assessing life history from commingled assemblages: the biogeochemistry of inter-tooth variability...

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Journal of Archaeological Science 47 (2014) 10e21

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Journal of Archaeological Science

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Assessing life history from commingled assemblages: thebiogeochemistry of inter-tooth variability in Bronze Age Arabia

Lesley A. Gregoricka*

Department of Sociology, Anthropology, & Social Work, University of South Alabama, HUMB 34, 5991 USA Drive North, Mobile, AL 36688, USA

a r t i c l e i n f o

Article history:Received 2 November 2013Received in revised form11 February 2014Accepted 5 April 2014Available online xxx

Keywords:Strontium isotopesOxygen isotopesInter-tooth samplingMobilityLife historyComminglingOman peninsula

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a b s t r a c t

Stable isotopes represent a remarkable means of assessing the life histories of individuals in thearchaeological record, primarily by taking advantage of multiple skeletal tissues that form and remodelthroughout life. However, this methodological tool also has the potential to provide crucial informationon the life histories of individuals represented only by commingled, fragmentary, and otherwise isolatedhuman skeletal remains, particularly when more traditional methods of evaluation are not possible. Inthis study, a sequential analysis of radiogenic strontium (87Sr/86Sr) and stable oxygen (d18O) isotopes ofassociated molars from the in situ mandibular or maxillary fragments of 19 commingled individuals frommultiple Bronze Age tombs in the United Arab Emirates was undertaken in order to examine temporalchanges in mobility at the level of the individual. The majority of individuals display little difference ininter-tooth isotope ratios, suggesting a general lack of residential mobility between early childhood andlate adolescence, an unexpected result given abundant evidence (artifactual and written) for a popula-tion actively engaged in both regional and interregional trade in the third millennium BC. However, twoindividuals from the Emirate of Fujairah demonstrate disparate inter-tooth 87Sr/86Sr values e likelyindicative of some migration event e which calls into question the supposed cultural isolation of theregion during the second millennium BC.

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1. Introduction

A recent bioarchaeological shift in emphasis from the popula-tion level to that of the individual e variously described as osteo-biography (Mayes and Barber, 2008; Stodder and Palkovich, 2012)or life history (Barrett and Blakey, 2011; Zvelebil and Weber, 2013)e has introduced new perspectives into the interpretation of pastlifestyles and behaviors. Increasingly refined methodologicaltechniques have for the first time permitted a detailed assessmentof individual life histories and the temporal shifts associated withparticular periods of the life cycle, all of which can be utilized toform a more holistic and comprehensive understanding of pastpopulations as a whole.

Radiogenic and stable isotopes incorporated into human skele-tons during life represent a remarkable means of assessing the lifehistories of individuals in the archaeological record. Primarily bytaking advantage of multiple skeletal tissues that form and remodelat different stages throughout life, bioarchaeologists have begun to

rely more heavily on biogeochemical data as a way to elucidate notonly changes in diet within the life course but also geographic or-igins in childhood and residential mobility later in life. In particular,intra-tooth sequential sampling has come to the forefront ofarchaeological chemistry as a means of improving our under-standing of specific life histories. Previous studies have used intra-tooth stable isotope analysis of faunal enamel in an attempt toreconstruct seasonal migrations and changes in patterns of dietaryintake (e.g., Balasse, 2003; Balasse et al., 2002; Bocherens et al.,2001; Britton et al., 2009; Fricke and O’Neil, 1996; Montgomeryet al., 2010; Stevens et al., 2011; Weidemann et al., 1999). Farfewer applications to human enamel have been conducted (e.g.,Holt, 2009; Lovell and Dawson, 2003; Sandberg et al., 2012;Wright,2013), although as with faunal biogeochemical studies, the goal ofmore precisely identifying and understanding particular processese including the process of weaning, developmental defects, andmigration patterns e remains the same.

While these intra-tooth sampling methods have gained popu-larity over the past decade, especially because of their ability tominimize the destructive nature of isotope sampling for rare orsmall samples, a number of issues make the use of intra-toothsampling and its associated technology problematic. First, studies

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L.A. Gregoricka / Journal of Archaeological Science 47 (2014) 10e21 11

comparing 87Sr/86Sr enamel values derived from both solution-based MC-ICP-MS and laser ablation report conflicting measuresof accuracy because of CaePeO isobaric interference; as the com-binedmass of calcium, phosphorus, and oxygenmimics that of 87Sr,87Sr/86Sr values are artificially inflated during laser ablation(Richards et al., 2008; Simonetti et al., 2008). Some (e.g., Copelandet al., 2010) have reported better accuracy and precision, suggestingthat enamel matrix diagenesis may play a role in methodologicaldifferences in strontium values, while others point to analyticalchallenges in producing reliable 87Sr/86Sr ratios from regions withlow strontium concentrations (w100e200 ppm) (Horstwood et al.,2008). This uncertainty makes it necessary to approach intra-toothsampling using laser ablation analysis with caution.

Secondly, and most pertinently, establishing a definitive timeresolution for specific growth increments of enamel is not yet fullyunderstood due to the process of amelogenesis itself. After theformation of a preliminary enamel matrix during the secretorystage, the second phase of enamel formationematuratione can becharacterized by consecutive but irregular, multi-directionalmineralization fronts that do not adhere to discrete, incrementalunits of growth (Balasse, 2002; Fisher and Fox, 1998; Hillson, 1996;Montgomery et al., 2010). Furthermore, the initiation and termi-nation of mineralization, in addition to overall mineralization rateand intensity, can vary substantially between enamel layers(Sandberg et al., 2012; Suga, 1979; Suga et al., 1979). As a result,isotopic signatures derived from intra-tooth sampling proceduresreflect attenuated values that are better presented as a product oflong-term averaging than as exact gauges for short-term behavior(Balasse, 2002; Passey and Cerling, 2002). Consequently, whilepreliminary patterns reflecting seasonal changes, mobility, anddietary change have been tentatively identified (Balasse et al., 2002,2005; Fricke and O’Neil, 1996; Kohn et al., 1998; Sharp and Cerling,1998) and point to the potential of intra-tooth sampling, a singlesample nevertheless represents months of enamel mineralization(Hoppe et al., 2004).

Another means of obtaining isotope values associated withspecific periods of growth, development, and maturation insteadlies with the bulk sampling of different teeth and bone with knownformation and remodeling rates (Dupras and Tocheri, 2007). Unlikeintra-tooth sampling, this method does not seek to pinpoint theexact timing (in weeks or months) of isotope incorporation intoskeletal tissues, but instead takes a broader life history approach bycomparing two or more extended periods (e.g., infancy, adoles-cence, adulthood) within the life cycle. While the utilization ofnumerous skeletal tissues to achieve such an evaluation supplies awealth of information when more complete individual skeletonsare present and well preserved, commingled skeletal materialpresents a challenge to even the most basic methods of evaluation(including age and sex estimation). Further, without direct associ-ation of skeletal elements, commingled remains do not tradition-ally lend themselves to extensive examinations of individual lifehistories.

Nevertheless, for millennia in the Near East, communal burialwas a common form of disposing of the dead, and as a result,commingled collections are frequently represented in the archae-ological record in the Levant (Al-Shorman, 2003, 2004; Amiranet al., 1986; Blau, 2006; Chesson, 1999; Haskins, 1988; Lev-Tovet al., 2003; Nagar et al., 1999; Ortner and Frolich, 2008; Sher-idan, 1999; Ullinger et al., Early view), Arabia (Blau, 1998, 2001a,2001b, 2007; Frifelt, 1991; Gregoricka, 2011, 2013a; Haerinck, 1991;Porter and Boutin, 2012; Potts, 1993), and Anatolia (Boz and Hager,2014; Losch et al., 2006; Whitcher et al., 2009). Commingled col-lections can provide surprisingly abundant data on variables suchas health, non-specific stress indicators, activity patterns, and diet,and as such, have the potential to provide crucial information on

communities in the past (Adams and Byrd, 2008; Adams andKonigsberg, 2004; Osterholtz et al., 2014). Moreover, despite thecommingled nature of these interments, mandibular and maxillaryfragments commonly contain multiple in situ teeth from the sameindividual, permitting an appraisal of temporal change in the lifecourse using isotopes incorporated into enamel hydroxyapatite.

Here, a sequential analysis of radiogenic strontium (87Sr/86Sr)and stable oxygen (d18O) isotopes of consecutively forming molarsfrom the in situ mandibular and maxillary fragments of 19 com-mingled individuals from multiple Bronze Age tombs in the UnitedArab Emirates was undertaken in order to examine temporalchanges in mobility at the level of the individual in early life.

2. Mobility and mortuary traditions in Bronze Age Arabia

Considerable changes in funerary practices over the course ofthe third and second millennia BC e exhibited by differences intomb placement, architecture, and membership e are inextricablylinked to fluctuating patterns of mobility across the Arabian land-scape. These mortuary traditions have customarily been classifiedas Umm an-Nar (ca. 2700e2000 BC) and Wadi Suq (ca. 2000e1300 BC), and are discussed in further detail below.

The commencement of the Umm an-Nar period brought with itwidespread sedentism in response to the domestication of the datepalm, instigating the nomadic pastoralists of the preceding Hafit(ca. 3100e2700 BC) period to seek out sources of fresh water tosupport oasis agriculture and permanent settlement (Mery andTengberg, 2009; Potts, 1990). Villages constructed by Umm an-Nar communities were typically centered around large mud-brickfortress towers, which served to protect wells critical to the sur-vival of crops grown in this arid environment (Blau, 1999; Potts,2001, 2009). In addition to garden crops, domesticated herds andmarine resources supplemented the varied diet of these thirdmillennium populations (Al Tikriti, 1985; Aspinall, 1998; Potts,1998; Uerpmann, 2001). Located in close proximity to these set-tlements, monumental circular tombs 4e14 m in diameter con-tained hundreds of commingled individuals representingapproximately 200e300 years of use (Al Tikriti and Mery, 2000;McSweeney et al., 2008; Potts, 1997). The fine craftsmanshipevident in the expertly-shaped ashlar façades of these tombs,coupled with their generally large size, speaks to the communaldivision of labor and hierarchical system of management requiredto construct such funerary structures (Cleuziou, 2007).

The fragmentary and commingled nature of human remains inUmm an-Nar tombs can be attributed to a number of factorsaffecting the postmortem environment, including natural tapho-nomic processes (e.g., water, rodent burrows), human disturbancefrom the looting of metals and stones in antiquity, and intentionalmortuary practices (Blau, 2001a,b). In rare instances, one or morefully articulated individuals have been recovered, buried in a flexedposition and variably laid on either their right or left sides (Benton,1996; Blau, 2001a,b, 2001c; McSweeney et al., 2008). These in-dividuals are consistently found at the lowest levels of the tomb’sfoundations and point to a funerary strategy consisting of an initialprimary interment, with whole bodies placed into these gravesuntil a need for space required that those already buried werepushed aside in order to make more room (Blau and Beech, 1999).Articulated limbs and other skeletal elements recovered fromUmman-Nar tombs lend credence to this interpretation (e.g., Mery et al.,2004; Schutkowski, 1988). In some cases, it appears that small fireswere lit within the tomb to clear additional human debris, and in atleast one instance, postmortem cut marks suggest deliberatedisarticulation (Al Tikriti and Mery, 2000; McSweeney et al., 2008).

While interregional relations reemerged in the preceding Hafitperiod, it was the intensification of trade with the far-reaching city-

L.A. Gregoricka / Journal of Archaeological Science 47 (2014) 10e2112

states of Mesopotamia, the Indus Valley, Dilmun, and beyond thatmarks the important role of local Umm an-Nar communities assuppliers of copper to the larger Persian Gulf (Carter, 2003, 2008;Hellyer, 1998; Mery, 1997; Potts, 1990, 2001). Geographically,southeastern Arabia offered an ideal, centralized location for mer-chants following sea trade routes, and the extensive assemblages ofimported Mesopotamian and Indus wares found at third millen-nium sites across the United Arab Emirates indicates that trade waslikely conducted along its coastline (Mery, 1997; Potts, 2000, 2009).Coastal settlements not only provided easy access to maritimetraders, but also connected the Gulf with the mountainous interiorof the Oman Peninsula where rich copper deposits were minedbefore export (Begemann and Schmitt-Strecker, 2009; Begemannet al., 2010; Carter, 2003; Possehl, 1996). The distribution of foreigngoods in both mortuary and domestic settings corroborates theheavy involvement of southeastern Arabia in these exchange sys-tems (Cleuziou and Vogt, 1983).

Drastic changes in mortuary practices, material culture, traderelations, subsistence, and social organization mark the beginningof the Wadi Suq period in the early second millennium BC. Rapidaridification across the whole of the Near East instigated a numberof regional shifts in environment, most notably a lowering of localwater tables necessary to support oasis agriculture and sedentaryUmm an-Nar communities (Cleuziou, 1989; Parker et al., 2006a;Parker and Goudie, 2008). This climatic shift prompted Wadi Suqpopulations to congregate along the northern coast of the penin-sula, evidenced by shell middens interpreted as reflecting a majortransition from a mixed subsistence economy to one dominated bymaritime resources (Blau, 2007; Carter, 2003; Crawford, 1998;Potts, 2009). Archaeologically, a dramatic reduction in the num-ber, size, and permanence of settlements is apparent, reflective oflarger changes in social organization and interpreted by many asindicative of the adoption of a more mobile lifestyle (Hellyer, 1998;Potts, 1990).

Concurrent with these changes in climate, interregional traderelations quickly broke down between southeastern Arabia andMesopotamia as well as the Indus in the early Wadi Suq period(Kenoyer, 2000; Oates, 2008; Potts, 2009; Reade, 2008). Little evi-dence for copper mining and smelting activities exists in south-eastern Arabia during this time, suggesting that this collapse largelyreflects a marked decline in the demand for copper from the OmanPeninsula in favor of its acquisition from Dilmun, which had cometo dominate Gulf trade in the early second millennium (Carter,2003; Crawford, 1998; Weeks, 1997). While ties were notcompletely severed between these regions, the placement offoreign goods in second millennium tombs is rare (Carter, 1997,2003; Potts, 1992).

While some continuity in tomb form between the Umm an-Narand Wadi Suq speaks to population continuity (and not replace-ment) during the Bronze Age, the highly standardized mortuarystructures of the third millennium were replaced by more variableforms, both in architecture and in number of interments. Singleinhumations increased markedly in the second millennium, andalthough communal burial also persisted, those interred numberfar less than in the previous period (Hellyer, 1998). As in the Umman-Nar, the dead were originally placed in a flexed position beforebeing disturbed by later burials, contributing to the commingledand fragmentary nature of these assemblages, although cremationwas no longer performed (Vogt, 1998). Likewise, tombmembershipincluded individuals of all ages and both sexes (Potts, 1990;Schutkowski and Herrmann, 1987; Wells, 1984).

While it had long been assumed that these widespread inter-regional exchange networks resulted in the considerable presenceof non-locals at Umm an-Nar sites before the so-called cultural“collapse” of the Wadi Suq period, recent biogeochemical analyses

of human skeletal material suggest that e while a few non-localswere indeed present e the majority of individuals interred inlocal tombs from both periods were native to the region(Gregoricka, 2013a, 2013b). Moreover, the homogeneity of bothradiogenic strontium and stable oxygen isotope ratios indicate thatmobility did not play an important role in the daily lives of theBronze Age inhabitants of southeastern Arabia. Nevertheless,because these studies generally utilized permanent first molars,residential mobility later in life would not be evident isotopically.Thus, it was hypothesized that mobility would increase with agedue to the more likely involvement of older individuals insubsistence-based or commercial enterprises. This hypothesis wastested using strontium and oxygen isotope analysis; if mobilityincreases with age, these values should exhibit temporal variabilitybetween molars forming earlier and later in life. Subsistence-related patterns of mobility may reflect a transhumant lifestyleinvolving the seasonal migration of herds between the coast andthe mountains, so that a subset of the population tending to theseherds would display variable isotope values in later-forming teeth.Economic influence as the result of involvement in both regionaland/or interregional exchange systems centered on copper exportmay also produce isotopic variability at the level of the individual.

3. Evaluating prehistoric patterns of mobility usingbiogeochemistry

Strontium (87Sr/86Sr) isotope analysis has been established as aneffective means of identifying patterns of mobility and migrationamong individual members of ancient communities. While thestable, non-radiogenic isotope 86Sr remains constant in local eco-systems, the radiogenic 87Sr is the outcome of rubidium (87Rb)decay, and as such, varies both by the composition of local bedrockas well as the age of its minerals (Bentley, 2006; Budd et al., 2000;Ericson, 1985). Correspondingly, different regions exhibiting adistinct geologic makeup will possess dissimilar 87Sr/86Sr ratios.

Unlike their lighter counterparts (e.g., d13C, d15N, d18O), stron-tium isotopes do not undergo any meaningful fractionation as theyescape bedrock andmove throughout the various trophic levels of agiven environment, so that their values remain unaltered (Beardand Johnson, 2000; Graustein, 1989). Eventually, strontium iso-topes from this local environment pass into humans via the foodsthey consume. Because of the structural similarities betweenstrontium and calcium, dietary strontium is incorporated into teethduring enamel formation, with resultant 87Sr/86Sr values indicativeof childhood residence in a particular geographic area (Bentley,2006; Price et al., 2002).

Ratios from archaeological enamel samples deserve specialmention as a proxy for 87Sr/86Sr bioavailability across Arabia andSouth Asia. In the United Arab Emirates, enamel from fauna datingto the Bronze Age (n ¼ 39) provides a local 87Sr/86Sr baseline of0.7086e0.7090 for the region (Gregoricka, 2011). This contrastsdramatically with local ranges produced in regions known to haveengaged in trade with the Oman Peninsula during the third mil-lennium BC, including the Indus Valley and Mesopotamia.Archaeological fauna from Harappa, one of the largest Bronze Agecities of the Indus Civilization located along the Ravi River in themodern-day province of Punjab, Pakistan, exhibit a local range of0.7158e0.7189 (Kenoyer et al., 2013). Furthermore, while no faunal87Sr/86Sr ratios are currently available for Mesopotamia, two hu-man values from the Royal Cemetery at Ur span 0.7080e0.7081(Kenoyer et al., 2013). Taken together, these three regions displaydifferent local strontium ratios, making comparative assessmentsof geographic origins appropriate.

Along with strontium, stable oxygen (d18O) isotopes can offerclues into temporal changes in geographic residence among past

Fig. 1. Geologic map of the United Arab Emirates illustrating the locations of mortuarysites dating to the Umm an-Nar and Wadi Suq periods discussed in this study.


populations. Oxygen isotope ratios are influenced by a multitude ofcomplex variables related to water and the hydrologic cycle,including altitude, air temperature, humidity, and distance fromthe coast (White et al., 2000). The unique combination of thesevariables at a given locale produces meteoric water with d18O sig-natures distinct from other areas. In humans, d18O values incor-porated into bone and enamel hydroxyapatite are largelyinfluenced by drinking water (Dupras and Schwarcz, 2001; Luzet al., 1984; Luz and Kolodny, 1985). At a constant body tempera-ture of 98.6�F, body water and bone d18O values are in equilibrium,so that the d18O ratios incorporated into the skeleton reflect thegeographic area in which they were ingested (Luz et al., 1984;Sponheimer and Lee-Thorp, 1999).

Additional fractionation of oxygen isotopes occurs duringbreastfeeding. As water is ingested by humans and becomes bodywater, a subsequent enrichment in 18O isotopes takes place, withlighter 16O isotopes preferentially lost as water vapor is exhaled(White et al., 2004; Williams et al., 2005; Wright and Schwarcz,1998). Because body water acts as the source from which breastmilk is created, breast milk correspondingly possesses elevatedd18O values relative to drinking water that is passed on to infantsuntil weaning (i.e., the cessation of breastfeeding) is complete(Wright and Schwarcz, 1998). Subsequently, throughout the dura-tion of the weaning process, subadult d18O values graduallydecrease with the inclusion of environmental water sources untiladult d18O ratios are reached (Dupras and Tocheri, 2007). However,unlike the more extreme fractionation of nitrogen isotopes as aresult of breastfeeding (although other physiological factors can beat play here as well), which results in higher d15N values in the bonecollagen of breastfeeding infants by approximately 2.0e3.6&(Dupras et al., 2001; Katzenberg et al., 1996; Schurr, 1997, 1998;Schurr and Powell, 2005), d18O values appear elevated by only0.5e0.7& (Dupras and Tocheri, 2007; White et al., 2000; Wrightand Schwarcz, 1998).

Both strontium and oxygen isotopes are integrated into humanteeth during enamel formation in childhood, and because enameldoes not remodel, represent biogeochemical indicators ofgeographic residence at a young age. The development of humanpermanent first molars begins in utero and continues until thecrowns have completely formed at approximately 2.5e4.5 years ofage (AlQahtani et al., 2010; Hillson, 1996). Permanent second mo-lars begin to form around ages 2.5e3.0 and are complete between7.0 and 8.5 years of age, while the initiation of crown formation inpermanent third molars takes place at approximately 7e10 years ofage, with crown completion occurring around ages 12e16(AlQahtani et al., 2010; Hillson, 1996).

Isotopic values of dental enamel are then compared againstratios local to the burial environment to determine if residentialmobility had taken place. However, assessing locality differs be-tween strontium and oxygen isotopes. Strontium bioavailability isevaluated using the mean 87Sr/86Sr ratio generated by local faunasharing the same environment as the humans under study, �2standard deviations (Bentley, 2006). Fauna permit a broader eval-uation of biologically available strontium than geologic or plantsampling because their consumption of local food resources resultsin the homogenization of 87Sr/86Sr values (during enamel forma-tion) that may vary within a single site because of variable geology(Bentley et al., 2004; Price et al., 2002; Sillen et al., 1998). As such,87Sr/86Sr ratios from archaeological fauna provide a local baselineagainst which human values may be assessed.

Conversely, fauna may not be used as a baseline for local d18Osignatures, primarily due to the considerable variability in d18Ovalues exhibited by animals as a result of the complex interplaybetween diet, metabolism, body size, and drinking behavior.Instead, modern precipitation maps enable an estimate of d18O

ratios that may have been available to past human communities. Inthe Oman Peninsula, arid climatic conditions coupled with littleannual rainfall produce d18Odw(VSMOW) signatures spanning �4.0e0.0& (Waterisotopes.org), while bottled water (�1.9&) valuessourced from the Emirate of Abu Dhabi corroborate these elevatedsignatures (Bowen et al., 2005). Bronze Age human dental enamel(n¼ 114) sampled from across the Emirates generally support thesemodern ranges, with converted human ratios (see Chenery et al.,2012) displaying a mean d18Odw value of �3.6 � 1.3& (1s;d18Oc(VDPB) ¼ �2.5 � 0.8&), although lower values from multiplesites are evident and will be discussed further below (Gregoricka,2013b).

Outside of southeastern Arabia, human enamel d18Oc(VPDB) ratiosfrom Bronze Age Harappa (n ¼ 32; �4.8 � 0.9&, 1s) and Ur (n ¼ 2;�3.4 � 0.9&, 1s) confirm that some overlap may exist between theUAE (n¼ 114;�2.5� 0.8&, 1s) and its trading partners in the IndusValley and Mesopotamia (Kenoyer et al., 2013). Correspondingly,for modern precipitation, these locations have yieldedmean d18Odwvalues of�4.6� 0.5& and�3.9� 1.0&, respectively (Bowen, 2012;Bowen and Revenaugh, 2003). While variability between thesemeans is present, such small differences make it difficult to detectindividuals of non-local geographic origin using stable oxygenisotopes alone; the usefulness of d18O ratios in this region is thuslimited at best. Subsequently, oxygen isotope values will beexamined in conjunction with strontium isotope analysis.

4. Regional geology of the Emirates

Numerous geologic zones characterize southeastern Arabia.Fromwest to east, these include coastal sabkhas, sand dunes, gravelfans, and the Hajar Mountains, all of which are discussed in furtherdetail below.

The geology of the United Arab Emirates is clearly dominated byQuaternary sediments (Fig. 1). Along the arid western coastline ofthe peninsula, low-lying supratidal zones known as sabkhas, or saltflats, consist predominantly of carbonate sediments, includingcalcareous but also gypsiferous silt and sand (Glennie,1998; Goudie

http://Waterisotopes.org


et al., 2000). Seawater 87Sr/86Sr values off the shoreline of theEmirate of Abu Dhabi (0.70915) closely resemble expected valuesfor modern seawater (0.70923), and carbonate samples taken fromcoastal sediments along the sabkha reflect this marine influence asfar inland as 4.4 km due to cyclic coastal recharge, generating87Sr/86Sr ratios ranging from 0.70909 to 0.70913 (Müller et al.,1990). While these salt flats border the eastern coastline of theEmirates as well, the Hajar Mountains to the immediate west resultin only a narrow strip of sabkha in this region.

The sand dunes of the Rub’ al-Khali e the largest sand desert onEarth e extend from southern Arabia into the northern Emirates,adjacent to the coastal sabkhas in the west before ending at thealluvial fans of the Hajar Mountains in the east (Glennie, 1998).Shaped by Quaternary aeolian deposits, the landscape of the dunesis interrupted by small areas of exposed calcareous sandstone aswell as by marl and sandy limestones representative of Tertiary-period evaporite sequences (Government of the UAE Ministry ofPetroleum and Mineral Resources, 1979). Widespread Quaternarydeposits along the western foothills of the Hajar Mountains reflectthe presence of ancient wadi systems, once-active streambeds thatresulted in the accumulation of sediment eroded from the HajarMountains and transported by flowing water to form alluvial fans(Al Farraj, 1995; Parker and Goudie, 2008). Some of these deposits,which are largely composed of dolomite, limestone, and chert, havebeen dated from the Plio-Pleistocene to approximately 30,000years ago (Al Farraj and Harvey, 2004; Juyal et al., 1998; Parker et al.,2006b; Sanlaville, 1992).

Finally, the Hajar Mountains dominate the eastern Emirates andrepresent the most complex and geologically variable region of theOman Peninsula (Fig. 1). To the north, the CretaceouseJurassicdolomitic and gray limestones comprise the Musandam Group,while Triassic and Permian intrusions of limestone, dolomite, shale,chert, and marl characterize the eastern portion of the range(Government of the UAE Ministry of Petroleum and MineralResources, 1979; Mery, 1991; Parker and Goudie, 2008). Incontrast, the more geologically heterogeneous mountains of thesouth consist primarily of Silutrian ultrabasic and gabbro rockswith small limestone, sandstone, granite, and metamorphic per-meations (Government of the UAE Ministry of Petroleum andMineral Resources, 1979; Parker and Goudie, 2008).

5. Sample descriptions

Archaeological enamel from Bronze Age fauna (n ¼ 39) in theUnited Arab Emirates was utilized to generate a local 87Sr/86Srbaseline as part of a previous study (Gregoricka, 2013a). Faunawereselected from both domestic andmortuary contexts, including fromthe third and second millennia BC settlements of Tell Abraq(n¼ 12), Shimal (n¼ 9), and Umm an-Nar Island (n¼ 15), as well asthe tombs at Unar 1 (n ¼ 1), Dibba 76 (n ¼ 1), and Qidfa 4 (n ¼ 1).

Of the 114 commingled individuals sampled for a morecomprehensive analysis of mobility patterns using a single molar

d18OcðVSMOWÞ ¼�1:03091� d18OcðVPDBÞ

�þ 30:91 Coplen et al:; 1983

d18Odw ¼�1:590� d18OcðVSMOWÞ

�� 48:634 Chenery et al:; 2012

(Gregoricka, 2013a, 2013b), only 19 of these teeth were both (a)located in situ within a fragment of the jaw, and (b) were accom-panied by another in situ molar so that temporal changes might beassessed at the level of the individual. To prevent repetitive

sampling of the same individual, samples were extracted from thesame dental quadrant for each tomb (e.g., right mandibular molarsonly). These individuals were recovered from Early and MiddleBronze Age tombs at Umm an-Nar Island in the Emirate of AbuDhabi (n ¼ 5), Unar 1 at Shimal in the Emirate of Ra’s al Khaimah(n¼ 3),Mowaihat TombB in the Emirate of Ajman (n¼ 6), Tell Abraqin the Emirate of Sharjah (n ¼ 1), and three tombs in the Emirate ofFujairah, including Bidya 1 (n ¼ 1), Qidfa 4 (n ¼ 1), and Dibba 76(n ¼ 2) (Fig. 1).

6. Methodology

The surfaces of all teeth were mechanically abraded using a car-bide drill bit attached to a Dremel Tool, eliminating surface enamelsusceptible to diagenesis (Budd et al., 2000). Following this cleaning,3e5mg of powdered dental enamel from each tooth were removedfor sampling for both strontium and oxygen isotope analysis.

Sample preparation of enamel for strontium isotope analysiswas drawn from Perry et al. (2008, 2009). Samples were preparedand analyzed in the Department of Geological Sciences at theUniversity of North Carolina at Chapel Hill Isotope GeochemistryLaboratory. Enamel powder was dissolved in 3.5 M HNO3 beforecolumn extraction of strontium using EiChrom Sr-Spec resin.Extracted samples were treated with 0.1 M H3PO4, dried down, andre-dissolved in TaCl5 for preparation of sample placement onto Refilaments. After liquid samples were dried using an electrical cur-rent, stable isotope ratios were analyzed on a VG Micromass Sector54 thermal ionization mass spectrometer (TIMS) in quintuple-collector dynamic mode. To correct for mass fractionation, an in-ternal ratio of 86Sr/88Sr ¼ 0.1194 was used. Ratios are reportedrelative to a value of 0.710270 � 0.000014 (2s) for the NBS-987standard. Internal precision for strontium runs is typically�0.000012e0.000018% (2s) standard error based on 100 dynamiccycles of data collection.

Sample preparation of enamel carbonate for oxygen isotopeanalysis was drawn from Krigbaum (2003) and Garvie-Lok et al.(2004). Enamel powder was treated with 2% NaOCl and later rinsedtoneutrality.Diageneticcontaminants andadsorbedcarbonateswerethen removed by soaking samples in 0.1 M acetic acid. After lyophi-lization for 48 h, carbonate samples were analyzed for d18Oc(VPDB)usinganautomatedCarbonateKieldevice coupled toaFinniganDeltaIV Plus stable isotope ratio mass spectrometer at The Ohio StateUniversity Stable Isotope Biogeochemistry Laboratory. Samples wereacidified under vacuum with 100% ortho-phosphoric acid, theresulting CO2 cryogenically purified, and normalized using NBS-19.The standard deviation of repeated measurements of an internalstandard was �0.06& for d18O. Non-parametric statistical analyseswere conducted using Statistical Analysis Software (SAS) version 9.3.

d18Oc(VPDB) values were converted to drinking water (d18Odw)ratios using equations derived by Coplen et al. (1983) and Cheneryet al. (2012) so that ancient human values might be directlycompared to local d18Odw precipitation values (Table 1):

Recent investigations into the error introduced by theseregression equations have brought to light the considerable un-certainty involved in calibration (Chenery et al., 2012; Kendall et al.,2013; Pollard et al., 2011). Pollard et al. (2011) estimate error

Table 1Strontium and oxygen isotope ratios for Bronze Age human enamel samples from the United Arab Emirates. Converted oxygen (VSMOW) isotope ratios are also provided.

Individual Site Tooth 87Sr/86Sr D87Sr/86Sr d18Oc(VPDB) d18Odw(VSMOW) Dd18Odw(VSMOW)

UaN 123/124 Umm an-Nar Island I M1 0.70891 0.00002 �1.9 �2.7 0.5M3 0.70893 �2.3 �3.2

UaN 125/126 Umm an-Nar Island II M1 0.70890 0.00001 �2.2 �3.2 1.5M2 0.70891 �3.2 �4.7



UaN 144/145 Umm an-Nar Island V M1 0.70892 0.00001 �2.3 �3.3 1.6M3 0.70893 �1.4 �1.7

TA 186/187 Tell Abraq M1 0.70887 0.00000 �2.5 �3.6 0.1M2 0.70887 �2.4 �3.5

MW 190/191 Mowaihat Tomb B M1 0.70887 0.00000 �2.0 �2.8 1.1M3 0.70886 �2.7 �3.9






RAK 226/227 Unar 1 M1 0.70882 0.00001 �3.5 �5.3 0.3M2 0.70881 �3.4 �5.0

RAK 233/234 Unar 1 M1 0.70875 0.00001 �2.8 �4.0 1.4M2 0.70876 �3.6 �5.4

RAK 239/240 Unar 1 M1 0.70882 0.00008 �3.0 �4.4 1.9M2 0.70890 �4.1 �6.3

Bid 245/246 Bidya 1 M1 0.70864 0.00041 �1.9 �2.6 0.1M2 0.70823 �1.8 �2.5

Qid 250/251 Qidfa 4 M1 0.70867 0.00002 �2.3 �3.3 0.1M2 0.70870 �2.4 �3.4

Dib 252/253 Dibba 76 M2 0.70880 0.00003 �2.7 �3.9 0.3M3 0.70883 �2.5 �3.6

Dib 254/255 Dibba 76 M1 0.70906 0.00030 �2.6 �3.8 1.4M2 0.70876 �1.8 �2.4


ranging from �1.0 to 3.5& (95% CI) for four common conversionequations (but do not include Coplen et al., 1983 in their assess-ment); similarly, while Chenery et al. (2012) improve these cali-brations by eliminating the need for multiple equations to convertd18Oc(VSMOW) to d18Odw, they nevertheless report an overall uncer-tainty of �1.0& (2s) for the equation they generate. Clearly, then,such error can have a substantial impact on the interpretation ofconverted ratios. Unfortunately, because local oxygen isotopebaselines for geographic areas are determined by drinking watervalues, such conversions are necessary until more extensive d18Opor d18Oc datasets can be produced (Pollard et al., 2011).

7. Results

A comprehensive listing of Bronze Age faunal strontium ratiosfrom the United Arab Emirates has been reported elsewhere(Gregoricka, 2011, 2013a). On Umm an-Nar Island, faunal enamelrecovered from the settlement site exhibit a 87Sr/86Sr range of0.70861e0.70897, with an associated mean of 0.70879 � 0.00009(1s). At Shimal, fauna both within the third millennium tomb ofUnar 1 as well as the nearby second millennium settlement displayan average 87Sr/86Sr ratio of 0.7088 � 0.0001 (1s) and range invalue from 0.7087 to 0.7090. In the Emirate of Sharjah, faunal teethcollected from the settlement near the Umm an-Nar tomb of TellAbraq produced a local range of 0.70866e0.70889 and a mean87Sr/86Sr value of 0.70878 � 0.00006 (1s). Finally, on the easterncoast of the Emirates, the second millennium tombs of Qidfa 4 andDibba 76 included fauna showing an average 87Sr/86Sr ratio of0.7086 � 0.0001 (1s), with values spanning 0.7085e0.7087. All

together, these fauna produce a local 87Sr/86Sr range of 0.70859e0.70896 and a mean value of 0.70878 � 0.00009 (1s).

Human strontium and oxygen isotope values are listed in Table 1and illustrated in Figs. 2e4. Overall, for the first tooth sampled (M1/M2), individuals exhibited an average 87Sr/86Sr ratio of0.7088 � 0.0001 (1s), while the second tooth sampled (M2/M3)showed a mean value of 0.7088 � 0.0002 (1s). Average 87Sr/86Srratios by tooth type forM1 (0.7088�0.0001),M2 (0.7088�0.0002),and M3 (0.70884 � 0.00008) did not vary significantly (KruskaleWallis; p ¼ 0.15). For oxygen, the first tooth sampled (M1/M2) dis-played amean d18O ratio of�2.3�0.6& (1s),while the second toothsampled (M2/M3) possessed an average value of �2.6 � 0.7& (1s).Like strontium,mean d18O ratios by tooth type forM1 (�2.3�0.6&),M2 (�2.8 � 0.7&), and M3 (�2.4 � 0.5&) did not vary significantly(KruskaleWallis; p ¼ 0.21).

8. Discussion

Themajority of individuals sampled exhibit relatively consistentstrontium and oxygen values throughout the life histories exam-ined here, as measured by the sequential sampling of two molarsfrom the same individual in a commingled context. These ratios, aswell as the presence of a few outlying values, are discussed furtherbelow.

8.1. Strontium isotopes

All but two individuals (Bid 245/246 and Dib 254/255) displayedtemporal changes in enamel 87Sr/86Sr ratios differing by less than

Fig. 2. Inter-tooth strontium isotope ratios from archaeological human dental enamel from the United Arab Emirates. The area between the dotted lines represents locally definedranges of bioavailable strontium using Bronze Age fauna (Gregoricka, 2011).


0.0001 (Table 1, Fig. 2). Of this majority, only one value falls justoutside of the locally defined range for strontium in the Emirates, asdetermined by archaeological fauna. The third molar of this indi-vidual (UaN 128/129) shows an elevated ratio of87Sr/86Sr ¼ 0.70900; however, this value is not considered anoutlier or indicative of residential mobility over time because of this

Fig. 3. Converted (VSMOW) inter-tooth oxygen isotope ratios from archaeological human dresents expected local ranges (d18Odw ¼ �4.0e0.0&) based on modern precipitation (Wate(�1.0e3.5&) involved in the use of conversion equations (Pollard et al., 2011).

tomb’s proximity to the coast. Seawater possesses a strontiumvalueof 0.70923, and as the inhabitants of Umm an-Nar Island frequentlyconsumed marine resources as suggested by zooarchaeologicalevidence (Hoch, 1979), human enamel 87Sr/86Sr values from thoseinterred in tombs on the island were expected to produce highersignatures than those more reliant on terrestrially-based sources

ental enamel from the United Arab Emirates. The area between the dotted lines rep-risotopes.org). Results should be treated with caution due to the analytical uncertainty

http://Waterisotopes.org

Fig. 4. Radiogenic strontium and stable oxygen (c-VPDB) isotope ratios for archaeological human tooth enamel from Umm an-Nar and Wadi Suq sites in the United Arab Emirates.


on the mainland. Subsequently, this individual appears to haveconsumed more littoral-based products over time than othersresiding on Umm an-Nar Island. In addition, the similarity betweenthis strontium ratio and that of the associated first molar from thesame individual suggests that mobility did not play a significantrole in the production of this elevated value.

An overall lack of inter-tooth isotopic variability for 17 of the 19individuals sampled suggests that mobility did not generally in-crease with age, despite expectations that interregional trade and/or transhumant patterns of subsistence would produce moremovement or migration in later stages of life. While incongruouswith the region’s involvement in such exchange systems, thesebiogeochemical patterns fit with what is known about changingsubsistence strategies in the region during the Umm an-Nar period.Major transitions from the preceding Hafit (ca. 3100e2700 BC) tothe Umm an-Nar included a growing reliance on domesticatedplants such as the date palm as fresh water wells were utilized tosupport oasis agriculture, which in turn permitted the inhabitantsof southeastern Arabia during the third millennium to becomeincreasingly sedentary (Potts, 1993; Cleuziou, 1996; Tengberg,2003). In addition to these palm gardens, coastal gathering and asteady supply of maritime resources provided additional incentivetowards a sedentary way of life, although with the exception ofUmm an-Nar Island, marine foods appear to have played a sup-plementary role in diet relative to terrestrial, agro-pastoral sources(Gregoricka, 2013b; Hoch, 1979; Uerpmann, 2001).

Of the 15 individuals sampled from the Umm an-Nar period,approximately half of these (n ¼ 7) are represented by a M1/M2sequence; in heavily commingled assemblages, it is unsurprisingthat adjacent molars survive together. Nevertheless, in an exami-nation of individual life histories, such a temporal comparison maynot be as meaningful as an evaluation of change between M1/M3,which represents a greater span of time between crown completion(4.5 to up to 16 years of age, a difference of about 11.5 years) relativeto M1/M2 (3 to up to 8 years of age, a difference of about 5 years).Particularly when bone is too poorly preserved for analysis, as is thecase in southeastern Arabia (a product of the extreme aridity of theregion), third molars also provide the latest-forming skeletal tissue

approaching adulthood and thus represent a more powerful meansof assessing changes in residential mobility over time. Conse-quently, it remains possible that at least some of the Early BronzeAge inhabitants of the Oman Peninsula did not engage in mobilebehavior until after the formation of M2 (or possibly even M3)enamel crowns.

Interestingly, the most variability between enamel samples wasassociated with individuals interred in tombs dating to the WadiSuq period. Two Middle Bronze Age inhabitants of the Emirate ofFujairahe Bid 245/246 and Dib 254/255e each exhibit one deviantstrontium value consistent with residential mobility, althoughthese occur at different stages in the life history of these individuals(Fig. 2). From the Wadi Suq tomb at Bidya 1, individual Bid 245/246depicts an intriguing pattern; while the strontium value of the firstmolar (87Sr/86Sr ¼ 0.70864) falls well within the local range, thesecond molar (87Sr/86Sr ¼ 0.70823) deviates from this range, bothfor Fujairah and for the Emirates as a whole. This pattern is unusualin that the individual appears to have spent the first three years oflife in the area, but that shortly thereafter e between approxi-mately 3 and 7e8 years of age e he/she migrated to a regiongeologically dissimilar (but not necessarily a great distance) fromthat of Fujairah. Nevertheless, at some point in this individual’s life(or death), he/she returned to Bidya and was interred in a localtomb. This case indicates that residential mobility was not limitedto adults, but involved children as well.

However, the inter-tooth difference (0.00041) from individualBid 245/246 is not particularly substantial, making it necessary toconsider other environmental or anthropogenic influences thatmay have played a role in the production of the low 87Sr/86Sr ratioexhibited by the second molar relative to the first. Irrigation is anunlikely cause of this difference in value, as falaj irrigation was notdeveloped in southeastern Arabia until the subsequent Iron Age(Potts, 2001). If utilized, fertilizers would have almost certainlyconsisted of manure from local fauna, so that non-local 87Sr/86Srsignatures would not be introduced. Windborne sea spray can alsoinfluence the values of terrestrial areas, which might signal a shiftin diet for this individual from more marine-based or coastally-grown foods to one more reliant on inland resources further


removed from sea spray. Nonetheless, the considerable homoge-neity characterizing the majority of human 87Sr/86Sr values frommultiple sites across the Oman Peninsula in both the third andsecond millennium BC, coupled with the correspondingly limitedlocal ranges generated by fauna, lend support to the idea thatsecond molar enamel crown formation took place in an area with aslightly different geologic makeup.

A similar non-local 87Sr/86Sr value of 0.70818 was also notedfrom Tell Abraq (TA 165; Gregoricka, 2013a), although only onemolar for this individual was available for sampling so that tem-poral change in residence could not be evaluated in this study. Bothoutlier values from Bidya 1 and Tell Abraq approach human87Sr/86Sr ratios reported for Ur in Mesopotamia (0.7080e0.7081;Kenoyer et al., 2013) and for the (Dilmun) A’ali Burial Mounds inBahrain (0.7082e0.7084; Gregoricka, 2011), a somewhat surprisingconclusion given the supposed collapse of interregional relationsbetween these powers and the Oman Peninsula during the secondmillennium BC, particularly as Dilmun appears to have supplantedsoutheastern Arabia as the Gulf’s primary entrepôt for the distri-bution of copper (Carter, 2003; Crawford, 1998; Weeks, 1997).

A second non-local (Dib 254/255) from the Wadi Suq tomb ofDibba 76 was also identified, but here, the deviant ratio occurs inthe first molar (87Sr/86Sr ¼ 0.70906), whose elevated value fallsoutside of the local range of Fujairah as well as the remainder of theUAE (Fig. 2). Conversely, the crown of the second molar, whichcompletes formation 4e5 years after that of the first molar, exhibitsa value (87Sr/86Sr ¼ 0.70876) consistent with local ranges as well asother human values from the same tomb and region. Correspond-ingly, this pattern represents an individual of nonlocal origin whosettled in Fujairah sometime after early childhood and who waseventually interred there. Interestingly, the strontium ratio of thefirst molar closely approaches signatures reported from Umm an-Nar Island on the western coast of the Emirates, and as such, maybe indicative of a regional and not interregional migrant whoseearly childhood diet emphasized maritime resources such as fishand shellfish, foods that (isotopically) did not appear to be a majorcomponent of diet for the second millennium communities whoburied their dead along the eastern shorelines of the Emirates(Gregoricka, 2013a, 2013b). While the regionally-based isotopicvariability exhibited by Dib 254/255 may point to an increase inmobility among Wadi Suq communities e possibly related to theadoption of a more nomadic lifestyle e this lone example is notcorroborated by strontium ratios from other Wadi Suq individuals,and more sequential data are needed before definitive conclusionscan be drawn from biogeochemical evidence.

More broadly, a comparison of individual life histories betweenthe Umm an-Nar (Umm an-Nar Island, Tell Abraq, Mowaihat, Unar1) and Wadi Suq (Bidya 1, Qidfa 4, Dibba 76) periods reveals that econtrary to archaeological evidence suggestive of a collapse in traderelations and subsequent cultural isolation e there are more non-locals in the later period. Sample size clearly plays a role in thisinterpretation; with only 19 individuals possessing multiple molarsfor assessment, this study provides only a very small window intoeveryday life in southeastern Arabia during the third and secondmillennia BC. Nevertheless, despite both a smaller sample size (4 of19 individuals sampled in this study) and the sequential samplingofM1/M2 (n¼ 3) andM2/M3 (n¼ 1) instead of a larger span of timerepresented by contrasting enamel from M1/M3, only individualsfrom the Wadi Suq period exhibited evidence of individualmigration events.

This is not to suggest that the Oman Peninsula was uninvolvedin interregional trade across the Gulf during the Umm an-Narperiod. In fact, nonlocals from both the Umm an-Nar and theWadi Suq have been identified using radiogenic strontium andstable oxygen isotope values from single, isolated molars, although

these are outnumbered by local interments (Gregoricka, 2013a,2013b). Together with this dataset, the sequential data presentedhere suggest that depicting theWadi Suq as a time of ‘collapse’ failsto recognize the continuity between the third and secondmillenniain the maintenance of socioeconomic relationships. Examples ofregional and interregional migrants not only entering but leavingthe region suggests that mobility did play a role in the lives of thosecomprising local communities.

8.2. Oxygen isotopes

For stable oxygen isotopes, inter-tooth variability was less than1.9& for all individuals sampled, with a mean difference (Dd18O) of0.9 � 0.6& (1s) (Table 1, Fig. 3). Even after recognizing knownfractionation rates (þ0.5e0.7&) for oxygen isotopes as a result ofbreastfeeding, which takes place during M1 enamel formation andsubsequently affects M1 d18O values, the relatively homogeneousnature of these values suggests that water was acquired fromisotopically similar sources over time. Like strontium, then, thesesequential d18O ratios indicate that residential mobility was not amajor part of everyday life during periods of enamel formation.

The most variable d18O values come from individuals interredwithin the third millennium BC tombs on Umm an-Nar Island (seeTable 1), with a mean difference of 1.2 � 0.4& (1s). Such variabilityis perhaps unsurprising given the nature of human occupation atthe site. Because this island possessed no source of fresh water, itsinhabitants would have been responsible for transporting water epossibly from different locales with slightly different d18O values eto its settlement (Frifelt, 1991). Additionally, as a likely trading postdue to its strategic, centralized location in the Gulf, Umm an-NarIsland may have attracted individuals from across southeasternArabia to participate in the production of goods for trade andtransport, individuals that may have originated from different lo-cales with disparate (but still local) d18O ratios.

It is evident that estimates of local d18O ratios (�4e0&) frommodern precipitation maps do not fully encompass the range ofvalues present in the Bronze Age. Instead, lower values from mul-tiple sites suggest that conditions were less arid than today. Whileconverted human d18Odw values from the Emirates (n ¼ 114;�3.6 � 1.3&, 1s) fall into this modern range, the variability evidenthere (�6.3e�1.3&) suggests that determining locality from mod-ern meteoric water may be problematic, and that archaeologicald18O ratios from humans may be more appropriate in estimatinggeographic origins (e.g., see Knudson and Tung (2011) for a similardiscussion of defining local strontium ranges using human values).Of course, this range encompasses the whole of the Emirates anddoes not account for patterns of site-specific d18O variability; forinstance, the mean d18Odw ratio for Tell Abraq(d18Odw ¼ �3.1 �1.4&, 1s) differs considerably from that of Unar 1(d18Odw ¼ �5.2 � 1.0&, 1s) due to the acquisition of water fromisotopically variable sources (Gregoricka, 2013b).

Alone, these d18O values could not be used to identify nonlocalmigration to or from the region. However, in conjunction withstrontium isotope ratios, these data can be further evaluated. Forinstance, individual Dib 254/255 e identified as a nonlocal by anelevatedM187Sr/86Sr value relative to a local M2 87Sr/86Sr signaturee possessed d18O ratios that differed by 1.4&, one of the largestchanges in d18O value to take place of any individual represented inthis study. While this difference falls within a 2& span of valuesexpected for local populations (Kenoyer et al., 2013), its breadthdoes fit with a similar change in strontium over time, and lendssupport to the conclusion that this individual is a non-local(although possibly a regional, and not interregional) migrant tothe Emirate of Fujairah. Conversely, individual Bid 245/246exhibited very little difference (0.1&) in d18O value between M1


and M2, despite a considerable change in 87Sr/86Sr ratios thatapproached local 87Sr/86Sr ranges observed at Ur and fell directlyinto ranges reported in Bahrain. Nevertheless, because of similar-ities in mean d18O values between southeastern Arabia(�2.5 � 0.8&), Ur (�3.4 � 0.9&), and Bahrain (�2.9 � 0.5&;Gregoricka, 2011), a relative absence of M1/M2 change does notexclude the possibility of residential mobility during childhood.

9. Conclusions

Radiogenic strontium and stable oxygen isotopes provide anincredible tool for examining patterns of mobility in the past at thelevel of the individual. In particular, by utilizing skeletal tissues thatform at different times, the life history of a single individual may berevealed. For bioarchaeologists working with fragmentary and/orcommingled assemblages, traditional methods of data collectionmay not be possible, yet valuable information about past behaviorcan still be gleaned from these remains. The sequential isotopicanalysis of such tissues offers a new way of answering questionsabout commingled interments, where evaluating individual lifehistories is not otherwise possible.

An overall lack of isotopic variability for both strontium andoxygen isotopes from associated molars suggests that residentialmobility during enamel formation (between early childhood andlate adolescence) did not generally play a significant role in sub-sistence strategies such as transhumant pastoralism or in socio-economic interaction as part of interregional exchange networks.Two individuals from the later Wadi Suq period demonstratedisparate inter-tooth 87Sr/86Sr ratios likely indicative of somemigration event; while this calls into question the supposed cul-tural isolation of the region during the second millennium BC, thehomogenous nature of isotope values from the majority of in-dividuals sampled here is indicative of a population that was nothighly mobile. This assessment fits with a transition in subsis-tence practices to oasis agriculture (and correspondingly, a moresedentary way of life) in the earlier Umm an-Nar period, but is notconsistent with speculation that local communities abandonedagriculture and adopted a more nomadic way of life in the sub-sequent Wadi Suq period in response to climate change and thecollapse of interregional trade. Additional sequential sampling,particularly from individuals interred in Wadi Suq tombs, andmore comparative regional samples are required to further definelocality across the Persian Gulf, Central Asia, and the Indiansubcontinent.

Acknowledgments

This research was supported by a National Science FoundationDoctoral Dissertation Research Improvement Grant (BCS-0961932),the Philanthropic Educational Organization (PEO) Scholar Award,the Ruggles-Gates Fund for Biological Anthropology, a Sigma XiGrant-in-Aid of Research, an Ohio State International Affairs Grant,and an Ohio State Alumni Grant for Graduate Research and Schol-arship. The following individuals also deserve recognition for theirsupport and assistance: Deb Martin, Christian Velde, Imke Moel-lering, Dan Potts, Michele Ziolkowski, Johanna Olafsdotter, SabahJasim, Margarethe and Hans-Peter Uerpmann, Richard Meadow,Flemming Højlund, Kim Aaris-Sørensen, Carl Phillips, Ali Moham-med Al Matroushi, Clark Larsen, Joy McCorriston, Paul Sciulli, JuliaGiblin, and Erica Chambers. Special thanks also goes to DrewColeman and the University of North Carolina at Chapel Hill IsotopeGeochemistry Laboratory, and to Andrea Grottoli and Yohei Matsuiat The Ohio State University Stable Isotope Biogeochemistry Labo-ratory. Finally, the author would like to thank the three anonymousreviewers whose comments improved this paper.

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assessing life history from commingled assemblages: the biogeochemistry of inter-tooth variability...

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