Journal of Archaeological Science (2002) 29, 341–349doi:10.1006/jasc.2002.0718, available online at http://www.idealibrary.com on

U-series Dating of Fossil Teeth and Carbonates fromSnake Cave, Thailand

Massimo Esposito

Laboratorio di Radiodatazione, ENEA, Via dei Colli 16, 40136 Bologna, Italy and Dipartimento di Fisica‘‘E. R. Caianiello’’, Universita di Salerno, Via S. Allende, 84081 Baronissi (Salerno), Italy

Jean-Louis Reyss

Laboratoire des Sciences du Climat et de l’Environnement, Laboratoire mixte CNRS/CEA, Domaine du CNRS,av. de la Terrasse, 91198 Gif-sur-Yvette Cedex, France

Yaowalak Chaimanee

Paleontology Section, Department of Mineral Resources, Rama VI Road, Bangkok 10400, Thailand

Jean-Jacques Jaeger

Institut des Sciences de l’Evolution, UMR 5554, CNRS-Universite de Montpellier II, Place Bataillon,34095-Montpellier Cedex 05, France

(Received 23 October 2000, revised manuscript accepted 28 April 2001)

Snake Cave (Thailand) is an important site for the study of the evolution in Southeast Asia, leading to a discovery ofrich and various faunas, including a human tooth. Fossils of large and small mammals converge to indicate, on the basisof faunal assemblage, a late Middle Pleistocene age for the fossiliferous layers; on the other hand, previous U-seriesdating of carbonates furnished a lower limit for the fossils of about 130 ky.

In order to achieve a better comprehension of the chronology of the cave, the Uranium-series method for absoluteage determination has been applied to some carbonates older than those studied previously and to fossils themselves.

We used the isochron technique approach to yield valuable ages because spleleothem samples consist in a mixture ofcalcite and detrital contamination. The results obtained suggest that the main fossiliferous level is older than about160 ky. The application of the U-series method to tooth samples from this tropical cave leads to erroneous ages,probably because of post depositional groundwater movements. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: U-SERIES DATING, THAILAND, CARBONATES, ISOCHRON TECHNIQUE, TEETH.

Introduction

T he Snake Cave is located in the Kon Sandistrict, Chaiyaphum Province, NortheasternThailand. It is a karstic cavity, opened in

Permian marine limestone. The cave entrance lies at anelevation of about 10 m above the surrounding alluvialplain. It consists of several interconnected chambers(Figure 1), three of them (Main Entrance, Main Layerand Upper Layer) have provided faunal remains, con-sisting mostly of teeth of large and small mammals.The Main Entrance Room is situated about 6 m belowthe present day entrance of the cave. A large quantity

3410305–4403/02/$-see front matter

of sediments (about 1 m in thickness) from this cham-ber had been excavated by villagers for use as fertilizer.The Main Layer, which lies about 2 m below the MainEntrance floor in a narrow cavity filling, correspondsto a rich fossiliferous sediment in continuity with thedeposits of the Main Entrance Room. The sedimentswere collected on about 3 m of thickness. As they weredecalcified, because of water percolation, they could bescreened much more easily and have therefore yield alarge amount of fossils. The Upper Layer lies about15 m away from the Main Entrance and 6 m higherthan the Main Entrance floor. Several meters ofpure layered calcite overlie the fossiliferous calcareous

� 2002 Elsevier Science Ltd. All rights reserved.

342 M. Esposito et al.

sediments, which suggested the proximity of a previousentrance of the cave.

This cave was excavated during several field seasonsby the Thai-French Paleontological Project inThailand. It led to the discovery of a large amount ofmammalian fossils including a human tooth (Tougardet al., 1998) and of teeth of 31 species of largemammals and of 30 species of small mammals(Tougard et al., 1996; Ginsburg, Ingavat & Sen, 1982and Chaimanee, 1998). The fauna is similar to that ofSouth China and Vietnam where it is known under thename of Ailuropoda-Stegodon mammalian assemblageassociated with remains of Pongo (Ginsburg, Ingavat& Sen, 1982 and Tougard et al., 1996). This associationis characteristic of the late Middle Pleistocene. Also, adetailed study of the rodents indicates that these fossi-liferous sediments are of late Middle Pleistocene age(Chaimanee, 1998). Unfortunately, the absence ofstone artefacts does not allow to put it more precisely

into the Southeast Asian prehistoric cultural timescale. Rodents and large mammals have been collectedfrom the three mentioned ‘‘Rooms’’ and indicate anhomogenous fauna suggesting no significant agedifferences between the different fossiliferous layers.

Previous studies (Esposito et al., 1998) on thechronology of Snake Cave lead to reliable ages for BedI of Main Entrance and for the calcite bed of UpperLayer. Both calcite matrices were dated by U-seriesmethods using a isochron approach. In the MainEntrance, Bed I has been dated at 96�4 ky, while anage of 137�7 ky has been obtained for calcitic bed ofUpper Layer. Because of the homogeneity of faunalassemblages in all rooms, a minimum age of 130 ky hasbeen evaluated for fossils of Snake Cave.

In the present work we successfully applied the230Th/234U dating technique to Bed II of MainEntrance and we attempt the direct dating, by thesame technique, of fossil teeth from the cave. Resultsconfirm that the age of these faunas is definitely olderthan the upper limit of the time range covered byradiocarbon method.

Old entranceSketch-map

Main layer Upper room

Carbonate

AA Main entrance

Present entrance

Section A-ABed I

Bed II

Fossiliferous layer

Figure 1. Sketch-map of Snake Cave.

Theoretical BasisAt the time of formation Uranium is trapped inauthigenic carbonate, such as speleothem, which nor-mally behaves as a closed system with respect to U andTh isotopes thereafter (Schwarcz, 1980). However, insome cases, carbonates also contain amount of detritalcontamination, e.g. clay, that contributes with unpre-dictable amount of U and Th isotopes, leading there-fore to erroneous dating. To detect this contaminationone can use 232Th, which is present only in the detritalfraction. Therefore to find out the true age ofsamples a correction procedure is needed, based onleachate/residue (Ku & Liang, 1984), leachate/leachate(Schwarcz & Latham, 1989) or total sample dissolution(TSD) analyses (Luo & Ku, 1991; Bischoff &Fitzpatrick, 1991), where the whole sample is dissolvedduring acid treatment and the results of the analysis ofcoeval samples are used to determine the true age.Because of the easy measurements and of their reliabil-ity, we have applied the TSD method to our samples.

The isochrons were constructed by plotting 230Th/232Th versus 234U/232Th and 234U/232Th versus 238U/232Th activity ratios. The slopes of the plot correspondto the radiogenic 230Th/234U and 234U/238U ratios ofthe sample and hence the age. The only implicitcondition is that samples of same age had equal230Th/232Th activity ratio at the time of formation.

The great similarity of bones and teeth and thescarcity of detailed informations allowed the extrapo-lation of conclusions from bones to teeth and vice-versawithout a loss of meaning.

Uranium is not present in living bones andteeth, while it is contained in fossils owing to apost-deposition, but the uptake mechanism is not

U-series Dating of Fossil Teeth and Carbonates from Snake Cave, Thailand 343

completely understood (Millard & Hedges, 1995). Ucontents of bones and teeth are generally much greaterthan in authigenic carbonates.

The ideal model of uptake predicts U assimilationinto bones and teeth soon after burial (EU, EarlyUptake), which behaves as in a closed system there-after. This may be the case for younger samples, thosewithin the range of radiocarbon control for whichconcordance can be demonstrated between U/Th andradiocarbon dates (Bischoff & Rosenbauer, 1981;Leitner-Wild & Steffan, 1993). EU behaviour has alsobeen demonstrated for some older samples (Chen &Yuan, 1988).

In some other cases bones and teeth lead to veryyoung dates, which are interpreted as a sign of alater assimilation of Uranium (Bischoff et al., 1988).Because of the lack of knowledge about the rate of Uincorporation in samples, an ideal linear mechan-ism from the time of burial to the present (LU,Linear Uptake) has often been proposed. Bischoff,Rosenbauer & Moench (1995) have derived age equa-tions for the LU process. It is difficult to quantify theprocess of assimilation of Uranium into bones andteeth, because it also can be discontinuous and epi-sodic, and it does not yet exist a manner to quantifythis process. Chen & Yuan (1988) supposed a two stageprocess: a first Uranium uptake followed by a latersecond one. Moreover, Uranium may be lost frombone, and an excess of 230Th activity, when comparedto 234U activity, has been measured by Rae &Ivanovich (1986). The radiometric age obtained inthese samples would be older than the real age.

Finally, the fossil bone analysed by Ayliffe & Veeh(1988) turned out to be relatively enriched in 234U inrelation to coeval speleothems and the modern cavewaters, results also recognized by Leitner-Wild &Steffan (1993). As a consequence of the enrichment in234U, they realized that bones preferentially accumu-late 234U in the long run from the sediment pore fluidsin contact with them.

The main point of the subject is to find out a methodable to understand and discover the way by which bonesamples have accumulated Uranium since the time ofburial. Concerning with samples that are beyond therange of radiocarbon dating, a good criterion is thecomparison between the 230Th/234U and 231Pa/235Udecay systems within a single sample. Discordancebetween the two methods indicates that the system isopen and leads to rejection of the dates. On the otherhand, the concordance test is limited to about 130 ky,beyond which the 231Pa/235U system is at equilibrium.Moreover, apparent concordance, in the limits ofanalytical uncertainties, would occur for a samplewhich acquired a large concentration of Uranium at asingle event late in its history, and the concordant datewould reflect only the time elapsed since the event.Therefore, apparent internal 231Pa/230Th concordanceis often not a sufficient criterion. Another validity testfor any kind of radiometric dates is concordance

among coeval samples and stratigraphic succession ofdates within the same deposit. Nevertheless these arenot sufficient criteria for U-series dating because allteeth at a site may be affected to same relative degree.

Recently, Millard & Hedges (1996) have developed adiffusion–adsorption model of Uranium uptake bybone which is able to predict the spatial distribution ofUranium in bones and teeth in terms of age, sitehydrology and groundwater Uranium concentration.Even if U-series application to fossil bones and teeth isfar from reliable, this kind of approach is promisingfor future developments.

Experimental ProcedureCarbonate samples were separated as much as possiblefrom clay by a surgical lancet, then sub-samples withdifferent amount of detritus were chosen for U-seriesanalyses.

Tooth surfaces were cleaned of detritus by carefulscraping. Dentine was separated from the enamel.

Samples were burned at 800�C for 12 h to removeorganic matter. The ashen sample was dissolved in 8 HNO3, then Al nitrate carrier and 232U and 228Thtracer solutions were added. Methods for separation ofU and Th, using an ion exchange column, are discussedin full in Wild & Steffan (1991). Thorium and Uraniumwere plated on aluminium foil and counted on�-spectrometer with a grid-chamber. The ages werecalculated using an iterative computer program (e.g.Ivanovich & Harmon, 1992).

Results and Discussion

a. Bed II of Main Entrance

Six calcite samples from the Bed II of the MainEntrance have been considered, and we noticed thatthey are characterized by visible degrees of contamina-tion, constitued probably by clay. The Uranium-seriesanalyses of travertine samples are shown in Table 1,where uncertainties (1�) are almost of statisticalnature. All isotope’s activities are expressed in disinte-grations per minute per gram of sample, while all ratiosrefer to specific activities. In theory we can considerthat a sample is sufficiently pure for direct datingwithout corrections if the amount of the non-radiogenic 230Th is smaller than the statistical error ofthe total 230Th measured. Bischoff & Fitzpatrick (1991)have shown that this is possible if the 230Th/232Thactivity ratio is greater than 16, with errors normallyobtained with traditional alpha spectrometry. Becauseof the relative abundance of 232Th, mirrored by the low230Th/232Th ratio in all samples, ranging from 1·2–2·9,we can infer about the need of a correction. The age ofeach sample in the last column is calculated withoutconsidering the detrital contamination and has to be,therefore, corrected. Thereafter we have applied thecorrection procedure described above, for which the

344 M. Esposito et al.

isochron plots are shown in Figure 2a & b. The slopeof the least-squares line for the plot of 234U/232Thversus 238U/232Th, which represents the 234U/238Uactivity ratio of the pure carbonate endmember, is1·151�0·016, while the 230Th/234U activity ratio of thepure carbonate, obtained from the plot of 230Th/232Thversus 234U/232Th, is 0·810�0·021. The errors for theslopes are calculated as in Luo & Ku (1991). The goodlinearity shown by the isochron plots (r2>0·99 in bothcases) confirms the assumptions of the same (230Th/232Th)0, the initial activity ratio, for all samples and ofthe same age of the samples themselves. From theseactivity ratios we obtain an age for the calcite compo-nent of 169�11 ky. This result is older than both theage of the Bed I of the Main Entrance, evaluated at96�4 ky, and the age of calcite from Upper Layer, of137�7 ky (Esposito et al., 1998), obtained with thesame isochron approach. Because of the homogeneityof the faunal remains in all the three chambers ofSnake Cave, this result allow to consider that the upperbound for the age of this fauna is actually 169�11 ky,older than the previous age of 137�7 ky obtainedearlier by Esposito et al. (1998).

Table 1. Radiochemical data and calculated ages for samples from Bed II of the Main Entrance

Sample

238Uppm 234U/238U 230Th/232Th 230Th/234U

Age(ky)

EP A41 2·24�0·07 0·886�0·027 1·57�0·09 0·909�0·047 >240EP C41 0·202�0·009 0·960�0·045 2·8�0·2 0·893�0·068 228+60

�38

EP A42 1·61�0·05 0·846�0·031 1·24�0·06 0·895�0·043 >245EP C42 0·206�0·007 1·020�0·041 2·9�0·2 0·844�0·051 199+31

�24

EP A43 1·43�0·05 0·787�0·033 1·18�0·06 0·918�0·048 —EP A44 1·53�0·07 0·822�0·028 1·30�0·05 0·920�0·047 —Isochron 1·151�0·016 0·810�0·021 169+11

�11

0.0 4.0

4.0234U/232Th

y = 1.151x – 0.549Age = 169 ± 11 ky

238U/232Th

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 4.0

4.0230Th/232Th

y = 0.810x + 0.139

234U/232Th

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Figure 2. Isochron plots for layer II of the Main Entrance: (a) 234U/232Th versus 238U/232Th diagram to determine 234U/238U activity ratio incarbonate endmember; r2=0·995. (b) 230Th/232Th versus 234U/232Th diagram to determine 230Th/234U activity ratio in carbonate endmember;r2=0·999. Values for the slopes of the least-square fits are indicated.

b. Teeth from Main Entrance

The importance of a well established chronology of theSnake Cave induces us to attempt the direct dating offossils themselves, keeping in mind the difficultiesconnected with the application of the U-series tech-nique to teeth. The working hypothesis in our work isthat all samples are supposed to be coeval and olderthan 169�11 ky, corresponding to the age of calcitefrom Bed II in Main Entrance. We therefore analysedseveral samples of well preserved large mammal teethfrom each room of Snake Cave.

For all samples dentine was separated mechanicallyfrom enamel and each portion was analysed separately.In some cases we performed a duplicate determinationof the same sample in order to test its homogeneity. Alluncertainties are calculated at one standad deviation.

Nine tooth samples, coming from the fossiliferouslevel of the Main Entrance, have been analysed.Results are shown in Table 2, where U concentrationunit is ppm and all ratios refer to activities.

A first look to results put in evidence that the spreadof radiometric ages is large, from 20 ky for sample KC

U-series Dating of Fossil Teeth and Carbonates from Snake Cave, Thailand 345

Table 2. U-series data for fossil teeth from the Main Entrance

Sample

238Uppm 234U/238U 230Th/232Th 230Th/234U Age (ky)

KA bovidEnamel 2·04�0·07 1·411�0·045 >50 0·869�0·041 184+22

�19

Dentine 8·9�0·3 1·649�0·035 >418 1·733�0·075 —

KB bovidEnamel 2·07�0·08 1·522�0·054 >65 0·830�0·042 162+18

�15

Enamel 2·3�0·2 1·490�0·142 >71 0·798�0·084 151+36�28

Dentine 10·9�0·3 1·610�0·021 >495 1·092�0·040 >300Dentine 12·8�0·4 1·414�0·025 >82 1·054�0·061 >270

KC bovidEnamel 0·071�0·008 1·022�0·163 Not detected 0·364�0·094 49+17

�15

Dentine 1·738�0·008 1·169�0·059 >23 0·167�0·014 20+2�2

KD bovidEnamel 4·18�0·16 1·630�0·054 >421 0·919�0·037 201+22

�19

Enamel 5·93�0·16 1·551�0·030 >122 0·866�0·044 177+22�19

Dentine 40·1�1·5 1·550�0·025 131�19 1·093�0·062 >290Dentine 6·5�0·2 1·557�0·033 36�4 1·273�0·063 —

KE bovidEnamel 1·97�0·05 1·453�0·042 >47 0·882�0·036 189+20

�17

Enamel 1·74�0·07 1·427�0·059 >79 0·928�0·057 217+43�32

Dentine 19·9�0·7 1·641�0·026 >368 1·220�0·063 —Dentine 1·00�0·04 1·234�0·050 4·3�0·4 1·110�0·068 —

KF bovidEnamel 2·43�0·11 1·406�0·060 >435 1·404�0·069 —Dentine 14·1�0·5 1·599�0·029 >1178 1·675�0·071 —Dentine 3·10�0·09 1·621�0·048 >54 1·209�0·051 —

KI rhinocerosEnamel 0·241�0·016 1·473�0·115 >20 0·750�0·067 134+24

�20

Dentine 17·0�0·5 1·366�0·035 >950 1·346�0·071 —Dentine 19·0�0·5 1·311�0·024 >1100 1·319�0·046 —

KN rhinocerosEnamel 0·89�0·04 1·367�0·064 >71 0·548�0·035 82+8

�8

Dentine 22·6�0·6 1·361�0·017 >1080 0·649�0·018 107+5�5

Dentine 24·4�0·5 1·315�0·022 >310 0·669�0·023 112+6�6

KP pigEnamel 1·37�0·06 1·470�0·064 54�18 1·109�0·059 >350Enamel 1·56�0·07 1·525�0·068 nd 1·194�0·066 >350Dentine 20·8�0·5 1·686�0·023 >1688 1·433�0·047 —Whole 3·33�0·06 1·665�0·029 >1680 1·638�0·030 —

to over 350 ky for sample KP, going until somesamples that can not be dated because of the 230Th inexcess with respect to 234U. Therefore the necessarycondition for the applicability of the Uranium-seriesdating, which foresees that the samples behave asclosed systems during their burial history, is obviouslyviolated. It has to be noted that all samples analysedhave been found in a few squared centimetres area,where the macroscopic conditions were apparentlyhomogeneous. But this has not be sufficient to guaran-tee the same behaviour of U in teeth. In particular,almost all the dentines show evidence of Uraniumleaching, with a large 230Th excess to 234U, differentlyfrom almost all the enamels, which do not have evi-dence of Uranium leaching. Moreover, the young ageof one bovid (KD sample) and two rhinoceros teeth(KI enamel and KD samples) induce us to suspect evena recent uptake of U. It is confirmed the greater

tendency for dentine to act as an open system versus Umigrations than the denser enamel. Our conclusionabout the fact that Uranium was leached more thanThorium incorporated is based on the much greatermobility of U than Th and is confirmed by the absence,in almost all samples, of 232Th, a non radiogenicisotope.

The range of the U content for enamels is from0·05 to 4·4 dpm/g, while for dentines is from 0·74 to30 dpm/g. Some dentines, KD, KE, KF show alarge non homogeneous distribution of Uranium.Differences of Uranium distribution in enamels are lesspronounced. Because of the little size of our samples, ithas not been possible to build the profile of U towardthe tooth, an index that has been helpful in some casesto understand some U patterns (Millard & Hedges,1996). Nevertheless, it is perhaps useful to gatherthe samples in two groups, following their different

346 M. Esposito et al.

enamel-dentine geometry, as suggested by Pike &Hedges (2001). Thus pig and rhinoceros teeth show asingle enamel layer surrounding a core of dentine,while the bovid teeth have a more complicategeometry, with two enamel-dentine sandwiches. Themodels of Pike & Hedges (2001) foresee that thediffusion of U into samples is dependent on its avail-ability in the microenvironment, such as in ground-water, in soil and, in the case of teeth, in dentine. In thesimple dentine-enamel system the diffusion of U intoone side of the enamel is limited by the diffusion of theU through the dentine, while in the dentine-enamel-dentine system, like in the bovid teeth, the U diffusionin enamel is entirely limited by diffusion into dentine.These considerations lead to the prediction that radio-metric ages of enamel in the simple dentine-enamelsystem should be younger than those in dentine-enamel-dentine system. Our data seem to promote thisapproach, although to estimate the parameter of themodel and, hence, the age of the samples, a U profileand at least one date is required. Nevertheless, asabove outlined, the little size of the sample and the useof alpha spectrometry did not allow the determinationof these quantities. Following the above consideration,it may however suggested that all enamel ages indicatethe minimum age of the fossils, in agreement withprevious results obtained on carbonate samples. Asexpected, no clear relationship exists between the Ucontents and the radiometric ages of samples, both inenamels and in dentines. The range of the 234U/238Uactivity ratio is from 1·41–1·68, except for the sampleKC, which displayed radioactive equilibrium. It isinteresting to observe that some samples (KA, KB, KFand KP) show a clear difference in the U isotope ratiobetween enamel and dentine, suggesting an iso-topic fractionation during the U uptake or leaching.Furthermore, these differences can be detected even inthe same fraction of the tooth (KB and KE dentines),depending probably on more factors than those nor-mally considered. Since all the samples, except KDdentine, are free of common Thorium, contaminationwith non-authigenic 230Th can be excluded.

c. Teeth from Main LayerThe samples from the Main Layer are of specialinterest because a human tooth was found (Tougardet al., 1998). Both enamel and dentine of nine samplesof large mammals were separately analysed, but in thisroom, because of the low U content and the little sizeof the samples, only one analysis has been conductedfor each tooth constituent. Teeth from Main Layerwere visibly altered, probably because of water perco-lation, but the great importance of these fossils forcedus to attempt their direct dating. Analytical results with1� uncertainties are shown in Table 3. Samples fromthe Main Layer show much less amount of Uraniumthan those in the Main Entrance. Even though Millard& Hedges (1995) suggest that a bad preservation could

be related to a high U content, our results indicate adifferent behaviour. The higher porosity of badly pre-served fossils and hence the decrease of the internalsurface area may have led to the decrease in the rate ofU adsorption, a mechanism also indicated by Millard& Hedges (1996). Therefore, the general young agesand the low U contents of most samples may be due toa rapid histological alteration of teeth, soon afterburial, as already described by Hedges, Millard & Pike(1995).

The range of 234U/238U activity ratio is from 0·94–1·45 for all the samples, with no apparent isotopicfractionation between enamel and dentine, in generalagreement with the hypothesis of short uptake time ofU. The relatively important errors in some cases aredue to the low level of Uranium activity and to thelimited amount of material available. In few cases wecan not rule out the presence of non authigenic 230Thbecause of the presence of 232Th, but this does notmodify the sense of our conclusions. As already noted,the spread of the ages is important, from 8 ky to morethan 350 ky, with nevertheless most samples veryyoung in relation to our starting point, i.e. fossils olderthan 169�11 ky. Even though the 230Th/234U ages areyoung enough to stay in the range of the 231Pa datingmethod, the low activities of the Thorium isotopes andthe small size of the samples did not permit themeasurement of the Protactinium via the 227Th, as isnormally done in this context (Shen, 1996). In threecases, samples JL1, JL6 and JL9, dentines displayedages which are non discordant with their respectiveenamels; nevertheless in at least two of these cases, forJL1 and JL9 samples, they are too young with respectto the stratigraphy previously accepted.

d. Teeth of Upper LayerFrom the Upper Layer we analysed two bovid teethembedded in calcite matrix, for which the isochronTh/U dating method has yielded an age of 137�7 ky(Esposito et al., 1998). For these samples too,dentine and enamel have been analysed separately.Radiometric results are shown in Table 4. TheUranium specific activities are higher in the dentinesthan in enamels, as it often happens when dealing withteeth. It is perhaps interesting to note that even in thiscase, where samples have been found in calcite matrixand not in sand or clay, the 234U/238U ratios varyconsiderably, from 1·07–1·56, both in dentines and inenamels. We can therefore suspect that the mechanicalprotection of teeth by calcite has not been sufficientto prevent the Uranium migration. Moreover, theseresults seem to give some confirmations to thediffusion–adsorption (D–A) model of Pike & Hedges(2001), in which the U diffusion into enamel follows itsdiffusion into dentine. The enamel should thereforeshow a recent U uptake and hence a younger age,with a U concentration lower than that in dentine.With the assumption of only this mechanism, both the

U-series Dating of Fossil Teeth and Carbonates from Snake Cave, Thailand 347

radiometric ages of enamel and of dentine should beregarded as the minimum age of the sample, about180 ky, consistent with the stratigraphy. Unfortunatelythe full application of the D–A requires, as alreadymentioned, an U concentration profile, which it hasnot been possible to obtain in our samples. Moreover,the wide spread of the ratios, not predicted by the D–Amodel, suggests, as it has been already noted by Michelet al. (2000), that a series of episodes of Uraniumleaching and accumulation may have taken place. Inboth samples contamination with detrital sediment canbe neglected because of the high 230Th/232Th activityratios.

Table 3. U-series data for fossil teeth from the Main Layer

Sample

238Uppm 234U/238U 230Th/232Th 230Th/234 Age (ky)

JL1 bovidEnamel 0·83�0·03 1·259�0·053 >9·2 0·493�0·038 72+8

�7

Dentine 5·5�0·3 1·318�0·040 >59 0·558�0·037 85+9�8

JL2 deerEnamel 0·161�0·010 1·008�0·096 >7·6 0·179�0·037 21+5

�5

Dentine 9·8�0·4 0·944�0·021 >23 0·070�0·008 8+1�1

JL3 deerEnamel 0·51�0·03 1·270�0·097 >6·1 0·620�0·063 100+17

�15

Dentine 9·0�0·4 1·231�0·028 >196 0·357�0·021 47+3�3

JL4 pigEnamel 0·162�0·013 1·347�0·135 Not detected 0·199�0·047 28+6

�6

Dentine 7·2�0·4 1·242�0·037 >31 0·076�0·005 9+1�1

JL5 pigEnamel 0·081�0·010 1·587�0·244 >4·2 0·284�0·053 35+8

�7

Dentine 6·6�0·2 1·273�0·022 >26 0·082�0·006 9+1�1

JL6 pigEnamel 0·54�0·02 1·162�0·063 >340 0·624�0·048 103+14

�12

Dentine 9·5�0·4 1·204�0·020 >23 0·612�0·037 99+10�9

JL7 pigEnamel 0·103�0·008 1·327�0·126 >8 0·233�0·034 28+5

�5

Dentine 5·0�0·3 1·356�0·036 >118 0·090�0·008 10+1�1

JL8 pigEnamel 1·35�0·04 1·328�0·026 >112 0·990�0·059 278+90

�52

Dentine 15·1�0·8 1·288�0·029 >708 1·049�0·071 >350

JL9 bovidEnamel 0·53�0·02 1·450�0·070 15�3 0·558�0·034 84+8

�7

Dentine 4·0�0·2 1·292�0·069 >175 0·682�0·041 117+13�12

Table 4. U-series data for fossil teeth from the Upper Layer

Sample

238Uppm 234U/238U 230Th/232Th 230Th/234U Age (ky)

XKA bovidEnamel 0·72�0·04 1·561�0·090 >112 0·618�0·043 97+11

�10

Dentine 17·9�0·8 1·297�0·045 >140 0·845�0·049 178+27�22

XKB bovidEnamel 1·29�0·09 1·175�0·086 >31 0·572�0·056 90+14

�13

Dentine 34·9�1·4 1·073�0·027 >283 0·525�0·034 80+8�7

Conclusions

The Snake Cave has yielded a very interesting fossi-liferous deposit, highlighted by the discovery of ahuman tooth attributed to Homo genus (Tougardet al., 1998). Three rooms contain fossil remains which,on the basis of faunal assemblage, are coeval and canbe attributed all together to late Middle Pleistocene.Two chambers contain carbonated levels associatedwith the fossils, which can help to understandthe chronological frame. Previous U-series studies(Esposito et al., 1998) established an age of 96�4 kyfor Bed I of Main Entrance and 137�7 ky for calcite

348 M. Esposito et al.

from Upper Layer. From these results and from thehomogeneity of the faunas it was attributed to fossilsof Snake Cave an age of more than 137�7 ky. Thenew Uranium-series date of Bed II of the MainEntrance has yielded an age of 169�11 ky, older thanand in stratigraphical concordance with previous U/Thdates. On the basis of stratigraphical relations, we cantherefore assume that the fossils of Snake Cave areolder than 169�11 ky. The whole radiometric resultsobtained from different rooms of Snake Cave havebeen summarized in Table 5.

To study the Bed II of the Main Entrance we usedthe method of total sample dissolution (TSD), since inall the samples analysed there was an important frac-tion of detritus. Nevertheless the relatively large spreadof the points, due to the important variability of degreeof contamination, permits the determination of a goodisochron line. Even if the environmental conditionswere not ideal for the application of the Uranium-series dating as a consequence of the important perco-lating phenomena in this tropical humid climate, wehave successfully applied the TSD method. The goodlinearity of the isochron plot confirms the two hypoth-esis of the method: all samples are coeval and theyhad equal 230Th/232Th activity ratios at the time offormation.

The application of the Uranium-series dating tofossil teeth from Snake Cave has not been useful forthe determination of the age of this site. Even ifmeasurements were numerous, we can not understandmigrations of the Uranium and its daughters throughthe teeth. The spread of radiometric ages is importantin all samples we analysed, and the U behaviour cannot be modelled in a single way. If all dentines from theMain Entrance show a leakage of Uranium, as clearlydemonstrated by the 230Th excess compared to 234Uactivity, some other samples from the same room seemto have experienced a late U uptake, giving their youngage. On the other hand, we can not imagine an Uleakage from fossil teeth of the Main Layer, where weprefer to imagine an episodic uptake of Uranium, inrecent times, to explain the apparent ages which arecertainly too young. We can suppose that in thisenvironment, with an extensive percolating activity, it

often occurs the settling down of oxidizing conditions,and therefore a series of frequent Uranium leakagesand uptakes. Fossils from Upper Layer, even if foundin calcite matrix, a protection that could minimize theU mobility, yielded no valuables ages, confirming thegreat difficulty when applying the U-series dating toteeth.

The migration of Uranium can be very various andcomplicated and until now no index was found for thecomprehension of the Uranium history. For thisreason it is often very difficult to confirm or discuss theassumptions normally at the basis of the Uranium-series dating of teeth. We think that the way suggestedby Millard & Hedges (1995), taking into account theU profile in teeth and some microenvironmentalparameters, can be helpful, in some cases, to apply onmore solid bases the U-series technique to bones andteeth. Unfortunately this work method is not evereasily applicable, because of little size of samples, lowisotope activities or leakage of data.

Moreover, we believe that, almost in tropical humidclimates, as those of Southeast Asia, the application ofthe Uranium-series dating to fossil teeth and bones isparticularly doubtful and all the results should be usedvery carefully, taking into account all possible outliers.

Table 5. Rooms dating from different rooms of Snake Cave

Main Entrance Main Layer** Upper Layer

*Bed I=96�4 ky *Calcite bed=137�7 kyBed II=169�11 ky

Late Middle Pleistocene

9 teeth 9 teeth 2 teeth in calciteDentine: 20�300 ky Dentine: 10�350 ky Dentine: 80�180 ky230Th excessEnamel: 50�350 ky Enamel: 20�280 ky Enamel: 90�100 ky

*Esposito et al. (1998).**Human tooth discovered (Tougard et al., 1998).

ReferencesAyliffe, L. K. & Veeh, H. H. (1988). Uranium-series dating of

speleothems and bones from Victoria Cave, Naracoorte, SouthAustralia. Chemical Geology 72, 211–234.

Bischoff, J. L. & Rosenbauer, R. J. (1981). Uranium series dating ofhuman skeletal remains from the Dal Mar and Sunnyvale sites,California. Science 213, 1003–1005.

Bischoff, J. L., Rosenbauer, R. J., Tavoso, A. & De Lumley, H.(1988). A test of uranium-series dating of fossil tooth enamel:results from Tournal Cave, France. Applied Geochemistry 3,145–151.

Bischoff, J. L. & Fitzpatrick, J. A. (1991). U-series dating of impurecarbonates: An isochron technique using total-sample dissolution.Geochimica Cosmochimica Acta 55, 543–554.

Bischoff, J. L., Rosenbauer, R. J. & Moench, A. F. (1995). U-seriesAge Equations for Uranium Assimilation by Fossil Bones.Radiochimica Acta 69, 127–135.

Chaimanee, Y. (1998). Plio-Pleistocene Rodents of Thailand. ThaiStudies in Biodiversity 3, 1–303.

U-series Dating of Fossil Teeth and Carbonates from Snake Cave, Thailand 349

Chen, T. & Yuan, S. (1988). Uranium-series dating of bones andteeth from Chinese palaeolithic sites. Archaeometry 30, 59–76.

Esposito, M., Chaimanee, Y., Jaeger, J.-J. & Reyss, J.-L. (1998).Datation des concretions carbonatees de la ‘‘Grotte du Serpent’’(Thaılande) par la methode Th/U. Comptes Rendus de l’Academiedes Sciences, Serie IIA, Sciences de la Terre et des Planetes 326,603–608.

Ginsburg, L., Ingavat, R. & Sen, S. (1982). Decouverte d’une fauned’age pleistocene moyen terminal (Loangien) dans le nord de laThaılande. Comptes Rendus de l’Academie des Sciences, Serie D294, 189–191.

Ivanovich, M. & Harmon, R. S. (1992). Appendix A. Derivation ofthe expressions for the 234U/238U and 230Th/234U activity-ratio ageequations. In Uranium-Series disequilibrium: applications to Earth,Marine, and Environmental sciences. Oxford: Clarendon Press,pp. 779–781.

Hedges, R. E. M., Millard, A. R. & Pike, A. W. G. (1995).Measurements and relationships of diagenetically altered bonefrom three archaeological sites. Journal of Archaeological Science22, 201–209.

Ku, T.-L. & Liang, Z. C. (1984). The dating of impure carbonateswith decay-series isotopes. Nuclear Instruments and Methods inPhysics Research 223, 563–571.

Leitner-Wild, M. & Steffan, I. (1993). Uranium-series dating of fossilbones from Alpine caves. Archaeometry 30, 137–146.

Luo, S. & Ku, T.-L. (1991). U-series isochron dating: Ageneralized method employing total-sample dissolution.Geochimica Cosmochimica Acta 55, 555–564.

Michel, V., Yokoyama, Y., Falgueres, C. & Ivanovich, M. (2000).Problems encountered in the U-Th dating of fossil Red Deer jaws(bone, dentine, enamel) from Lazaret Cave: a comparative studywith early chronological data. Journal of Archaeological Science27, 327–340.

Millard, A. R. & Hedges, R. E. M. (1995). The role of theenvironment in uranium uptake by buried bone. Journal ofArchaeological Science 22, 239–250.

Millard, A. R. & Hedges, R. E. M. (1996). A diffusion-adsorptionmodel of uranium uptake by archaeological bone. GeochimicaCosmochimica Acta 60, 2139–2152.

Pike, A. W. G. & Hedges, R. E. M. (2001). Sample geometry and Uuptake in archaeological teeth: implications for U-series and ESRdating. Quaternary Science Reviews 20, 1021–1025.

Rae, A. M. & Ivanovich, M. (1986). Successful application ofuranium series dating of fossil bone. Applied Geochemistry 1,419–426.

Shen, G. J. (1996). 227Th/230Th dating method: methodology andapplication to chinese speleothem samples. Quaternary ScienceReviews (Quaternary Geochronology) 15, 699–707.

Schwartz, H. P. (1980). Absolute age determination of archaeologi-cal sites by Uranium series dating of travertines. Archaeometry 22,3–24.

Schwarcz, H. P. & Latham, A. G. (1989). Dirty calcite 1. Uranium-series dating of contaminated calcite using leachates alone.Chemical Geology 80, 35–43.

Tougard, C., Chaimanee, Y., Suteethorn, V. & Jaeger, J.-J. (1996).Extension of the geographic distribution of the giant panda(Ailuropoda) and search for the reasons for its progressive disap-pearance in Southeast Asia during the Latest Middle Pleistocene.Comptes Rendus de l’Academie des Sciences, Serie II 323, 973–979.

Tougard, C., Jaeger, J.-J., Chaimanee, Y., Suteethorn, V. &Triamwichanon, S. (1998). Discovery of a Homo sp. tooth associ-ated with a mammalian cave fauna of Late Middle Pleistocene age,Northern Thailand. Journal of Human Evolution 35, 47–54.

Wild, E. & Steffan, I. (1991). Isolation of Uranium and Thorium forage determination of fossil bones by Uranium series method.Radiochimica Acta 54, 85–89.