A survey of some new approaches in maximum age limit and accuracy of luminescence application to archaeological chronometry

Abstract

In recent years considerable effort has been made to extend the dating
limits and accuracies of luminescence dating and its applications to
archeological sciences. These include, for example, the use of single
grains for dating (Duller 2008: 589-612; Jacobs and Roberts 2007:
210-223), research on athermal fading to explore the use of feldspars
(with higher saturation dose) for dating (Huntley 2006: 1359-1365;
Lamothe and Auclair 1999: 319-323), the use of red TL for the dating of
volcanic ash (Fattahi and Stokes 2003: 647-660; Visocekas and
Guérin 2006: 942-947) and direct dating of archeological contexts
(Chawla and Singhvi 1989: 416-418; Singhvi et al. 1986: 205-207). Use of
some of these have provided useful new data on chronometry and have
placed luminescence dating on the central stage amongst other
chronometric techniques available for the dating archeological sites and
several important results on aspects of human dispersal and chronometry
have been reported (Anikovich et al. 2007: 223-226; Mellars 2006:
796-800; Petraglia et al. 2007: 114-116; Roberts 1997: 819-892; Singhvi
et al. 1998: 23-83). This contribution outlines some new ideas that
offer prospects of developing the luminescence dating technique further
and help extend the age range of its applications in archeology. These
applications are based on consideration of the manner in which the
radiation dose is deposited in natural samples and aim to refine the
protocols that are currently used for the estimation of equivalent dose.

Full text

MediterraneanArchaeologyandArchaeometry,Vol.2010,No.4,pp.9‐15
Copyright©2010MAA
PrintedinGreece.Allrightsreserved.


ASURVEYOFSOMENEWAPPROACHESINEXTENDING
THEMAXIMUMAGELIMITANDACCURACY
OFLUMINESCENCEAPPLICATION
TOARCHEOLOGICALCHRONOMETRY
A.K.Singhvi,N.ChauhanandR.H.Biswas
GeosciencesDivision,PhysicalResearchLaboratoryAhmedabad380009,India
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Correspondingauthor:singhvi@prl.res.in
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
INTRODUCTION
Inrecentyearsconsiderableefforthasbeenmadetoextendthedatinglimitsandaccuraciesof
luminescencedatinganditsapplicationstoarcheologicalsciences.Theseinclude,forexample,the
useofsinglegrainsfordating(Duller2008:589‐612;JacobsandRoberts2007:210‐223),researchon
athermalfadingtoexploretheuseoffeldspars(withhighersaturationdose)fordating(Huntley
2006:1359‐1365;LamotheandAuclair1999:319‐323),theuseofredTLforthedatingofvolcanicash
(FattahiandStokes2003:647‐660;VisocekasandGuérin2006:942‐947)anddirectdatingof
archeologicalcontexts(ChawlaandSinghvi1989:416‐418;Singhvietal.1986:205‐207).Useofsome
ofthesehaveprovidedusefulnewdataonchronometryandhaveplacedluminescencedatingon
thecentrestageamongstotherchronometrictechniquesavailableforthedatingarcheologicalsites
andseveralimportantresultsonaspectsofhumandispersalandchronometryhavebeenreported
(Anikovichetal.2007:223‐226;Mellars2006:796‐800;Petragliaetal.2007:114‐116;Roberts1997:
819‐892;Singhvietal.1998:23-83).
Thiscontributionoutlinessomenewideasthatofferprospectsofdevelopingtheluminescence
datingtechniquefurtherandhelpextendtheagerangeofitsapplicationsinarcheology.These
applicationsarebasedonconsiderationofthemannerinwhichtheradiationdoseisdepositedin
naturalsamplesandaimtorefinetheprotocolsthatarecurrentlyusedfortheestimationof
equivalentdose.
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KEYWORDS:redTL,volcanicash,fading,IRSL

A.K.SINGHVIetal
10
DATINGTHEINTERIOROFLITHICIM‐
PLEMENTS
Inluminescencedating,quartzhasbeen
usedwidelytoprovidetheburialagesofsedi‐
ments.Themaximumachievableagedepends
onthedosecorrespondingtothesaturationof
theluminescencesignalandthelifetimeofthe
stablesignal.Onthebasisoflifetimemeasure‐
mentsithasbeenestimatedthatthelumines‐
cencesignalofquartzisstablefor108ka.How‐
ever,onsetofsaturationoftheluminescence
signalatlowradiationdoses,limitsitsrangeof
applicability.Thesaturationofthelumines‐
cencesignalistypicallyoccursat~250Gy
(Chawlaetal.1998:53‐63).Thisrestrictsmaxi‐
mumageobtainedfromquartzgrainsto
aroundhundredka,anddependslargelyupon
theabsorbeddoseratefromthesedimentma‐
trix.
Inasedimentmatrix,theradiationenviron‐
mentcomprisesalpha,betaandgammaradia‐
tions,emittedduringthedecayofnaturallyoc‐
curringradioactiveisotopesofU,Thseries,K
and87Rb.Thealphaparticlerangeislimitedto
onlyfewmicrons,betahasarangeoffewmm
andgammadosehasarangeofabout30cm.
Underinfinitematrixassumption(Aitken1985),
typicallyfora100μmgrain,thegammadoseis
25‐30%ofthetotaldose.Inlargequartzgrains
thataregenerallydevoidofinternalradioactiv‐
ity,thedistributionofdoseinsidethegrain
fromtheexternalradiationenvironment
changeswithdepthduetoverydifferentranges
ofthealpha,betaandgammaradiations.Asa
simpleapproximation,thealphaandbetadose
getattenuatedwithdepthandtheircontribu‐
tionisnegligibleatdepthsfewmm.Thusfora
quartzitepebble,theinteriorofthegrainre‐
ceivedoseduetoonlyduetogammaradia‐
tions.Thissuggeststhatininteriorregion,the
doseratereducestoathirdorafourthofits
originalvalueand,inturnimpliesthatthesatu‐
rationoftheluminescencesignalinthisregion
wouldoccurovermuchlongertimescales.We
exploredthefeasibilityofusingtheinteriorof
largesizequartzgrainstoextendtheagerange
achievablebyluminescence(Chauhanetal.
2009:629‐633).Inthiswork,therangeofbeta
particlesfromdifferentradioisotopes(i.e.40K,
212Bi,214Biand234Pa)andtheirrelativedosecon‐
tributionwascomputedanalyticallyandbyus‐
ingMonteCarlosimulations.Fig.1showsthe
variationofdoseinsidethequartzgrainrelative
toinfinitematrixdoseinsedimentmatrix.

Fig.1Attenuationofbetadoseinsideaquartzgrain
duetovariousnaturallyoccurringradionuclides.The
radioactivityisconsideredbeexternaltothegrain.
Noticethatatdepthsof2mmmostofthebetadose
getsattenuated.
Thesecomputationscomprised,
1. estimationofbetadoseprofileinsidea
quartzgrainwithdepthnormaltosurface
usingMonteCarlosimulation,
2. developmentofanalyticalmethodologyto
computetheannualradiationdoseand
comparewiththesimulationresults,
3. estimationofthedepthatwhichbetadose
insidequartzgrainisreducedtoanegligible
value,and
4. examinationofthepossiblecausesthatmay
inhibittheapplicabilityofthenewmethods.
Resultsuggestthatinlargecmsizequartz
grains,thenetbetadose(fromallthepossible
betaemitters)isconfinedtoouter~2mmskin.
Theinteriorofquartzgrainreceivesradiation
doseonlyduetogammarays.Thisisabout30%
ofthetotaldoseratewhichimpliesthatbyana‐
lyzingtheinteriorofalargesizequartzgrain,
datingofasamplewhichisthreetofourtimes
oldershouldbepossible.
Basedonthis,wesuggestthepossibilityof
directlydatingheatedorsunexposed,archaeo‐
logicalartifacts.Microlithsandsmalllithicim‐
plementsaregoodcandidatestobeusedfor
datingbythisapproach,consideringthatoften
thesewereheatedtoahightemperatureto

ASURVEYOFSOMENEWAPPROACHESINEXTENDINGTHEMAXIMUMAGELIMITANDACCURACY

OFLUMINESCENCEAPPLICATIONTOARCHEOLOGICALCHRONOMETRY
11
makeiteasytoshapethem.Further,attimes
someoftheimplementsaretranslucentandthis
factfurtherensuresthattheirgeologicallumi‐
nescencecanhephotobleachedbydaylight
duringtheirmanufacture,use,disposaland
eventualburial.Initialexperimentswithlarge
(~cmsized)piecesofquartzindicatedthattheir
interiorregionswerealsowellbleachedpartly
duetothepresenceofmultiplecleaveplains
andfractures,thatensuredmultiplereflections
oflightinthelatticeandhencebleaching.The
extractionoftheinteriorofthegrainsisnon‐
trivialandtwopossibleapproachesare,1)Hy‐
drofluoricacidetchingforextendedperiods,or
2)theimagingofaslice(takennormaltothe
grainsurface)towardstheinteriorandusing
imagingtechniquestomeasurethespatialdis‐
tributionofdosethroughthesamplematrix.
Thiscanbedoneusingthespatiallyresolved
luminescencemeasurementsystemanalogous
tothatinitiatedbyGreilichetal.(2005:645‐665).
Asystemforspatiallyresolvedluminescenceis
beingconstructedatourlaboratoryandsuch
casesoftheinterioroflithicimplementswill
thenbeexaminedviathepaleodosegradientin
aslicethroughitsinterior.Also,itshouldin
principlebepossibletodateslithicimplements
withfiniteinternalradioactivityduetothefact
thatonlyapartofthebetadosearisingfromthe
internalradioactivityistrappedinsidethevol‐
umeconsidered.Inthiscontextwealsoreferto
anearlyworksbyLiritzis(1980:242‐251)and
HuntleyandPrescott(2001:687‐699),whoex‐
ploitedlowdoseenvironmentsfordatingover
anextendedtimerange.

SENSITIVITYCHANGESDURINGNATU‐
RALLUMINESCENCEMEASUREMENTS‐
THEIMPROVEDSARPROTOCOL
Generally,theSingleAliquotRegeneration
(SAR)protocol(MurrayandWintle2000:57‐73)
isusedtoestimatethedosedepositedina
sedimentmatrix.Inthisprotocol,thesensitivity
changeduringmeasurementismonitoredbya
testdose(TD)signal.Thenaturalandregenera‐
tionluminescencesignalsarenormalizedbythe
TDluminescencesignal.Thisprocedurethere‐
forecorrectsforsensitivitychangeduringa
dosemeasurementcycle.However,ithasoften
beenobservedthattheintensityofanatural
aliquotlieswayabovetheintensityofthere‐
generatedcurveandoftentheregeneratedin‐
tensityevensaturateswellbelowthenatural
intensity.Suchasituationcanonlyarisewhen
thesensitivities(luminescenceintensityperunit
massperunitdose)aredifferent,thatis,the
naturalluminescenceandtheregeneratedlu‐
minescencesignalsarerecordedwithdifferent
sensitivities.Itwasthereforepointedoutthata
finitechangeinsensitivitycanoccurduringthe
pre‐heatingmeasurementofnaturalOSLand
consequentlytheSARprotocolasusednow
doesnotfullycorrectforthesensitivity
changes.Laboratorystudiesindicatedthatsuch
sensitivitychangesduringOSLreadoutand
preheatcyclescanbesignificant(upto30%or
more)andcanthereforecausesignificantsys‐
tematicerrorsorevenmakeasampleunsuit‐
ableforanalysis(e.g.thecaseofnaturalsignal
exceedingthesaturationlimitoftheregenera‐
tionsignal)asshowninFig.2.

Fig.2Changesinthesensitivityof110°Cpeakduring
themeasurementcycles.Thelargestepsofchanges
areduetopreheats.TheSARprotocolcorrectsfor
theseeffectivelybuttheinitialchangeisnotincluded.
Theareaunderthepeakfrom90°Cto120°Cwas
integratedtoobtaintheintensityofthe110°Cpeak
Withthisproviso,werecommendadditional
measurementstepsfortheanalysisofasample
usingSARprotocol.Thiscorrectionprocedure
assumesthatthe110°CquartzOSLpeak
(MurrayandRoberts1998:503‐515)canbeused
asasurrogatefortheOSLsensitivityofthe
sample(acorrelationthatideallyshouldbees‐
tablishedforeachsample).Inthisprotocolthe
ratioofthe110°CTLpeakresponseforTD,re‐

A.K.SINGHVIetal
12
cordedbeforeandafterthemeasurementof
naturalOSLandthisratioisusedtocorrectfor
thesensitivitychangeduringOSLmeasure‐
ments(Fig.3).

Fig.3Theprotocolforsensitivitycorrectionduring
thepreheatandreadoutofthenaturalOSL.The
intensitiesofsteps2and5providethecorrection
factorNCF.

Fig.4ChangesinthedistributionofSARpaleodoses
withandwithoutNCFcorrection.Noticethereduc‐
tioninthespreadandtheshiftinthemeanvalue.
ThisratioistermedtheNaturalsensitivity
CorrectionFactor(NCF)andisusedtoensure
thatthenaturalOSLandtheregeneratedOSL
arecomparedwiththesamesensitivity.The
remainderoftheprotocolremainssimilarasfor
theSARprotocol.Finallytheequivalentdoseis
obtainedbysubtractingtheTDvalue(given
priortonaturalOSLmeasurement)fromthe
obtaineddosevalue.Fig.4givesthepaleodose
distributionofasampleanditisclearthata
noteworthyreductioninthespreadofpaleo‐
dosesoccurs,duepossiblytohithertounac‐
countedforchangesinthesensitivityinthe
conventionalSARprotocols.
Thisstudysuggests,arevisitoftheagesus‐
ingtheSARprotocolandasimpletestofa
comparisonofthesensitivityof110°Cglow
peaktoatestdosebeforeandaftertheOSL
readout,isrecommended.Recentexperiments
withdosedistributionfromavarietyofenvi‐
ronmentsshowsimilartrends.Adecreasein
scatterinthepaleodosesusingthisprotocol
wasseeninallthesamplestestedandthissug‐
gestedthatinmanysituations,wellbleached
samplesmaybemisunderstoodasapoorly
bleachedsample.Thisaspecthasbeendealt
withelsewhere(ChauhanandSinghvi2010:in
process).

DATINGOFVOLCANICASH
InIndianarcheology,thepresenceofarcheo‐
logicalimplementsincentral–westernIndia
andthedatingofassociatedvolcanicashlayer
havebeenextensivelydebatedonmethodologi‐
calgrounds.Thusforexample,Korisettaretal.
(1989:564‐566)firstdatedtheBoriashbyK‐Ar
methodto1.4Ma.Thiswaslaterrevisedto
600kausingAr–Arages.Ontheotherhand,
basedongeochemicalsimilarity,thislayerwas
consideredtobetheYoungestTobatuff(YTT)
datedto73±4kabyChesneretal.(1991:200‐
203).Recently,Petraglia,etal.(2007:114‐116)
datedapreandpostTobaarcheologicalsam‐
ple,inJwalapuram,AndhraPradesh,bylumi‐
nescencedatingtechniqueas77±6and74±7ka
respectively.Theagebracketingshowsthatthe
ageofthisashis~74Ka.Directluminescence
datingofTobaashwasfirstattemptedbyHorn
etal.(1993:326‐329).Thethermoluminescence
ageoftheglasses,derivedfromBoriAsh,was
23.4±2.4kaandwasinterpretedtodatethesec‐
ondaryreworkingandconsequentphotobleach‐
ingoflight‐sensitiveTLsignalsinthetuff,post
deposition.Analternativeexplanationwasthat

ASURVEYOFSOMENEWAPPROACHESINEXTENDINGTHEMAXIMUMAGELIMITANDACCURACY

OFLUMINESCENCEAPPLICATIONTOARCHEOLOGICALCHRONOMETRY
13
thisashsufferedathermalfading.Notmany
ashbedshavebeendatedintheIndiancon‐
texts.
Toresolvesomeofthebasicissuesofdating
volcanicashinIndia,weinitiatedasystematic
evaluationoftheashsampleusingavarietyof
approaches.Towardsthis,avolcanicashlayer
samplefromdifferentstratigraphiccontexts
wastakensuchthattheageoftheashbedcould
beconstrainedwithSARageonquartzgrains
fromhorizonsthatsandwichtheashlayer.
Basedonfieldevidences,oneoftheashlayers
wasalsoconsideredastheTobaash.Datingof
sediments,aboveandbelowtheashlayer,us‐
ingSARprotocolforquartz,gaveagesof~52
and~76kasuggestingtheageoftheashlayer
tobewithintheagerangeof52‐76ka.These
ageshoweverdidnotpermitaclearresolution
ontheageexceptthatthedifferencebetween
thisandagesonhorizonsaboveandbelow.Di‐
rectdatingoftheashusingTL‐ MultipleAli‐
quotAdditiveDosemethod,inblueemission
window(430±30nm)yieldedanageof47±8ka.
ThisageistwicethatobtainedbyHorn,etal.
(1993:326‐329),suggestingthattheirinference
ofashbedbeingreworked/redeposited(and
hencenotinprimarycontext)waspossiblycor‐
rect.Usingacorrectionprocedurereportedby
Someshwararao(1996),thesensitivitycorrec‐
tionprocedureforathermalfadingindicateda
correctionfactorof1.6fortheunderestimated
ages.TheprotocolisoutlinedinFig.5.

Fig.5Correctionprocedureforfadingusingasample‐
basedanalysisfollowingSomeshwarararao,1996.
Thecorrectionfactoristheratioofsensitiv‐
ityatthetimeofburial(S2=S21‐S22)tothatas‐
receivedsample(S1=S11‐S12).Thisresultedina
valueof75±10kaandisclosertoitsbeingthe
YTTash.WethenattemptedthePostIRIRSL
SARprotocol,asintroducedbyBuylaertetal.
(2009:560‐565)(Fig6)andthepreliminaryre‐
sults,gaveanage~70ka.

Fig.6PostIRIRSLSARprotocolforthedatingofvol‐
canicash(followingBayleaurtetal.2009).
ItthusseemsthattheboththeSensitivity
correctionandthepostIRIRSLSARprotocol
workwell.Fig.7providestypicalopticaldecay
curvesobtainedusingIRSLandpostIRIRSL
elevatedtemperaturemeasurements.Theques‐
tionremainsthatastowhytheyprovidesimilar
resultsandthephysicsthatleadstothiscon‐
vergenceneedsfurtherelucidation.

Fig.7AtypicalIRSLandpostIRIRSLopticaldecay
curveforafinegrainsampleofvolcanicash.

A.K.SINGHVIetal
14
CONCLUSIONS
Theconceptualformalismofthreemethodo‐
logicalaspectsofluminescencedatinghasbeen
presented.Feasibilitystudiesonthese,
1. showthepossibilityofincreasingthe
datingrange,threefold,
2. indicatethepotentialfordecreasingthe
systematicerrorsthatcanoccurinSAR
protocolinOSL,and
3. providetheprospectsofimprovingthe
accuracyofagesinthecaseofvolcanic
ashbeds.Wesincerelyhopethatthisre‐
portwillinduceothergroupstotesttheir
efficacy.


ACKNOWLEDGEMENTS
AKSthanksS.StokesforearlycollaborationonthesensitivitycorrectionproceduresinSARpro‐
tocol.WethankDr.R.RajandDr.N.JuyalforhelpwithsamplecollectionsandMr.P.Adhayaru
forhissustainedhelpwiththeinstrumentation.Wethankstherefereesfortheirpainstakingcom‐
mentsandforpointingoutlanguageflaws.Butforthem,themanuscriptwouldnothavebeenas
palatable.

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