Taylor 50 y of Radiocarbon Dating Am Sci

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Fifty Years of Radiocarbon Dating: This widely applied technique has made major strides sinceits introduction a half-century ago at the University of ChicagoAuthor(s): R. E. TaylorReviewed work(s):Source: American Scientist, Vol. 88, No. 1 (JANUARY-FEBRUARY 2000), pp. 60-67Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27857964 .

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Fifty Years of Radiocarbon Dating

Thiswidelyapplied technique asmademajor strides ince its ntroductiona half-centurygo at theUniversityofChicagoR. E. Taylor

R. E. Taylor currently chairstheDepartment ofAnthropology at theUniversity ofCalifornia, Riverside, where he directstheradiocarbon laboratory.Taylor received his Ph.D. in anthropologyt theUniversityfCalifornia, Los Angeles in1970. He has written extensively on theapplication of radiocarbon and other datingmethods in archaeology, withparticular emphasis on issuessurroundinghepeopling f heNew World. Address: Radiocarbon Laboratory, Departmentof nthropology,nstituteofGeophysics and PlanetaryPhysics, University ofCalifornia, Riverside, CA 92521. Internet:[email protected]

Sincethe 1350s,when a linen fabricbearingfull-scale front nd back images ofwhat appeared tobe a crucifiedman was firstdisplayedina small village inFrance,many people debatedwhether this famous cloth?later enshrined

in theCathedral of St. John theBaptist inTurin,Italy?could have once served as the burialshroud forJesusofNazareth. Arguments ragedfor six centuries about the "Shroud of Turin;"then,during the 1980s, a group of scientists decided to seek a final answer to thequestion ofauthenticity. Investigators inEngland, Switzerland and theUnited States analyzed swatchesfromthe linen shroud (alongwith other ancienttextilesamples of known vintage). They appliedthenow-famous technique fordeterrnining theage of organic materials using the radioactivecarbon isotope 14C?or carbon-14. These measurements showed that theflax fromwhich thelinenwas produced grew sometime in the 13thor 14th century A.D.?far too recently tohavehad anything todo with thedeath of Jesus.Assigning a reliable age to thismedieval artifact is justone of a long stringofnotable accomplishments sinceWillard F. Libby and his colleagues at theUniversity ofChicago introducedradiocarbon dating inDecember of 1949 (anachievement thatbrought Libby the 1960Nobel Prize for chemistry). A 1974 article by thelate Elizabeth K. Ralph and Henry Michael oftheUniversity of Pennsylvania recounted forreaders ofAmerican Scientist the firstquartercenturyofprogress sinceLibby and his coworkersmade theirpioneering determinations of ageusing 14C. ere I review the entire fivedecadesof development of this remarkable technique,one that today requires no more carbon thanwhat can be found ina few strands of hair.

Long Ago and FarAwayRadiocarbon dating depends on a chain of natural events, some having taken place indeepspace long ago. The sequence begins invariousparts of thegalaxy,where charged particles areaccelerated to immense velocities, formingwhatare known as cosmicrays. fraction f thesepar

ticles eventually rain down on the earth andstrikemolecules of atmospheric gas, producingneutrons. Some of these neutrons in turn reactwith nitrogen, 14N,to form 14C, hich quicklycombines with oxygen to formmolecules of radioactive carbon dioxide (14C02).By the time theradioactive 14C02 reaches the earth's surface, ithas mixed fully ith normal carbon dioxide andaccounts for bout onemolecule in 1012.The vastmajority of this 14Ceventually enterstheoceans. But 1 or 2 percent goes into the terrestrialbiosphere, because plants absorb carbonfromC02 in theairduring photosynthesis. Thusvegetation, and the animals that feed on it, retaggedwith 14C.Living thingsmaintain a 14Ccontent that isabout equal to the atmospheric concentrationbecause the carbon atoms thatundergo radioactive decay within theirbodies are continuallyreplaced. But once an organism dies and itsmetabolic processes cease, theamount of 14C egins todiminish. The rateofdecline ismeasuredby the 14Chalf-life,about 5,730 years.Recognizing these phenomena, Libby realized that the age of ancient organicmatter canbe found from its residual 14Ccontent, asmeasured by the radiation emitted by thematter.For the radiocarbon age to correspond with itsactual age, several conditions must be satisfied.First, theatmospheric ratioof 14Ctonormal carbon (12C)must have remained essentially constant.Second, the ratioof carbon isotopes in themeasured material must not have changed?except by the radioactive decay of 14C?since thedeath of theorganism. Third, there should havebeen rapid and complete mixing of 14Cthroughthe various carbon reservoirs. Finally, themethod assumes that thepresumed half-life of14C is correct and that suitable analyses can reveal small concentrations of radiocarbon withappropriate levels of accuracy and precision.Libby's means ofmeasuring 14Cemployedelemental carbon inamorphous form ("carbonblack"), which he placed in a special type ofGeiger counter.He originally built this type ofapparatus forhis dissertation research atU.C.

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Figure 1.Testing material from the Shroud of Turin todetermine whether this linen is old enough tohave served as theburial cloth for Jesus of Nazareth is just one of thewidely known applications of radiocarbon dating. Sampling tookplace at the Turin Cathedral inApril 1988 under the gaze ofmany observers, including Paul Damon (standing at right),a radiocarbon expert from theUniversity of Arizona, and Cardinal Anastasio Ballestrero (farthest left), then Archbishop of Turin. (Photograph from the archive of Giovanni Riggi, provided courtesy of Paul Damon.)

Berkeley in themid-1930s, while hewas studying theradioactivity of rare-earth elements. Testimonyby thosewho labored with Libby at theUniversity of Chicago, James R. Arnold andErnest C. Anderson, records the great difficulties they encountered?and overcame?in making radiocarbon dating practical.The keywas finding an effectiveway todistinguishbetween background radiation and therelativelyweak and infrequentbeta decay from14Cinnatural samples. Their solution was anticoincidencecounting,a technique that had beenemployed in the study of cosmic rays since the1930s. Following this approach, they comparedpulses from the counter containing the samplewith pulses received from ringof surroundingGeiger counters, the "guard ring." Signals thatdid not also register on theguard ring, that is,those in "anticoincidence," reflected radioactive

decay inside the sample.With this scheme, Libby and his young collaborators were able todemonstrate the inverse relation between the14Ccontent and age fora series of known-agesamples, publishing their first "Curve ofKnowns" inDecember 1949.Successful as theywere, these procedureswere destined to change a great deal within justa decade. The production ofmassive amountsof artificial radiocarbon from the atmospherictestingofnuclear weapons complicated theuseof elemental carbon for low-level 14Cmeasurements. The problem is that solid carbon, like acharcoal filter, tends to absorb stray compounds, and many of theorganics in the environment at thatpoint had become mildly radioactive. So by themid-1950s, proportionalgasand liquidscintillationcounting, techniques thatinvolve only purified C02 orhydrocarbon gas

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Figure 2. Radioactive atoms of carbon, 14C, are created high in the atmosphere aftercosmic rays strike molecules of gas and give rise to short-lived spallation productsthat emit neutrons. Some of these neutrons (those slowed through collisions withatmospheric molecules)

can hit nitrogenatoms (14N) and transform them to 14C.

Quickly oxidized to 14C02, radiocarbon is taken up by terrestrial plants and, inturn, animals. Most of the radiocarbon generated in the atmosphere also mixes intothe sea. Recognizing these processes, Willard Libby, a chemist at the University ofChicago, realized that the age of an organic sample could be ascertained by measuring its radiocarbon content, which would decrease exponentially after death at arate governed by the half-life of 14C, about 5,730 years.

es (such as acetylene and methane) or liquids(such as benzene), had largely replaced theoriginalmethod. These strategies became the basison which tens of thousands of radiocarbondates have since been deterrnined inmore than40 laboratories scattered throughout theworld.

Unfortunately, none of these facilities candate with any precision samples that are lessthan about 300 years old, except under very special conditions. This limitation stems from thenatural fluctuations in 14Cproduction combined,more recently, ith two effects fmoderncivilization: the release of enormous quantitiesof fossil fuel carbon dioxide and theproductionof "bomb" 14C from atmospheric nuclear testing.Themaximum ages thatcan be inferred epend on characteristics of the instrumentationand otherdetails of theexperimental configuration?sample-blank values, counter size, lengthof counting and, to some degree, theamount ofmaterial available foranalysis.Modern laboratorieswith counters designedtowork with older material can readilymeasuremoderately sized pieces of organicmatterthatare asmuch as 40,000 to 50,000 years old.Employing somewhat largersamples, a few laboratories have developed the capability to obtainage estimates up toabout 70,000 years.Andwith isotopie enrichment, some investigatorshave measured a small number of specimensthatare asmuch as 75,000 years old?a trulyremarkable featconsidering that this interval represents 13 half-lives, or a diminution ofmorethan 8,000 times in theoriginal, tinyradioactivity f organicmatter. But improvements inmeasurement are justpart of the storyof thepast 50years: Specialists in radiocarbon dating havealsomade considerable progress inunderstanding the surprisingly complicated relationshipbetween 14Ccontent and elapsed time.Calibrating Radiocarbon TimeA early as 1958, thepioneering 14C investigatorHessel de Vries (of theUniversity ofGroningenin theNetherlands) noted certaindiscrepanciesbetween radiocarbon ages and true ages. Seekingan understanding of thegeophysical implications of thesevariations, several investigators,notably the lateHans Suess of theUniversity ofCalifornia, San Diego, along*with others, suchas Paul Damon at theUniversity ofArizona andMimze Stuiver at theUniversity ofWashington,began to study thisphenomenon. Suess quicklyrecognized a long-termtrendonwhich were superimposed shorter-term components, laterdubbed De Vries effectsr, less formally,"wriggles," "wiggles," "kinks," "windings" or "timewarps." The presence of these deviations in the14C time scale indicated that at least one of thepostulates of themethod?most probably the ssumption of a constant concentration of 14C intheatmosphere?is violated tovarying degrees.Among the investigators studying these socalled secular variationswere Ralph andMichael,who used their1974 article inAmerican Scientistto focus attentionon early sets ofmeasurementsthat illustrated clearly the systematic anomaliesin the 14C time scale. They and otherswere abletodocument theamount of offsetbetween "14C



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time" and "real time" for thepast 7,000 years bymeasuring the radiocarbon ages ofwood fromCalifornia giant sequoia and bristlecone pine?samples for hich the trueages could be determined by counting themany yearly growthrings in these venerable trees. These findingssparked a series of further ffortstocalibrate theradiocarbon method, allowingtrue ages tobecalculated by adding or subtracting the appropriate offset.When Ralph and Michael wrote theirreview,many workers were assuming that the longtermcomponent of thevariation follows a sinewave function,with amaximum deviation of800 to 1,000 years formaterials living about7,000 years ago. But that surmise proved premature. In thepast 25 years, investigatorshavesucceeded in extending the 14C calibrationmuch furtherback in time. For instance, com

parisons between 14C and tree-ring counts ofIrish oak and of German oak and pine pushedthe limit toabout 11,800years ago.Probing the discrepancy between conventional radiocarbon ages and trueages by counting treerings is currentlynot possible for arlierperiods. But investigatorshave nevertheless extended the calibration using othermaterials andother radioisotopic clocks. For instance, themeasurement of trace amounts ofuranium-234and thorium-230 in samples of ancient coral(calcium carbonate) provide estimates of theirantiquity. So by pairing 234Tj/230xh ges wim14Cages from such coral samples, geologists,geochemists and oceanographers working together over thepast decade have extended thecalibration to about 24,000 years ago (or,as expressed inuncorrected 14Ctime, toabout 20,300years before 1950, theunspoken referencedatespecialists commonly use when they give anumber of radiocarbon years "beforepresent").Rather than displaying a sinewave, the longtermoffsetappears to slowly diminish, from itsvalue of about 3,700 years for that time to lessthan200 years for thepast two irtillennia.For earlier intervals, the offsetsbetween agesobtained from 14C and other dating methodsshow discrepancies inmany cases. For example, using materials from aboriginal hearthsfound near Lake Mungo, Australia, investigators have compared radiocarbon ages withthose obtained fromhearth stones using thermoluminescencedating, a technique thatmeasuresthephotons released fromquartz when internal high-energy electrons are freedby heat. Because sunlight can also liberate these trappedelectrons, thermolurrtinescence dating gaugesthe amount of time an object has been buried.The differences between thermoluininescenceand 14Cages for these hearths suggest that theoffset in the radiocarbon time scale for about29,000 years agomay have been in the range of3,500 to 5,000 years. But comparisons between14C results and ages obtained frommineral

Figure 3.Measurement of trace quantities of 14C in a sample is complicated by background environmental radiation, which sets off the instruments used to count the number of atoms of radiocarbon that decay and release an electron. The solution Libby andhis colleagues used in the late 1940s was to surround the decay-counting apparatus withheavy shielding and a ring of small detectors (photograph). They could thus distinguishpulses caused by radiation that had penetrated the shield (A-A' on diagram) from pulses originating inside the detector ( - '),which would be "anticoincident" with signalsfrom the "guard ring. "(Photograph courtesy of theUniversity of Chicago Library.)



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SAMPLES OF KNOWN AGE

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Figure 4. Success was initially demonstrated in December 1949, when Libby and ayoung colleague, James Arnold, published their first "Curve of Knowns" (above).These results showed that radiocarbon content in samples of known age (expressedhere as decay counts per minute per gram of carbon) did, in fact, decrease over time ata rate commensurate with the half-life of 14C. This achievement depended on helpLibby (below, at right) received from another young colleague, Ernest Anderson (below, at left). (Photograph courtesy of theUniversity of Chicago Library; graph reprinted with permission from Science.)

specimens by assessing the radioactive decay ofpotassium into argon suggest less of an offsetfor this same period.Studies of ancient changes in the intensityofthe earth's dipole magnetic field (and thus itsability to shield theplanet fromcosmic rays)?currently considered tobe amajor cause of thelong-termoffsets in 14Cages?also cast doubt onthemagnitude of thediscrepancy found usingmermoluminescence as a yardstick. One interpretation of thegeomagnetic evidence argues fora slowly decreasing offset in radiocarbon ages,one thatstarts in therange of 1,500 to2,700 yearsformaterial living between 25,000 and 40,000years ago. Interestingly, eductions drawn fromthegeomagnetic measurements predict that 14Cages should be largely correct forcarbon that isabout 45,000 to50,000 years old.Further work will be needed to confirm thatthesystematic error inradiocarbon ages for 00century-oldmaterial is trulyso small and toexamine the short-term secular variations at suchremote times.But investigatorsnow have a goodhandle on the "wiggles" in the calibration forthe last twelve millennia. These variations arewell documented, yet they cause considerable

problems because they introducebuilt-inuncertainties indating: Calendar ages can oftenbe expressed only as broad ranges, evenwhen the 14Cconcentrations are known precisely. Thus radiocarbon dates forsome time periods will be inherently lessprecise than those for thers.Radiocarbon specialists have considered various strategies to increase precision. Some employ Bayesian statistics?a technique forcombining stratigraphie ordering and 14Cages?toprevent theerrorbars ina ordered series of samples fromoverlapping. Among other things,such a strategyassumes thatsamples from lowerbeds must be older than those obtained higher in the stratigraphie column. Another novelcalibration procedure is "wiggle-matching."Thismaneuver requires a sequence of 14Cmeasurements for closely spaced tree rings takenfrom sample ofwood of unknown age.Matching the shifts in these values against the knownpattern of short-termde Vries effects in the calibration curve permits a more precise age of thesample tobe found. For example, Austin Longand his colleagues at theUniversity ofArizonaused this strategy todetermine the cuttingdatefora timber froma 4th-century Japanese tombwith an uncertainty of only five years.Accelerated ProgressSo in some cases radiocarbon dates are clearlybecoming more accurate. They are also becoming easier to obtain. The chief difficulty in thepastwas thata considerable amount ofmaterialwas needed tomeasure radiocarbon concentration?and thus age?by counting decay eventswith an ionization or scintillation detector.Decaycounting registersonly a tiny fractionof the 14Catoms present. For example, of the 60 billionatoms of 14C in 1 gram ofpre-industrial carbon,fewer than 14 of these atoms will, on average,decay during each minute ofmeasurement.Practitioners tolerated the inherent inefficiency in decay counting from Libby's firstwork in 1946 until 1977, when physicistRichard Muller of theUniversity ofCalifornia,Berkeley proposed that radiocarbon concentrations could be found directlywith a cyclotron.This high-energy accelerator,which sends particles along a spiral trajectory, ould be used asan ultrasensitive mass spectrometer to distinguish ionized carbon isotopes by theirchargeto-mass ratio. Detecting 14C in thisway waspossible, but consistent results proved difficulttoachieve despite years of effort.Another type of acceleratormass spectrometry,as this approach was called, was described independently by two groups ofphysicists withinmonths of Muller's initial publication. Bothgroups?one led by D. Erie Nelson at SimonFraser University and the second involving acollaboration of investigatorsat theUniversitiesofToronto and Rochester, and theGeneral IonexCorporation?noted that therewere no known



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stablenegative nitrogen ions.By using negativeions, the acceleratorwould discriminate against14N,the ion thatwould create themost difficulty inmeasuring 14C.Both groups also used tandem electrostaticccelerators. s thename implies,these devices employ two stages:A negative ionbeam isfirstaccelerated and passes through a"stripper,"which removes electrons, convertingthe beam topositive ions. These particles arethen further ccelerated. The stripping processbreaks up molecules ofmass 14,which wouldotherwise interfere ith the detection of 14C.Amass spectrometer, essentially a large electromagnet, separates the isotopes of carbon so thatthe 14C toms can be counted directly.The advent of acceleratormass spectrometryat the end of the 1970s brought about an enormous boost in detection efficiency, one thatpromised three important advantages. First, theamount of carbon required should plummetfromgrams tomilligrams. Second, countingtimes should be reduced fromdays, weeks orevenmonths to justminutes. Finally, the sensitivityof detection should increase so that themaximum age datable with radiocarbon mightextend from the then routine 40,000 to 50,000years toasmuch as 100,000 years.

Over the last 20 years, investigators have indeed realized the first two of these anticipatedbenefits. For both sample sizes and countingtimes, thousand-fold reductions have beenpossible. However, theprojected third advancehas not materialized. Why not? It turns outthat sensitivity is limited not by the detectorbut by the tainting of samples with moderncarbon,which is commonly introduced as thematerials are prepared for analysis. Much ofthis contamination stems from the current requirement inmost laboratories that samples beconverted tographitic carbon formeasurementwith an accelerator mass spectrometer. So investigators at these facilities face the sameproblem that hampered Libby's use of solidcarbon samples a half-century ago. Even a fewparts permillion ofmodern carbon ina samplewill limit the effectivemaximum ages thatcanbe resolved. So inpractice, 14Cdating with accelerator mass spectrometry has not brokenthrough the age barrier established by decaycounting: In routine application, it still liessomewhere between 40,000 and 50,000 years;only rarely can 60,000 be achieved.Also, theages estimated with particle accelerators are nomore or less accurate orprecise than

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Figure 6.Gas and liquid-scintillation counting replaced solid-carbon counting duringthe 1950s, in part because of themassive amount of artificial 14C created by the thenfrequent testing of nuclear bombs in the atmosphere. Solid carbon samples wouldquickly absorb trace amounts of "bomb 14C" from the air, just as a carbon filter absorbs many other gaseous organic compounds. Vacuum lines topurify and handle gassamples for radiocarbon dating, such as the apparatus pictured here at the IsotopeLaboratory of theUniversity of California, Los Angeles, were widely employed. (Photograph courtesy of theUCLA Department of Chemistry.)

those obtained by decay counting. But thenewapproach provided away tomeasure sampleswith extremely small amounts of carbon. Experiments thatwould not have been mountedorwould not have been practical with conventional decay countingwere suddenly possible.In thebiomedicai sciences, forexample, clinicians could use natural concentrations of 14Ctotrace the flow of a compound within a patient'sbody, rather thanhaving to administer a morehighly radioactive solution. This advance thuspermitted studies thatwere previously deemedtobe overly dangerous.In archaeology, the leapwas no smaller.Veryquickly, scientists could be quite selective aboutwhat theywanted tomeasure. A good illustrationof thegains thatensued comes froma 1989study ofmaize specimens excavated from tworock shelters in the Tehuacan Valley ofMexico.The Zea mays found at those siteshad been re

garded as among the earliest example of cultivated maize in theNew World, seemingly asmuch as sevenmillennia old. But this ideawasestablished at thebeginning of the 1970s,when itwas not possible tomeasure the radiocarboncontentof the small amount ofmaize recovered.Archaeologists had toobtain age estimates frompieces of charcoal that ere thoughttodate fromthe same period. (Charcoal, unlike unburnedwood, can survivemany centuries ofburial.)In 1989, Long and his colleagues measuredthe 14Ccontent inmilligram amounts ofmaizewith accelerator mass spectrometry and foundthat the seeds were less than 5,000 years old.So over the past decade, archaeologists havehad to rethink their ideas about justwhere thecenter ofmaize domestication inMesoamericamay have been.Accelerator mass spectrometry has alsohelped to address one of themost acrimoniousdebates inNew World archaeology?the natureand timing of the peopling of theWesternHemisphere. Historically, arguments have centeredon two issues: thestrengthof the evidenceforhuman presence and itsdating. Many archaeologists have questioned thevalidity ofmaterialswith radiocarbon dates in excess of thewell-documented Clovis period, forwhich theages of artifacts range from about 10,890 to11,570 radiocarbon years.Beginning in themid-1970s, my colleaguesand I undertook a direct examination of bonefrom human skeletons found at different archaeological sites inNorth American that,onvarious grounds, had been declared tobe preClovis inage. The sensitivityof acceleratormassspectrometry eventually permitted us toobtain14C ages from different chemical fractions ofthese bones, including amino acids and otherhighly specific organic constituents. We couldthus judge whether the bones might have absorbed older carbon, for example, from theground inwhich theywere buried.The work of several laboratories revealed thatall of the alleged pre-Clovis ages were unreliable. Although therenow is evidence that atleastone site inSouth America (Monte Verde inChile) may have been occupied about 1,000years before theClovis period, no human skeleton fromNorth America has yielded ages anyolder than 11,000 radiocarbon years.So thepeopling ofNorth America probablybegins, as most prehistorians have long believed, at the very end of the last ice age. Butthestory iscertainlynot a simple one, and the final chapter has not been written. Our recent 14Cmeasurements on human skeletons recoveredfromKennewick inWashington and SpiritCaveinNevada hint at some of the complications.These remains, between 8,000 and 10,000 yearsold, seem to show skeletal features similar tothose of early South Asian populations, unlikemore recentNative Americans. It appears that



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Figure 7.Direct measurement of 14C became possible after1977, when physicists began to couple particle acceleratorswith mass spectrometers. One of the first demonstrations of this technique took place using an acceleratoratMcMaster University (above). D. Erie Nelson and hiscolleagues combined this impressive instrument with alarge analyzing magnet (far right), allowing the differentisotopes of carbon tobe separated. They shared their pioneering ideas for conducting radiocarbon measurements in thisway with a diagram (right), generated usingone of the firstMacintosh computers. (Photographs anddiagram courtesy ofD. Erie Nelson.)

the late ice-age inhabitants of North Americamay have been much more genetically diversethanwas previously imagined.An Impressive Half-CenturyNow, at the 50th anniversary, it is difficult formany people toappreciate justhow dramaticallythe advent of radiocarbon dating transformedarchaeology. Libby's inventionmade the studyofworld prehistory truly ossible, contributingatime scale thatmay not have been perfect (particularlyat theoutset) but that transcended theformer jumble of regional schemes.Radiocarbon dating continues toprovide thechronological foundation, directly or indirectly,formost investigations into the past 500 centuriesofprehistory.And itregularly contributestomany other fields of science, fromgeology toastrophysics, a breadth that is perhaps fittingforamethod thatowes its current level of sophistication toworkers in somany disciplines.AcknowledgmentThe author would like to thank r. JohnSouthon(CenterforAcceleratorMass Spectrometry, aw

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renceLivermoreNational Laboratory) or his valuable assistance in thepreparationof this rticle.BibliographyArnold, J.R., and W. R Libby. 1949. Age determinationsby radiocarbon content: Checks with samples ofknown age. Science 110:678-680.Bard, E., B.Hamlin, R. G. Fairbanks and A. Zindler. 1990.Calibration of the 14C timescale over the past 30,000years using mass spectrometric U-Th ages from Barbados corals. Nature 345:405-410.Kline, J.,J. . Lerman, P. E. Damon and E. K. Ralph. 1982.Calibration of radiocarbon dates: Tables based on theconsensus data of theworkshop on calibrating the radiocarbon timescale. Radiocarbon 24:103-150.Ralph, E. K,.and H. N. Michael. 1974. Twenty-five years ofradiocarbon dating. American Scientist 62:553-560.

Links to Internet resources for further explorationof "Fifty Years of Radiocarbon Dating" areavailable on the American ScientistWeb site:http: / /www.arnsci.org/amsei/ articles/

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Taylor 50 y of Radiocarbon Dating Am Sci