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Beyond the radiocarbon limit inAustralian archaeology andQuaternary researchJOHN CHAPPELL,OHN HEAD& JO HN M A G E E *

Al l en (1994) and Al len b Holdaway (1995),not ic ing the pa t tern in early radiocarbondates f rom Austra l ia , hav e advanced the no t ion the ir limit records the hum an se t t lementof the cont inent . A critical analysis of conte xt and c onte nt in thos e carbondeterminat ions leads to a d i fferen t v iew. Th e resul ts m a y be d isconcert ing for everyregion w hich build s its late Pleistocene chronologies on radiocarbon!IntroductionThe question of hum an antiquity in Australiahas recently become a debate largely aboutdating. On one side, Allen (1994) and Allen &Holdaway (1995) argue that humans have beenin Australia for some 40,000 years, essentiallybecause no I4Cages greater than 40,000 b.p. havebeen obtained from Australian archaeologicalsites. On the other side, Roberts et al. (1990a;1994) have published thermo- an d opticallystimulated luminescence (TL and OSL) agemeasurements that indicate human presence5340,000 years ago.To support their view, Allen& Holdaway (1995) show that ages significantlygreater than 40,000 b.p. have been reported frommany geological sites in Australia but there arenone from archaeological sites.

We believe that the reliability of 14C ages,when close to the dating limit, has not beensufficiently examined. There is no more prac-tical way of testing any dating method than tocompare it with another method, using the sameor stratigraphically associated specimens. Con-cordant ages may indicate that both methodsare correct; disparate ages demonstrate that a tleast one of the methods is incorrect. Unfortu-nately, the I4C and luminescence dating meth-ods have not been well tested against each otherat Australian archaeological or geological sites,for the critical age-range of about 35,000 to50,000 b.p.

The fundamental assumption in I4C datingis that a specimen has remained closed to al lexchanges of carbon, from the time of its for-mation to the time of its measurement. Althougha variety of methods are used to isolate the origi-nal carbon in a sample, it is not easy to dis-cover whether decontamination is perfect when14C itself is the only tracer of the process. Inthis paper, we examine the reliability of LatePleistocene 14Cage estimates derived from vari-ous sites, in the light of both I4C data per seand comparisons of I4 Cdates with ages derivedby other techniques, and we show that a largenumber of 14Cdates in the range 30-45,000 b.p.do not represent the true age of their parentdeposits. This is not a new conclusion, but bydocumenting the data we hope that proceduresfor obtaining reliable ages for late Pleistocenesites- rchaeological and geological - illbe clarified. Although most of our case-stud-ies are drawn from Australia, our conclusionsprobably have a wider relevance.A problem revisitedThe question of the age of human antiquity inAustralia is reminiscent of an earlier debate,concerned with the position of sea-level some30-40,000 years ago. FIGURE shows sea-levelestimates in the age range of 20-38,000 b.p.,from more than 60 studies reviewed by Thom(1973). Sample materials included shells, cor-

* Division of Archaeology & Natural History, Research School of Pacific & Asian Studie s, Australian NationalUniversity, Canberra ACT 0200, Australia.Received 15 January 1996, accepted 9 February 1996.ANTIQUITY 70 (1996):543-52


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544 JOHN CHAPPELL, JO H N H E A D & JO H N MAGEE

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0

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100

E2 -10a>Q)Q)v)

Icb -20-30

-40

-50

I 00 0

0 . 0 00

0

0 .

0 .0

0

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FIGURE. Sea-levelest imates based on14C-duted alaeo-20 22 24 26 28 30 32 34 36 38 40 shorelinedepositsrepor ted f rom m an y

counir ies (data fromThorn 1973).thousand years b.p.als, wood and other organic matter. Many ofthe putative sea-level estimates are within 10m of the present level but altogether they scat-ter over more than 50 m. The scatter does notreflect tectonic displacements of individual sites;most of the data come from places that aretectonically stable on Pleistocene time-scales.Thom (1973) considered that most- f notall - he sea-level estimates represented inFIGURE are unreliab le, except perhaps for thethree points ar oun d45 to -50 mat about 30,000yr b.p. Except for the same three points, noneare remotely compatible with widely accepteddata which show that sea-level varied between-50 to -130 m, 40,000 to 20,000 years ago,according to 23nTh/Z34U-datedaised coral ter-races in Huon Peninsula, Papua New Guinea,and oxygen-isotope data from deep-sea cores(Chappell & Shackleton 1986; Shackleton 1987;Chappell 1994). In some cases, Thom (1973)considered that dated samples may have beenmisassociated with sea-level events; in most

cases the 14Cdates themselves were in error. Inshort , FIGURE represents a set of late Pleisto-cene I4C ages that have missed the target.

We now examine sources of dating error thathave been revealed by selected cross-datingstudies.14Cversus 230ThP34Uating of coral reefsAncient corals are assumed to have been closedgeochemical systems provided they retain 100%of their original aragonite skeleton and havethe same trace element and isotopic composi-tion as modern corals, including the same ini-tial 23nTh/Z34Uatio. These criteria are met incoral specimens from drill cores from PapuaNew Guinea and Barbados, dated by the I4CandZ3nTh/234Uating techniques, that range inage from about 8000 to about 3 8,000 b.p. Theresults, judged reliable, have been used to ex-tend 14C age calibration beyond the tree-ringcalibration limit (Bard et al. 1990; Edwards eta]. 1993).


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BEYOND THE RADIOCARBON LIMIT IN AUSTRALIAN ARCHAEOLOGY AND QUATERNARY RESEARCH 545Not all 14Cages from corals are equally reli-able. Chappell & Polach (1972) reported results

from 10diagenetically-altered corals in whichthe proportion of original aragonite skeletonranged from 85% down to 5%. All these speci-mens were known to be older than 40,000 b.p.,but the I4C ages ranged from 11,500 to 30,000b.p. The discrepancy between real and appar-ent ages generally increased with the degree ofalteration, but the relationship was not uniform.Although these particular corals would not beaccepted as reliable for 14Cdating because theywere not 100% aragonite, they clearly demon-strate that diagenesis leads to false 14Cages.The same is true for molluscs, even massivetypes such as the giant clam, Tridacna gigas.Chappell et a l . (1974) described five 7: gigasspecimens known to be much older than 40,000b.p., which X-ray diffraction showed to be 100%aragonite (like modern Tridacna) and micro-scopically show only the most subtle diageneticalteration of the original aragonite fibres. Withlong counting periods, 14Cbackground was re-corded for three specimensand twoothers showedapparent ages of 40,000 and 43,000 b.p. (I4, back-ground is a measurement that is statistically in-distinguishable from the background level of the14Ccounting instrument.) The false ages- 0,000and 43,000 b.p.- ere believed to reflect theeffect of minor contamination, owing to diagenesisof the structure but not the mineralogy.Corals and shells are prone to diagenesis,making it difficult to obtain reliable ages closeto the practical 14Cdating limit. With their highspecific surface area, corals may be more proneto diagenesis than the more massive varietiesof molluscs. 230Th/234Uating and uranium iso-tope diagenesis criteria, used to check I4C agesfrom corals, cannot be applied to molluscs asthey contain negligible original uranium. Dia-genesis can be detected by combined X-raydiffraction, microscopic thin-section examina-tion and trace-element analysis (Curtis & Krin-sley 1965; Chappell & Polach 1972; Zhu et al.1994); t indicates that I4C exchange may haveoccurred. Regrettably, such data often are notdocumented for Pleistocene 14C ages. Thom(1973)judged that almost all studies representedin FIGUREwere deficient in this respect.14Cversus other dating methods at Lake EyreA cross-dating study at Lake Eyre in centralAustralia by Magee et al. (1995) found I4C ages

of organic sediments to be unreliable in the range35-45,000 b.p. These authors describe a se-quence of lacustrine, lake-shore a nd dune de-posits at the Williams Point site, Lake Eyre, thathave been cross-dated by I4C (both conventionaland AMS), TL, 230Th/Z34Und amino-acidracemization (AAR). The AAR ages, based onegg-shell from emu (Drornaius)and extinct gi-ant, flightless Geniornis,were calibrated experi-mentally a nd from AMS I4C-dated egg-shellyounger than 30,000 b.p. (Magee et a l . 1995).The base of the sequence is correlated with theclimax of the Last Interglacial (oxygen isotopestage 5e , 125,000 b.p.); 14C and AAR resultsindicate that the top of the sequence is 40-50,000years old. Stratigraphically, the sequence repre-sents lake-level variations that occurred duringearly and middle stages of the last glacial cycle.Results from all dating methods used atWilliams Point are plotted against stratigraphicdepth in FIGURE . Except for the radiometric230Th/234Uate at the top of the sequence (basedon a mixed sample with no systematic pre-treatment; regarded as less reliable than thecarefully-pretreated TIMS-based 230Th/234Uam-ple: Magee et al . 1995), he TL, 230Th/234UndAAR dates increase uniformly with depth; thetrend line passes through the stratigraphicallyassigned age of 125,000 b.p. at the base of thesection. In contrast, the I4C esults are all youngerthan 45,000 b.p., to the base of the section. Fiveof the I4C dates were done by AMS using re-fractory carbon residues extracted by a rigor-ous oxidative cleaning procedure, including fourfrom impervious, laminated organic clays inthe lower part of the sequence (Magee et a l .1995). In the light of the consistency betweenexternal evidence and all the other dating re-sults, the 14C dates are misleading - et theAMS samples represent materials and pre-treat-ments commonly regarded as reliable.Asymptotic series of 14C datesRadiocarbon ages of several wood samples foundunderneath basalt flows at Auckland, NewZealand, were compared with potassium-argon(K/Ar) dates from the overlying basalts byMcDougall et al. (1969). Several of the K/Arages were much greater than the I4C dates; theauthors were confident the 14Cdates wereessentially correct, because different chemicalextracts from a given wood specimen gavesimilar results. However, the Lake Eyre study


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54 610

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J OHN CHAPPELL, JOH N HEAD &JOHNMAGEE

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td w w 0 4 - I \\\\\

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age (thousand years b.p.)FIGURE. Sam ple ages , ob ta ined b y various da t ing me thod s , versus depth a t the Wi l l iams Point s i t e,Lake Eyre. The I4C ages tend to an asymp to t ic va lue of 45,000 b.p. whereas resul ts fro m thc othermeth ods increase un i formly wi th dep th .AMS = accelerator ma ss spectrometry; U/Th = 23nTh/2U; IM S = thermal ion iza t ion m ass spec trometry;TL = thermoluminescence; AAR = amino acid racemizat ion.Note: the radiometric a30Th/Z34Uate w as fro m a col lect ion of eggshell fragm ents an d is judged to beless reliable than th e T I M S 230Th/234Uate ( see t ex t ](summarized in FIGURE ) indicated thatconcordant I4 C ages for different fractions of agiven specimen do not necessarily prove thatthe age is correct, when the results are around

age closeto the dating limit is illustrated by a set of meas-urements by the Australian National Univer-sity Radiocarbon Dating Laboratory from kauri(Agathis sp.) logs, preserved in coastal sandbarriers in northern New South Wales. The logswere deposited when the sand barriers wereformed (B.G.Thom pers. comm.), 120-130,000years ago according to 23Th/234Uates fromcorals found at other localities in the same barrierformations (Marshall & Thom 1976). However,14Cages from 16 different extracts, separatedfrom the wood samples and humified matrix,

40-50,000 b.p.The difficulty of confirming a

ranged from 11,100 to 43,430 b.p., includingtwo greater than ages (~37 ,800and >39,690)and two background results (TABLE).The data in TABLE show several instruc-

tive features. Firstly, only the background andgreater than ages are technically compatiblewith the true age of the deposit; the other 1 214C ages are erroneous. Secondly, four prepa-rations that gave ages below 40,0130 b.p. are oftypes normally regarded as highly reliable, twocellulose preparations and two clean woodresidues. The cellulose in these samples waslater shown to be diagenetically altered, by elec-tron microscopy and other methods describedby Head (1979; 1980). Regrettably, this type ofinformation is rarely sought or reported. Finally,any of the 1 2 finite I4Cages normally would beaccepted unless the dating laboratciry was asked


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BEYOND THE RADIOCARBON LIMIT IN AUSTRALIAN ARCHAEOLO GY AND QUATERNARY RESEARCH 547sample site l material age (years b.p.) errorANU-918BANU-12 3 7ANU-918CANU-1162ANU-918AANU-9 18DANU-973ANU-971EANU-971DANU-971BANU-971AANU-968ANU-1161ANU-971 CANU-970ANU-969

resinhumatehumic & fulvichumateuntreated woodcellulosecelluloseclean wood residueclean wood residuecellulose Bcellulose Acellulosehumatehumiccellulosecellulose

11,10015,05015,90022,25023,29035,20036,10036,65036,76041,44043,40043,430>37,800>39,690backgroundbackground

k880k750k180f 3 0 0k560+2000/-1500+2670/-2000+1460/-1240+1500/-1260+2140/-1690+2590/-1950+3200/-2300

1because the data are unpublished.TABLE . I4C d a t e s of wood and wood extracts from ast interglacial coastal barriers in N e w S o u t h W a l e s.[Unp ubl i shed da tes from A N U l a b a r c hi ve ,from a m p l e s s u b m i t te d b y B.G. Thorn.)

Field sites are indicated to allow comparisons between samples from same site; sites are coded by their in itial letters

to persevere until the possibility of a back-ground result was confirmed. Without this, andwithout the independent 230Th/Z34Uates, anage around 3545,000 b.p. would have beenaccepted.

A second set of I4Cages, represented in FIG-URE 3 , are from a 450-cm section at PulbeenaSwamp, northwest Tasmania, reported by Col-houn et al. (1982). At depths below 180 cm,the I4C ages vary between 40,000 and 55,000

FIGURE. I4Cagesversus s a m p l e d e p t ha t P ul b ee n a S w a m p ,nor thwe s t Tasmania(data from C o l h o u n etal. 1982).Arrowrepresents constants e d i m e n t a t io n a t 4cm p e r 1000 yearsfrom 0 o 200 c m .

60

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0 6 I 1 I I0 100 200 300 400 500

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548 JOHN CHAPPELL, JOHN HEAD & JOHN MAGEEA: DECREASING FREQUENCY

6: RECTANGULAR

I

FIGURE . Schematic age-frequency histograms.(triangular or decreasing distribution).increasing age (rectangular distribution).peak at high-age end (high-age cluster).

ABC Rectangular distribution plus significant

Frequency decreases with increasing ageFrequency is statistically constant with

b.p. The dates from samples below 200-cm depthare almost certainly erroneously young. At facevalue, the results imply an extraordinary changeof sedimentation rate around 45,000 b.p. (FIG-URE 3); this seems most unlikely, as no dramaticchanges of sedimentary facies were reported

by Colhoun et al. (1982).The pollen record it-self does not dictate that the dates below 200cm are correct; the true age of the Pulbeenadeposit below 200 cm almost \certainly wellexceeds the I4Cages.

The I4C ages in FIGURES & 3 and TABLEall approach asymptotes at c. 40-50,000 b.p.Other data-sets follow this pattern. Although oc-casionally a result may lie beyond the asymp-tote for a particular data-set, the asymptotictendency shows that I4C very commonly be-comes quite unreliable at around 40,000 b.p.,owing to traces of younger carbon. The asymp-totic tendency is manifest by radiocarbon agesthat are cited as finite (i.e. citecl as ages k anage error), and not by greater than ages as de-fined by Stuiver & Polach (1977 .

The asymptotic tendency is a general but notinvariant limitation; it is always possible thatcompletely uncontaminated specimens beyondthe 14C imit can be found. In general, however,the practical asymptotic age varies with the typeof sample material and sample pretreatment.It is about 30,000 b.p. for the cliageneticallyaltered corals reported by Chappell & Polach(1972); about 45,000 for the org,anic clays atLake Eyre (FIGURE); and about 50,000b.p. forthe kauri wood samples (TABLE) nd PulbeenaSwamp (FIGURE) . In each case, some sampleswhose true age lay well beyond their groupasymptote gave 14Cages significs ntly youngerthan the asymptotic value. All these illustratethe difficulty of obtaining a true age for sam-ples older than about 40,000 b.p.Age-frequency distributions of I4C datesAge-frequency histograms of rad;ometric agesoften are used to evaluate the time-distributionof past events. Allen & Holdaway (1995) usedage-frequency histograms of late Pleistocenearchaeological and geological 14Cdates to sup-port the inference that humans probably havebeen in Australia for no more than 40,000 years.Before discussing their histograms, some effectsof sampling strategies and other factors areoutlined.

Three I4C age-frequency histogram patternsare shown in FIGURE ; 4A shows frequencydecreasing with time, 4B a uniform (rectangu-lar) distribution and 4C a concentration of datesaround 40,000 b.p. Ensembles of dates, fromwhich background and greater-than dates areexcluded, can show any of these patterns. Given


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BEYOND THE RADIOCARBON LIMIT IN AUSTRAL IAN ARCHAEO LOGY AND QUATERNARY RESEARCH 549Al : SWAMP& BOG CORES

>W3Y

se0 3 5 8 10 13 15 18 20

thousand years b.p.

A2: VALLEY-FILL DEPOSITS

3 6 9 12 15 18 21 24thousand years b.p.

B: CORA L-REEF DRILL CORE

7.5 8 8.5 9 9.5 10 10.5 11thousand years b.p.

C: PULBEENA SWAMP CORE

10 15 20 25 30 35 40 45 50thousand years b.p.

FIGURE. Four age-frequency his tograms from I4C-dated s i tes . Distribut ion type s matc h s chem atic for mss h o w n in FIGURE3 .A-1 Triangular dis tr ibut ion of dates fro m eight swa m p an d bog cores cores in sou theas t Aus t ral ianhigh land s (data source, pol le n s i te data-base he ld a t Divis ion of Archaeology b Natural History, RS PA S,

ANU).Wales (da ta rom Gi llesp ie e t al. 1992).1991;Edwards et al. 1993).

A- 2 Triangular dis tr ibut ion of dates fro m val ley- f il l dep osi ts at W angrah Creek, southern N ew Sou thB Rectangular dis tr ibut ion of dates fro m 53 -m coral-reef core (d ata sources Chappel l 6. PolachC High end-pea k d i s t ribu tion o f da tes f rom Pulbeena Sw am p (da tes shown in FIGURE ) .

uniform sampling and accurate dates, pattern4A arises either when the events being datedbecome more frequent towards the present, orwhen the occurrence of carbon material de-creases as site age increases. Pattern 4B ariseswhen carbonaceous deposits that are uniformlydistributed in time are uniformly sampled andaccurately dated. Pattern 4C arises when carbon-aceous deposits that extend beyond the 14Cdating limit are uniformly sampled, but resultsnear the I4C limit tend to a dating asymptote.Finally, the histogram of an assemblage thatrepresents a mixture of the above three casesmay approximate to a rectangular pat tern, thatof 4B , depending on the mix.

Actual examples of these three patterns areshown in FIGURE. Alternative origins for pat-

tern 4A are illustrated by I4C dates from bogand swamp deposits in the southeastern Aus-tralian highlands (FIGUREA-1) and from lateQuaternary slope and valley-fill deposits insouthern New South Wales (FIGUREA-2). Thefirst represents regional expansion of swampydeposits in post-glacial times, while the sec-ond represents diminishing preservation ofcarbon material with increasing time rather thantemporal changes in the extent of slope andvalley-fill deposits. Pattern 4B is illustrated byresults from a closely dated coral-reef drill-core(FIGUREB). The data from the Pulbeena site(discussed earlier) conform statistically to pat-tern 4C (FIGUREC).

In the suites of Australian la te PleistoceneI4C dates compiled as histograms by Allen &


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550 JO H N CHAPPELL, JOHN HEAD & JOHN MAGEEHoldaway [1995), he archaeological dates showdecreasing frequency with increasing age; thereare no dates beyond 40,000 b.p. (pattern 4A),whereas the geological dates approximatelyconform to a rectangular pattern that extendsbeyond 50,000 b.p. [pattern 4B). Allen & Hold-away did not filter dates for reliability, implic-itly assuming that unreliable dates would notaffect the significance of the differences betweenthe archaeological and geological histograms.The rectangular histogram of geological datesis a n amalgam containing 14C dates from hill -slope and alluvial deposits with a type 4A dis-tribution, long lake-core deposits with a type4B distribution, and asymptotic sequences (in-cluding Pulbeena Swamp) that tend to a type4C distribution. In comparison, the archaeo-logical histogram may reflect either: diminishingpreservation of carbon material with increas-ing age; or human arrival at about 40,000 b.p.followed by slow expansion of the number ofsites and, presumably, of the human popula-tion. These alternatives were not closely ex-amined by Allen & Holdaway (1995). Until moreis known about all the dates involved, the dif-ference between the archaeological and geologi-cal distributions cannot be used as firm evidencefor human arrival at 40,000 b.p.The 14Cevent horizon in AustralianarchaeologyCarbon, in combination with a few other ele-ments, forms a very considerable variety ofcompounds, including those essential to andstemming from all life processes. While thisreactivity generates a large suite of datablematerials, it also militates against the chancethat carbon materials will readily persist asclosed chemical systems in surficial deposits.The studies reviewed above indicate that ourability to measure low levels of 14Coften sur-passes our ability to remove contamination bysample pretreatment; at the very least, all I4Cdates near 40,000 b.p. require close scrutiny.

Various schemes have been proposed for fil-tering I4C dates. Meltzer & Meads (1985) wasused by Baynes (1995) to scrutinize I4C dateslinked to extinct Australian megafauna: Baynesregarded 83 dates as doubtful out of a total of91, including all those younger than 28,000 b.p.Probably no filtering scheme is perfect. In as-sessing north American dates, Meltzer & Meadranked charcoal as the most reliable dating

material and shell as almost the least reliable.However, charcoal has proven m indifferentdating material in many Australian sites olderthan 30,000 b.p., owing to its cspacity to ad-sorb mobile organic compound:;. While mostcontaminants are removed from charcoal bystandard acid and alkali pre-treatment, it hasbeen found that repeated, successively morerigorous cleanings can produce 8l eries of dif-ferent ages, showing the persistence of low levelsof contamination (Gillespie et al. 1991; 1992;Magee et a l . 1995). Furthermore, contrary tothe ranking adopted by Meltzer & Mead (1985),shells in Australia have often given older andmore consistent 14C dates than have charcoalsamples from the same deposits; they are re-garded as at least equally reliable (Bowler &Wasson 1983).Finally, no screening procedureis likely to succeed unless all forms of samplediagenesis are taken fully into account.

The asymptotic tendencies o F different I4Cdata-sets indicate that there is a 14C event ho-rizon, difficult to detect by screening proce-dures that do not include crosj-dating. Thealternative dat ing techniques that are used tocross-check 14C esults must themselves be in-dependently validated, and here in lies the rubin Australian Pleistocene archaeology. Agemeasurements at the Malakunanja I1 and Nawal-abila sites in northern Australia, by TL and OSLrespectively, indicated that the oldest artefact-bearing horizons were deposited 50-60,000years ago (Roberts et a l . 1990a; 1994). The as-sociation of TL dates and artef,x ts at Mala-kunanja I1 [debated by Hiscock ,1990; Robertset al . 1990b) no longer appears to be at issue,particularly since Roberts et al. (1994)reporteda similar association at Nawalabils; the remain-ing question is whether the luminescence datesfrom these sites are realistic.

The critical TL/OSL dates from the lowerdeposits at Malakunanja I1and Navvalabila couldnot be cross-checked against I4C,as carbon wasabsent ; the methods agreed reasonably well athigher stratigraphic levels in the range 15-30,000b.p. [Roberts et al. 1990a; 1994). Furthermore,luminescence age measurements o~~110-130,OOOb.p. have been reported from Australian coastalsand barriers of Last Interglacial age, and L30Th/234Umeasurements gave comparable results[Huntley et a l. 1994; Marshal & Thom 1976;Roy et al. 1994). The dated material in boththe archaeological sites and the coastal barri-


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BEYOND THE RADIOCARBON LIMIT IN AUSTRALIAN ARCHAEOLOGY AND QlJATERNARY RESEARCH 551

ers was quartz sand. As the luminescencemethod has been supported well beyond theage-range of the early Australian archaeologi-cal sites, with the same type of sample mate-rial, there is no good reason to doubt the TLand OSL dates reported by Roberts et al. (1990a;1994).

In conclusion, the I4C age of 40,000 b.p. forfirst humans in Australia is likely to reflectnothing more than an event horizon, whicharises partly through the tendency of I4C datesto cleave to an asymptote near the effectivedating limit and partly because carbon is scarcein the oldest sites. For the 14C-based ge favouredby Allen & Holdaway to win acceptance, ear-liest archaeological sites that are carbon-richto the bottom need to be found, in order thatcross-dating against other methods and com-prehensive 14C dating of multiple carbon frac-tions can be done, as has been done whereconditions have permitted at geological sites.Furthermore, it would be necessary for lumi-nescence ages to be shown independently tohave a tendency to err in the old direction.

ReferencesALLEN,. 1994. Radiocarbon determinations, luminescence dat-

ing and Australian archaeology, Antiquity 68: 339-43.ALLEN,. & S. HOLDAWAY.995. The contamination of radio-carbon determinations in Australia, Antiquity 69: 101-

12.BARD,E. , B. HAMELIN,.G. FAIRBANKS,. ZINDLER,. MATHIEU& M. ARNOLD. 990. U/Th and 14C ages of corals fromBarbados an d their us e for calibrating th e C time scalebeyond 9000 years BP, Nuclear Instruments 6.PhysicsResearch B52: 461-8.BAYNES, A. 1995. The question why did t he Australianmegafauna become extinct is unansw erable. Abstracts,Taphonomy Sy mpos ium, Department of Anthropology& Archaeology, ANU, Canberr a, April 1995 : 12-14.BOWLER, .M. & R.J. WASSON. 983. Glacial age environ mentsof inl and Australi a, in J.C. Vogel (ed.), Late Cainozoicenvironments of the Southern Hemisphere: 183-208.Rotterdam: Balkema.

CHAPPELL,. 1995. Upper Quaternar y sea-levels, coral terraces,oxygen isotopes an d deep-sea temperatures, Journal ofGeography (Japan) 103: 828-40.

CHAPPELL, J. , W.S. BROECKER,.A. POLACH B.G. THOM. 974.Problem of dating upp er Pleistocene sea-levels from coralreef areas, Se con d Internat ional Coral Reef Symposium2: 563-71.CHAPPELL, J. & H.A. POLACH. 972. Som e effects of p artialrecrystallisation on C-14 dating of late Pleistocene cor-als and molluscs, Quaternary Research 2: 244-52.

1991. Post-glacial sea-level rise from a coral record a t HuonPeninsul a, Papua New Gu inea, Natu re 349: 147-9.CHAPPELL,. & N.J. SHACKLETON.986. Oxygen isotopes and

sea-level, Nature 324: 137-40.COLHOUN, E.A., G. VAN DE GEER& W.G. MOOK.1982. Stratigra-phy, pollen analysis, and paleoclimatic interpretation ofPulbeena Sw amp, northwestern Tasmania, QuaternaryResearch 18: 108-26.

However, the asymptotic tendency of I4C datesindicates that these tend to err in the youngdirection while cross-dating supports lumines-cence dates to at least 120,000 b.p. Hence, weconsider that the luminescence-based age of 50-60,000 is the best available estimate of the ageof human presence in Australia.

Finally, the problems illustrated here withreference to Australian evidence are likely tooccur widely. When a set of 14Cdates tend toan asymptote, there is a high possibility thatages close to the limit are unreliable. This isbest checked by cross-dating studies as well asI4Cdating of successive chemical extracts froma given sample. Furthermore, no particularmaterial should be assumed to be best for 14Cdating close to the limit; it is more importantto examine the limits of a given type of sampleby a range of pretreatment studies and to verifyresults by cross-dating beyond the 14C limit.Acknowledgements. We particularly wi sh to thank Mat-thew Spriggs, Sue OConnor and other colleagues at AN17for their encouragement to prepare this paper.

CURTIS , .D. & D. KRINSLEY.965. The detecti on of minordiagenetic alteration in shell material, Geochimico Cosmo-chimica Acta 29: 71-84.EDWARDS, .L., J.W. BECK, G.S. BURR,D.J. DONAHUE,.M.A.CHAPPELL,.L. BLOOM,E.R.M. DRUFFEL F.W. TAYLOR.1993. A large dr op in atmospher ic 14C/2C nd reducedmelting i n the Younger Dryas, documented with Thages of corals, Science 260: 962-8.GILLESPIE,., E. DLUGOKENCKY ,.J. SPARKS, . WALLACE,.P.PROSSER J. CHAPPELL.992. Alluvial s ediments: AMSdating at Lanyon and Wangrah Creek, Radiocarbon 34(1):21-8.

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beyond the radiocarbon limit in - chappell

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