Radiocarbon dating

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Radiocarbon dating

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  • RadiocarbondatingFromWikipedia,thefreeencyclopedia

    Radiocarbondating(alsoreferredtoascarbondatingorcarbon14dating)isamethodofdeterminingtheageofanobjectcontainingorganicmaterialbyusingthepropertiesofradiocarbon(14C),aradioactiveisotopeofcarbon.ThemethodwasinventedbyWillardLibbyinthelate1940sandsoonbecameastandardtoolforarchaeologists.LibbyreceivedtheNobelPrizeforhisworkin1960.Theradiocarbondatingmethodisbasedonthefactthatradiocarbonisconstantlybeingcreatedintheatmospherebytheinteractionofcosmicrayswithatmosphericnitrogen.Theresultingradiocarboncombineswithatmosphericoxygentoformradioactivecarbondioxide,whichisincorporatedintoplantsbyphotosynthesisanimalsthenacquire14Cbyeatingtheplants.Whentheanimalorplantdies,itstopsexchangingcarbonwithitsenvironment,andfromthatpointonwardstheamountof14Citcontainsbeginstoreduceasthe14Cundergoesradioactivedecay.Measuringtheamountof14Cinasamplefromadeadplantoranimalsuchaspieceofwoodorafragmentofboneprovidesinformationthatcanbeusedtocalculatewhentheanimalorplantdied.Theolderasampleis,theless14Cthereistobedetected,andbecausethehalflifeof14C(theperiodoftimeafterwhichhalfofagivensamplewillhavedecayed)isabout5,730years,theoldestdatesthatcanbereliablymeasuredbyradiocarbondatingarearound50,000yearsago,althoughspecialpreparationmethodsoccasionallypermitdatingofoldersamples.

    Theideabehindradiocarbondatingisstraightforward,butyearsofworkwererequiredtodevelopthetechniquetothepointwhereaccuratedatescouldbeobtained.Researchhasbeenongoingsincethe1960stodeterminewhattheproportionof14Cintheatmospherehasbeenoverthepastfiftythousandyears.Theresultingdata,intheformofacalibrationcurve,isnowusedtoconvertagivenmeasurementofradiocarboninasampleintoanestimateofthesample'scalendarage.Othercorrectionsmustbemadetoaccountfortheproportionof14Cindifferenttypesoforganisms(fractionation),andthevaryinglevelsof14Cthroughoutthebiosphere(reservoireffects).Additionalcomplicationscomefromtheburningoffossilfuelssuchascoalandoil,andfromtheabovegroundnucleartestsdoneinthe1950sand1960s.Fossilfuelscontainlittledetectable14C,andasaresulttherewasanoticeabledropintheproportionof14Cintheatmospherebeginninginthelate19thcentury.Conversely,nucleartestingincreasedtheamountof14Cintheatmosphere,toamaximum(reachedin1963)ofalmosttwicewhatithadbeenbeforethetestingbegan.

    Measurementofradiocarbonwasoriginallydonebybetacountingdevices,whichcountedtheamountofbetaradiationemittedbydecaying14Catomsinasample.Sampleswereconvertedtosolidcarbonfortheearliestdevices,butitwasquicklydiscoveredthatconvertingthemtogasorliquidformgavemoreaccurateresults.Morerecently,acceleratormassspectrometryhasbecomethemethodofchoiceitcountsallthe14Catomsinthesampleandnotjustthefewthathappentodecayduringthemeasurementsitcanthereforebeusedwithmuchsmallersamples(assmallasindividualplantseeds),andgivesresultsmuchmorequickly.Datesareoftenreportedinyears"beforepresent",orBPthisactuallyreferstoabaselineof1950AD,sothatadateof500BPmeans1450AD.

    Thedevelopmentofradiocarbondatinghashadaprofoundimpactonarchaeology.Inadditiontopermittingmoreaccuratedatingwithinarchaeologicalsitesthandidpreviousmethods,itallowscomparisonofdatesofeventsacrossgreatdistances.Historiesofarchaeologyoftenrefertoitsimpactasthe"radiocarbonrevolution".Occasionally,themethodisusedforitemsofpopularinterestsuchasthe

  • ShroudofTurin,whichisclaimedtoshowanimageofthebodyofJesusChrist.Asampleoflinenfromtheshroudwastestedin1988andfoundtodatefromthe13thor14thcentury,castingdoubtonitsauthenticity.[1]

    Contents

    1Background1.1History1.2Physicalandchemicaldetails1.3Principles1.4Carbonexchangereservoir

    2Datingconsiderations2.1Atmosphericvariation2.2Isotopicfractionation2.3Reservoireffects2.4Contamination

    3Samples3.1Materialconsiderations3.2Preparationandsize

    4Measurementandresults4.1Betacounting4.2Acceleratormassspectrometry4.3Calculations4.4Errorsandreliability4.5Calibration4.6Reportingdates

    5Useinarchaeology5.1Interpretation5.2Notableapplications

    5.2.1Pleistocene/HoloceneboundaryinTwoCreeksFossilForest5.2.2DeadSeaScrolls

    5.3Impact6Notes7References8Sources9Externallinks

    Background

    History

    Intheearly1930sWillardLibbywasachemistrystudentattheUniversityofBerkeley,receivinghisPh.D.in1933.Heremainedthereasaninstructoruntiltheendofthedecade.[2]In1939theRadiationLaboratoryatBerkeleybeganexperimentstodetermineifanyoftheelementscommoninorganicmatterhadisotopeswithhalfliveslongenoughtobeofvalueinbiomedicalresearch.Itwassoondiscoveredthat14C'shalflife

  • wasfarlongerthanhadbeenpreviouslythought,andin1940thiswasfollowedbyproofthattheinteractionofslowneutronswith14Nwasthemainpathwaybywhich14Cwascreated.Ithadpreviouslybeenthoughtthat14Cwouldbemorelikelytobecreatedbydeuteronsinteractingwith13C.[3]AtsometimeduringWorldWarIILibbyreadapaperbyW.E.DanforthandS.A.Korff,publishedin1939,whichpredictedthecreationof14Cintheatmospherebyneutronsfromcosmicraysthathadbeensloweddownbycollisionswithmoleculesofatmosphericgas.ItwasthispaperthatgaveLibbytheideathatradiocarbondatingmightbepossible.[4]

    In1945,LibbymovedtotheUniversityofChicago.Hepublishedapaperin1946inwhichheproposedthatthecarboninlivingmattermightinclude14Caswellasnonradioactivecarbon.[5][6]LibbyandseveralcollaboratorsproceededtoexperimentwithmethanecollectedfromsewageworksinBaltimore,andafterisotopicallyenrichingtheirsamplestheywereabletodemonstratethattheycontainedradioactive14C.Bycontrast,methanecreatedfrompetroleumshowednoradiocarbonactivity.TheresultsweresummarizedinapaperinSciencein1947,inwhichtheauthorscommentedthattheirresultsimplieditwouldbepossibletodatematerialscontainingcarbonoforganicorigin.[5][7]

    LibbyandJamesArnoldproceededtoexperimentwithsamplesofwoodofknownage.Forexample,twosamplestakenfromthetombsoftwoEgyptiankings,ZoserandSneferu,independentlydatedto2625BCplusorminus75years,weredatedbyradiocarbonmeasurementtoanaverageof2800BCplusorminus250years.TheseresultswerepublishedinSciencein1949.[8][9]In1960,LibbywasawardedtheNobelPrizeinChemistryforthiswork.[5]

    Physicalandchemicaldetails

    Innature,carbonexistsastwostable,nonradioactiveisotopes:carbon12(12C),andcarbon13(13C),andaradioactiveisotope,carbon14(14C),alsoknownas"radiocarbon".Thehalflifeof14C(thetimeittakesforhalfofagivenamountof14Ctodecay)isabout5,730years,soitsconcentrationintheatmospheremightbeexpectedtoreduceoverthousandsofyears,but14Cisconstantlybeingproducedinthelowerstratosphereanduppertropospherebycosmicrays,whichgenerateneutronsthatinturncreate14Cwhentheystrikenitrogen14(14N)atoms.[5]Thefollowingnuclearreactioncreates14C:

    wherenrepresentsaneutronandprepresentsaproton.[10]

    Onceproduced,the14Cquicklycombineswiththeoxygenintheatmospheretoformcarbondioxide(CO2).Carbondioxideproducedinthiswaydiffusesintheatmosphere,isdissolvedintheocean,andistakenupbyplantsviaphotosynthesis.Animalseattheplants,andultimatelytheradiocarbonisdistributedthroughoutthebiosphere.Theratioof14Cto12Cisapproximately1.5partsof14Cto1012partsof12C.[11]

    Inaddition,about1%ofthecarbonatomsareofthestableisotope13C.[5]

    Theequationfortheradioactivedecayof14Cis:[1]

  • Byemittingabetaparticle(anelectron,e)andanelectronantineutrino(e),oneoftheneutronsinthe14C

    nucleuschangestoaprotonandthe14Cnucleusrevertstothestable(nonradioactive)isotope14N.[12]

    Principles

    Duringitslife,aplantoranimalisexchangingcarbonwithitssurroundings,sothecarbonitcontainswillhavethesameproportionof14Castheatmosphere.Onceitdies,itceasestoacquire14C,butthe14Cwithinitsbiologicalmaterialatthattimewillcontinuetodecay,andsotheratioof14Cto12Cinitsremainswillgraduallydecrease.Because14Cdecaysataknownrate,theproportionofradiocarboncanbeusedtodeterminehowlongithasbeensinceagivensamplestoppedexchangingcarbontheolderthesample,theless14Cwillbeleft.[11]

    Theequationgoverningthedecayofaradioactiveisotopeis:[5]

    whereN0isthenumberofatomsoftheisotopeintheoriginalsample(attimet=0,whentheorganism

    fromwhichthesamplewastakendied),andNisthenumberofatomsleftaftertimet.[5]isaconstantthatdependsontheparticularisotopeforagivenisotopeitisequaltothereciprocalofthemeanlifei.e.theaverageorexpectedtimeagivenatomwillsurvivebeforeundergoingradioactivedecay.[5]Themeanlife,denotedby,of14Cis8,267years,sotheequationabovecanberewrittenas:[13]

    Thesampleisassumedtohaveoriginallyhadthesame14C/12Cratioastheratiointheatmosphere,andsincethesizeofthesampleisknown,thetotalnumberofatomsinthesamplecanbecalculated,yieldingN0,thenumberof

    14Catomsintheoriginalsample.MeasurementofN,thenumberof14Catomscurrently

    inthesample,allowsthecalculationoft,theageofthesample,usingtheequationabove.[11]

    Thehalflifeofaradioactiveisotope(usuallydenotedbyt1/2)isamorefamiliarconceptthanthemeanlife,soalthoughtheequationsaboveareexpressedintermsofthemeanlife,itismoreusualtoquotethevalueof14C'shalflifethanitsmeanlife.[note1]Thecurrentlyacceptedvalueforthehalflifeof14Cis5,730years.[5]Thismeansthatafter5,730years,onlyhalfoftheinitial14Cwillhaveremainedaquarterwillhaveremainedafter11,460yearsaneighthafter17,190yearsandsoon.

    Theabovecalculationsmakeseveralassumptions,suchasthatthelevelof14Cintheatmospherehasremainedconstantovertime.[5]Infact,thelevelof14Cintheatmospherehasvariedsignificantlyandasaresultthevaluesprovidedbytheequationabovehavetobecorrectedbyusingdatafromothersources.[14]

    Thisisdonebycalibrationcurves,whichconvertameasurementof14Cinasampleintoanestimatedcalendarage.Thecalculationsinvolveseveralstepsandincludeanintermediatevaluecalledthe"radiocarbonage",whichistheagein"radiocarbonyears"ofthesample:anagequotedinradiocarbonyearsmeansthatnocalibrationcurvehasbeenusedthecalculationsforradiocarbonyearsassumethatthe14C/12Cratiohasnotchangedovertime.Calculatingradiocarbonagesalsorequiresthevalueofthehalf

  • Simplifiedversionofthecarbonexchangereservoir,showingproportionsofcarbonandrelativeactivityofthe14Cineachreservoir[5][note3]

    lifefor14C,whichformorethanadecadeafterLibby'sinitia