besimilar to that in the Liwa aquifer based on hydrologicanalyses (Imes and Wood 2007).

Tectonic rifting in the Red Sea basin, with itssurrounding elevated topography in close proximity tothe sea, limits overland runoff to small local drainagebasins immediately adjacent to the sea. Similarly, theOman and Yemen Mountains restrict drainage to smallbasins that discharge to the Arabian Sea. Thus, waterisotopes from runoff reflect local, not regional precipita-tion. This topographic condition is believed to have beenin effect since middle Miocene time.

Currently the Indian monsoons regularly impact a portionof southern Oman providing more precipitation than otherareas in southern Arabia. The monsoon intensity and pathare a complex function of the amount of Indian Oceanupwelling along the coast, temperature of the water, strengthof Indonesian-Australia monsoon, wind-stress-curl anoma-lies, and other factors (Izumo et al. 2008). It is proposed thatthe increase in monsoon precipitation observed at severalintervals in the Pleistocene and Holocene were associatedwith 500-km northward movement of the IntertropicalConvergence Zone into Arabia (McClure 1976).

Methods and results

All wells were located by a hand held GPS (globalpositing satellite) instrument and have a horizontalaccuracy of ±20 m. Samples of groundwater werecollected using a peristaltic pump fitted with silicontubing that forced the water through a disposable flow-through 0.45-micrometer Gelman “filter capsule.” TheGWPwells (Table 1) flowed and required no pump to collect

Fig. 1 Map showing locationof groundwater samples fromthe Gachsaran and Liwaaquifers in the Emirates ofAbu Dhabi, UAE. Wellnumbers refer to Table 1.Three and four digit numbersare from the Gachsaranaquifer, others from the Liwaaquifer

156

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

samples. A minimum of 1 pipe volume of water was allowedto flow prior to collection. At each site, samples for waterisotope analysis were collected in 60 ml glass bottles withscrew-top polyseal caps and sent to the U. S. GeologicalSurvey for analysis (Révész and Coplen 2008).

Deuterium excess or d is defined as:

d ¼ d2H� 8d18O ð1Þδ notation is defined as RX=RSð Þ � 1½ � � 1000. Rx is theisotopic ratio of the sample, and Rs is the isotopic ratio ofVienna Standard Mean Ocean Water (VSMOW) in partsper thousand (permil, ‰). Graphically d is the δ2Hintercept of δ2H/δ18O line with a slope of 8. Thedeuterium excess is controlled by the temperature-dependent equilibrium of water vapor with the water

source and the relative humidity of the atmosphere(Merlivat and Jouzel 1979). During evaporation, theisotopic fractionation occurs as the result of molecularexchange between the seawater and atmospheric watervapor, under non-equilibrium conditions, where relativehumidity is less than 100%. For the global meteoricwater line (GMWL), deuterium excess d (Dansgaard1964) has the value of +10. The parameter d has beenshown to be diagnostic for measuring the contributionsof evaporated moisture (Gat et al. 1994) wherenumerically lower values are associated with samplesfractionated by evaporation (Gat and Matsui 1991).

Liwa aquifer samples have a mean d value of −15, aδ2H/δ18O slope of 4.9, a mean δ18O of +2.19‰ and meanspecific conductivity of 4,700 μS (Fig. 2; Table 1).Gachsaran aquifer samples have a d value of −28, a

Table 1 Sampling details and isotope data

Field identification UTM UTM UTM Date Sp. Cond. Temp δ 2H δ 18O dEasting Northing Zone (d/m/y) μS at 25°C °C ‰ ‰ ‰

Liwa aquiferLQ-1 767 256 2 635 100 39 2/12/92 16,200 1.0 2.30 −17.4LQ-2 773 785 2 621 805 39 7/12/91 11,400 29.7 0.0 2.10 −16.8LQ-3 777 267 2 613 448 39 3/12/91 9,100 33.7 −1.5 1.65 −14.7LQ-4 778 773 2 600 768 39 3/12/91 7,100 34.6 −3.0 1.60 −15.8LQ-5 783 606 2 584 458 39 3/12/91 2,000 31.5 3.5 2.20 −14.1LQ-6 784 547 2 577 688 39 3/12/91 1,300 31.4 7.5 2.60 −13.3LQ-7 785 931 2 566 889 39 3/12/91 1,700 32.5 4.0 2.05 −12.4LQ-8 219 995 2 539 176 40 4/12/91 16,000 32.6 4.5 3.10 −20.3LQ-9 221 270 2 540 164 40 4/12/91 11,600 32.5 6.0 2.10 −10.8LQ-10 210 946 2 547 106 40 4/12/91 4,100 32.2 4.5 2.55 −15.9LQ-11 203 470 2 551 955 40 4/12/91 3,800 32.9 5.0 2.45 −14.6LQ-12 806 671 2 552 229 39 4/12/91 7,200 32.6 7.0 2.75 −15.0LQ-13 789 180 2 554 772 39 4/12/91 4,500 29.1 6.5 2.50 −13.5LQ-14 799 720 2 559 277 39 4/12/91 8,300 31.8 3.5 2.30 −14.9LQ-15 782 556 2 558 652 39 4/12/91 10,600 30.1 4.5 2.55 −15.9LQ-16 749 309 2 543 855 39 5/12/91 10,700 30.9 3.0 2.60 −17.8LQ-17 755 990 2 548 570 39 5/12/91 6,400 31.1 3.5 2.30 −14.9LQ-18 770 754 2 561 800 39 5/12/91 1,700 31.6 4.5 2.10 −12.3LQ-19 764 298 2 557 633 39 5/12/91 1,700 32.5 4.5 2.15 −12.7LQ-20 774 827 2 557 749 39 5/12/91 3,300 33.4 3.0 2.00 −13.0LQ-21 779 758 2 559 433 39 5/12/91 7,600 32.4 0.0 1.95 −15.6LQ-22 795 951 2 556 944 39 6/12/91 3,000 32.4 6.0 2.40 −13.2LQ-23 783 074 2 582 469 39 7/12/91 1,613 4.0 2.25 −14.0LQ-24 784 520 2 574 773 39 7/12/91 1,754LQ-25 783 144 2 585 688 39 7/12/91 1,825 4.0 2.05 −12.4LQ-26 782 275 2 591 224 39 7/12/91 3,400 0.5 1.75 −13.5LQ-27 781 263 2 593 994 39 7/12/91 4,025 0.0 1.65 −13.2LQ-28 781 577 2 597 261 39 7/12/91 5,400 −1.0 1.75 −15.0LQ-29 778 756 2 605 864 39 7/12/91 5,770LQ-30 778 188 2 608 558 39 7/12/91 8,750 −3.5 1.70 −17.1LQ-31 777 845 2 616 028 39 7/12/91 10,000 −2.0 1.95 −17.6LQ-32 770 959 2 627 670 39 7/12/91 12,400 −2.0 2.20 −19.6LQ-33 769 198 2 631 874 39 7/12/91 14,160 0.0 2.10 −16.8LQ-34 784 428 2 575 353 39 19/1/92 1,774 4.0 1.95 −11.6LQ-35 772 776 2 624 602 39 19/1/92 24,900 6.0 3.70 −23.6LQ-36 774 530 2 621 312 39 19/1/92 9,370 0.0 1.75 −14.0LQ-37 778 834 2 606 574 39 19/1/92 7,700 −1.5 1.60 −14.3Gachsaran aquiferGWP-287A 600 328 2 647 191 39 Jan-98 181,590 −42.29 −2.89 −19.2GWP-288A 687 801 2 666 649 39 Jan-98 57,070 30.8 −6.49 2.44 −26.0GWP-289 777 587 2 655 712 39 Jan-98 33,750 30.8 0.67 3.07 −23.9GWP-290A 210 989 2 655 365 40 Feb-98 77,340 31.3 5.72 4.29 −28.6GWP-291A 205 481 2 670 574 40 Feb-98 147,500 30.6 5.9 4.66 −31.4GWP-292 236 909 2 671 579 40 Feb-98 225,000 31.2 9.81 5.51 −34.3GWP-306A 208 465 2 668 318 40 Mar-99 164,500 6.75 5.02 −33.4

157

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

δ2H/δ18O slope of 6.3, a mean δ18O of +3.16‰ and meanspecific conductivity of 127,000 μS (Fig. 2; Table 1).Mean current δ18O precipitation is −0.05‰, with a mean dof +7 and δ2H/δ18O slope of 5.3 (at the InternationalAtomic Energy Agency station in Bahrain (IAEA 2010))and is assumed to represent modern (Shamal or northern)rainfall in the Emirate of Abu Dhabi (Fig. 3).

Discussion

Origin of the waterPaleoclimate analyses indicate that the current hyper-aridity prevalent across much of the Arabian Peninsula hasbeen punctuated by episodes of increased rainfall duringthe late Pleistocene (∼25 to 30 ka; McClure 1976; Clarkand Fontes 1990; Wood and Imes 1995; Burns et al. 1998;Burns et al. 2001; Neff et al. 2001) and Holocene (∼5 to9 ka; McClure 1976; Wood and Imes 1995; Burns et al.1998; Lezine et al. 1998; Wehenmeyer et al. 2000; Neff etal. 2001; Wood and Imes 2003; Stokes et al. 2003). TheHolocene moisture source remains elusive, althoughgroundwater temperatures from noble gases suggest thatPleistocene precipitation in this area may have beenmonsoonal (Wehenmeyer et al. 2000; Fleitmann et al.2003), as do dune morphology, age, and distribution(Preusser et al. 2002; Stokes and Bray 2005). Summermonsoon-associated precipitation during the mid-Holo-cene in southern Arabia is thought to be the result ofnorthward extension of the intertropical convergencezone, with moisture derived from the Indian Ocean(McClure 1976). This monsoon hypothesis is consistentwith other sectors of the African paleo-monsoon belt(deMenocal et al. 2000; An et al. 2000).

The presence of water that is isotopically heavy δ18O,has a large negative d and a δ2H/δ18O slope approximated

by the local meteoric water line (LMWL) demandsunusual conditions for the origin of this paleo-water.Typically isotopically heavy oxygen values with largenegative d represent post precipitation evaporation ashydrogen and oxygen respond differently to evaporation.The δ2H/δ18O slope decreases coupled with heavieroxygen results in a more negative value of d. For example,current winter Shamal rains are the source of the water inthe UAE coastal sabkha (Wood et al. 2002) and arestrongly evaporated causing a decrease in δ2H/δ18O slopefrom 5.3 to 1.7 and a mean increase in δ18O from 0.05 to6.47‰ resulting in decrease in d from +7 to −24 (Fig. 3).

The Gulf (winter Shamals) or occasional the IndianOcean (summer/fall monsoons) are currently the moisturesources for the area. Thus, one must look to see if these werethe moisture sources in the past. It is extremely improbablethat stable isotopes from the Liwa aquifer representevaporation of precipitation derived from the Gulf. TheLiwa aquifer has a δ2H/δ18O slope is 4.9 compared with theShamal slope of 5.3 yet the mean δ18O difference is 2.14‰.That is, if evaporation had increased the δ18O one wouldexpect a much greater change in slope associated with such alarge difference in δ18O. Further, the lack of correlation(r2=0.04) between specific conductance and δ18O in theLiwa aquifer (Fig. 4) suggests that post precipitationevaporation is not controlling the increase in δ18O or δ2H/δ18O slope in this aquifer. That is, if evaporation wereincreasing the value of δ18O, one would expect a comparableincrease in specific conductance. It is also unlikely thatevaporation of the current monsoonmoisture from the IndianOcean (δ2H/δ18O slope of 3.4, mean δ18O equal 0.21‰ anda d of + 5; Fig. 5; Clark et al. 1987) would produce theobserved distribution by post precipitation evaporation.

It is also extremely improbable that stable isotopesfrom the Gachsaran aquifer represent precipitation derivedfrom the Gulf as the δ2H/δ18O slope is 6.3, or greater than

Fig. 2 Graph showing slope,intercept, mean δ18O, andd water isotopes (VSMOW)from local precipitation(Shamals)(IAEA 2009) , andthe Liwa and Gachsaranaquifers

158

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

the Shamal slope of 5.3, and thus the observed heaverδ18O is unlikely to have been derived by post precipitationevaporation. It is also unlikely that evaporation of thecurrent Oman monsoon moisture would produce theobserved isotopic relation in the Gachsaran aquifer. Onecould argue that the large negative d is simply the result ofevaporation near 100% relative humidity, thus retainingthe δ2H/δ18O slope of LMWL but becoming isotopicallyheaver. This seems improbable in an arid area with lowrelative humidity, and one does not see this occurring inthese environments today. This model also seems physi-cally unlikely in the case of recharge in the elevatedmountains representative of the Gachsaran aquifer.

The simplest explanation for the observed isotopicallyheavy oxygen with normal δ2H/δ18O and large negative

value of d is that moisture was derived from previouslyevaporated water, not standard mean ocean water. Largenegative values of d have been observed in rivers in thenorthern tier of the United States and the moisture sourcehas been interpreted to result from evaporation of lakesand wetlands formed from previous precipitation (Kendalland Coplen 2001). It is hypothesized that during periodsof enhanced precipitation the low density, warm con-tinental water floated out of the Red Sea onto the surfaceof the Indian Ocean and was driven northeastward alongthe coast of modern Yemen and Oman under the influenceof Indian monsoons (Fig. 6). This water would havebecome the moisture source for precipitation in southern

Fig. 3 Graph showing slope, intercept, mean δ18O, and d waterisotopes (VSMOW) from local precipitation (Shamals) and thecoastal sabkha aquifer

Fig. 4 Graph showing the lack of correlation between specificconductance and δ18O (VSMOW) from the Liwa aquifer

Fig. 5 Graph showing 1987 monsoon rains from Oman (Clark etal. 1987). Note the first day (open symbols) has a significantdifferent d value than the rains that followed

Fig. 6 Map showing proposed source of low-density continentalwater floating out of the Red Sea onto the Indian Ocean where it isevaporated providing a source of monsoon precipitation to the area

159

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

Arabia during the mid-Holocene, and presumably earlierepisodes of enhanced precipitation. Evaporation of thiscontinental runoff and consequent re-precipitation wouldbe consistent with the isotopes observed in the paleo-groundwaters. The amount of water discharging from theRed Sea during times of enhanced precipitation wouldhave been significantly greater than present and dominatedthe isotopic signature of the near-surface water in thisarea. Thus, the surface-water discharge and stable isotopesfrom the Red Sea would have been linked by a positivefeedback processes to precipitation in the catchment area.To a minor degree, the current monsoon follows thispattern and the isotopic fingerprint of the first day of the12−13 August 1985 monsoon (Clarke et al. 1987)illustrates the values of heaver δ18O (0.55‰) and negativedeuterium excess d (−4) (Fig. 5) that is believed torepresent a small amount of fresher water currentlyfloating out of the Red Sea.

The difference in δ18O (∼1‰) and d (−13) between theLiwa and Gachsaran aquifers is hypothesized to be afunction of distance from the moisture source. TheGachsaran aquifer is believed to be recharged largely inthe Oman Mountains within 100 km of the Indian Oceanwhile the Liwa aquifer is approximately 600 km from theIndian Ocean. That is, the heavier isotopes of oxygenwould have precipitated first as the moisture mass movedfrom the Indian Ocean over the Oman Mountains north-east over the Arabian Peninsula.

Implications for continental carbonate speleothemrecordsWhen carbonate precipitates from groundwater it incor-porates oxygen from the water in the carbonate (CO3

−2)ion. By dating the carbonate and determining the oxygenisotopes, a record of the oxygen isotopes with time isavailable to infer the environmental history of the water.For many continental paleo-climatic records, speleothems,travertines, tufas, lacustrine, and capillary-zone depositsprovide an excellent proxy of previous climatic conditions.Because of a general lack of deuterium from fluid inclusionsdata in the rock record, this approach implicitly assumes thatthe moisture source for the oxygen isotopes was derivedfrom marine water of known isotopic ratios. The observedvariation in oxygen isotope values in the rock record is, thus,assumed to be a function of environmental conditions ofelevation, latitude, evaporation, temperature, rainfall fluxand other factors.

The mean difference in isotopic values of the Liwaaquifer δ18O (2.19‰) and Gachsaran aquifer (3.16‰)compared to current Oman monsoons (0.05‰) isextremely large. At 30°C this difference amounts toapproximately 2.0‰VPDB (Vienna Pee Dee Belemnite)or greater than the range observed in the rock record inthis time interval (Clark and Fontes 1990; Burns et al.2001; Neff et al. 2001). That is, many of the observedvariations in oxygen at the critical time intervals could bedue to changes in moisture source, not local environ-mental conditions. Unless there are fluid inclusions, there

is no permanent record of the hydrogen ion in the rockrecord, making it difficult to check the assumption ofmoisture source. Fleitmann et al. 2003 evaluated thesource of the isotopically light Holocene fluid inclusion inspeleothems in Oman and concluded that the carbonaterecord represented monsoonal conditions likely derivedfrom the intertropical convergence zone 500 km north ofits current position. They did not report observing anyunusually negative d or isotopically heavy oxygen in theirfluid inclusion work of this age, however isotope signatureof negative d and heavy oxygen isotopes have beenobserved in some of the paleo-groundwaters of Oman(Wehenmeyer et al. 2000) and in the eastern Rub Al Khaliof Saudi Arabia (Sultan et al. 2008). Other areas in whichsignificant surface water runoff occurs to the Earth’soceans or seas that in turn act to recycle the source ofcontinental moisture may be expected to produce isotopeswith low value of d and isotopically heavy oxygenanalogous to those postulated to occur in Arabia duringthe last “wet” phase of 5000–9000 BP.

Conclusions

The oxygen and deuterium isotopic data from both theLiwa and coastal Gachsaran aquifer systems suggest thatHolocene moisture sources were derived from previouslyevaporated continental waters. Hence, the data suggestoxygen isotopes in speleothems of carbonate rock insouthern Arabia during enhanced times of precipitationmay have variations in water isotope sources that aresignificantly larger than environmental effects; thus, caremust be taken in their use as proxies for environmentalchanges.

Acknowledgements This is unfunded ‘curiosity’ research thatutilizes data collected from a number of projects over a 20-yearperiod from an ongoing study of the water resources of the Emirateof Abu Dhabi by the United States Geological Survey (USGS) andthe National Drilling Company of the Emirate of Abu Dhabi.Thanks go to D. Clark, J. Imes, and J. Tamayo of the USGS/NDC,A. Ain, for great field companionship and help in sample collectionover 20years. Reviews on an earlier version by J. Gat and I. Clarksignificantly improved the thinking and presentation. I. Clark kindlyprovided detailed isotopic data of the Oman monsoon. E. Harvey, T.Kraemer and M. Sultan reviewed the current manuscript, as did twoanonymous reviewers. The National Drilling Company and theDirector of the USGS have authorized publication.

References

Ahmed EA, Soliman MA, Alsharhan AS, Tamer S (1998)Mineralogical characteristics of the Quaternary sand dunes inthe eastern province of Abu Dhabi, United Arab Emirates. In:Alsharhan AS, Glennie KW, Whittle GL, St CG, Kendall C(eds) Quaternary Deserts and Climatic Change. Balkema,Rotterdam, The Netherlands, pp 85–90

An Z, Porter SC, Kutzbach JE, Xihao W, Suming W, Xiaodong L,Xiaoqiang L, Weijian Z (2000) Asynchronous holoceneoptimum of the East Asian monsoon. Quat Sci Rev 19:743–762

Burns SJ, Matter A, Frank N, Mangini A (1998) Speleothem-basedpaleoclimate record from northern Oman. Geology 26:499–502

160

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy

Burns SJ, Fleitmann D, Matter A, Neff U, Mangini A (2001)Speleothem evidence from Oman for continental pluvial eventsduring interglacial periods. Geology 29:623–626

Clark ID, Fontes JC (1990) Paleoclimactic reconstruction inNorthern Oman based on carbonates from hyperalkaline groundwaters. Quat Res 33:320–336

Clark ID, Fritz P (1997) Environmental isotopes in hydrogeology.Lewis, New York, 318 pp

Clark ID, Fritz P, Quinn OP, Rippon PW, Nash H, bin Ghalib alSaid B (1987) Modern and fossil groundwater in an aridenvironment: a look at the hydrogeology of Southern Oman.Use of Isotopes in Water Resources, Symposium 299, IAEA,Vienna, pp 167−187

Dansgaard W (1964) Stable isotopes in precipitation. Tellus16:436–468

deMenocal P, Ortiz J, Guilderson T, Adkins J, Sarnthein M, BakerL, Yarusinsky M (2000) Abrupt onset and termination of theAfrican humid period: rapid climate responses to gradualinsolation forcing. Quat Sci Rev 19:347–361

Fleitmann D, Burns SJ, Neff U, Mangini A, Matter A (2003)Changing moisture sources over the last 330,000years innorthern Oman from fluid-inclusion evidence in speleothems.Quat Res 60:223–232

Gat JR, Matsui E (1991) Atmospheric water balance in the AmazonBasin: an isotopic evapotranspiration model. J Geophys Res96:13179–13188

Gat JR, Bowser CJ, Kendall C (1994) The contribution ofevaporation from the Great Lakes to the continental atmosphere:estimate based on stable isotope data. Geophys Res Lett21:557–560

Hadley DG, Brouwers EM, Bown TM (1998) Quaternary paleo-dunes, Arabian Gulf coast, Abu Dhabi Emirate: age andpaleoenvironmental evolution. In: Alsharhan AS, Glennie KW,Whittle GL, St CG, Kendall C (eds) Quaternary deserts andclimatic change. Balkema, Rotterdam, The Netherlands, pp123–140

IAEA (International Atomic Energy Agency) (2010) (Completedaily isotope data 1961−1988). http://www-naweb.iaea.org/napc/ih/IHS_resources_gnip.html. April 2010

Imes JL, Wood WW (2007) Solute and isotope constraint of ground-water recharge simulation in an arid environment, Abu DhabiEmirate, United Arab Emirates. Hydrogeol J 15:1307–1315

Izumo T, de Boyer Montegut C, Luo JJ, Behera SK, Masson S,Yamagata T (2008) The role of the western Arabian Seaupwelling in Indian monsoon rainfall variability. J Climate21:5603–5623

Kendall C, Coplen TB (2001) Distribution of oxygen-18 anddeuterium in river waters across the United States. HydrolProcess 15:1363–1393

Lezine A-M, Saliege J-F, Robert C, Wertz F, Inizan M-L (1998)Holocene lakes form Ramlat as Sab’atayn (Yemen) illustrate theimpact of monsoon activity in Southern Arabia. Quat Res50:290–299

McClure HA (1976) Radiocarbon chronology of late Quaternarylakes in the Arabian desert. Nature 263:755–756

Merlivat L, Jouzel JJ (1979) Global climatic interpretations of thedeuterium-oxygen 18 relationship for precipitation. J GeophysRes 84:5029–5033

Neff U, Burns SJ, Mangini A, Mudelsee M, Fleitmann D, Matter A(2001) Strong coherence between solar variability and themonsoon in Oman between 9 and 6kyr ago. Nature 411:290–293

Preusser F, Radies D, Matter A (2002) A 160,000-year record ofdune development and atmospheric circulation in SouthernArabia. Science 296:2018–2020

Révész K, Coplen TB (2008) Methods of the Reston Stable IsotopeLaboratory. US Geol Surv Tech Methods 10–C1 and 10-C2

Stokes S, Bray HE (2005) Late Pleistocene eolian history of theLiwa region, Arabian Peninsula. Bull Geol Soc Am 117:1466–1480

Stokes S, Bray H, Goudie A, Wood WW (2003) Later Quaternarypaleorecharge events in the Arabian Peninsula, in, waterresources perspectives: evaluation, management and policy. In:Alsharhan AS, Wood WW (eds) Developments in waterscience, vol 50. Elsevier, Amsterdam, pp 371–378

Sultan M, Sturchio N, Al Sefry S, Milewski A, Becker R, Nasr I,Sagintayev Z (2008) Geochemical, isotopic and remote sensingconstraints on the origin and evolution of the Rub Al Khaliaquifer system. J Hydrol 356:70–83

Wehenmeyer CE, Burns SJ, Waber NN, Aeschbach-Hertig W,Kipfer R, Loosli HH, Matter A (2000) Cool glacial temperaturesand changes in moisture source recorded in Oman ground-waters. Science 287:842–845

Whybrow PJ, Hill A (1999) Fossil vertebrates of Arabia. YaleUniversity Press, New Haven, CT, 523 pp

Wood WW, Imes JL (1995) How wet is wet? Constraints on lateQuaternary climate in the southern Arabian Peninsula. J Hydrol164:263–268

Wood WW, Imes JL (2003) Dating of Holocene ground-waterrecharge in the Rub al Khali of Abu Dhabi: constraints onglobal climate-change models. Water resources perspectives:evaluation, management and policy. In: Alsharhan AS, WoodWW (eds) Developments in water science, vol 50. Amsterdam,Elsevier, pp 379–385

Wood WW, Sanford WE, Al Habschi ARS (2002) The source ofsolutes in the coastal sabkha of Abu Dhabi. Bull Geol Soc Am114:259–268

Wood WW, Rizk ZS, Alsharhan AS (2003) Timing of recharge, andthe origin, evolution, and distribution of solutes in a hyperaridaquifer system, in water resources perspectives: evaluation,management and policy. In: Alsharhan AS, Wood WW (eds)Developments in water science, vol 50. Elsevier, Amsterdam,pp 295–312

Wood WW, Clark DW, Imes JL, Councell TB (2010) Eoliantransport of geogenic hexavalent chromium to ground water.Ground Water 48(1):19–29

161

Hydrogeology Journal (2011) 19: 155–161 DOI 10.1007/s10040-010-0626-9

Author's personal copy