Precision and accuracy in applied 14 C dating: some findings from the Fourth International Radiocarbon Inter-comparison

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1 Journal of Archaeological Science 31 (2004) 1209e Precision and accuracy in applied 14 C dating: some findings from the Fourth International Radiocarbon Inter-comparison E.M. Scott a, ), C. Bryant b, I. Carmi c, G. Cook d, S. Gulliksen e, D. Harkness a, J. Heinemeier f, E. McGee g, P. Naysmith d, G. Possnert h, H. van der Plicht i, M. van Strydonck j a Department of Statistics, University of Glasgow, Glasgow, G128QW, UK b NERC Radiocarbon Laboratory, Glasgow, UK c Weizmann Institute, Rehovot, Israel d SUERC, Glasgow, UK e NUST, Trondheim, Norway f University of Aarhus, Aarhus, Denmark g University College Dublin, Dublin, Ireland h University of Uppsala, Uppsala, Sweden i University of Groningen, Groningen, The Netherlands j KIK, Brussels, Belgium Received 30 September 2001; received in revised form 5 April 2002 Abstract Users in the Quaternary and Archaeological Sciences have expressed a general desire for significant improvements in the accuracy and precision of radiocarbon dating results in general but also allied to the measurement of small samples. The accuracy and precision of measurement has also been the focus of some attention within the 14 C community. As a result, the 14 C community has undertaken a wide-scale, far-reaching and evolving programme of inter-comparisons, to the benefit of laboratories and users alike, the most recent being completed in The information arising from the studies is important for the appropriate interpretation of the scientific evidence provided by 14 C analyses in calibration and construction of chronologies where assemblages of dates are frequently assessed. In this paper, some preliminary findings from the Fourth International Radiocarbon Inter-comparison, completed in 2001, will be reviewed and some conclusions drawn with regard to accuracy and precision of 14 C dates. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Radiocarbon dating; Inter-comparison; Precision; Accuracy 1. Introduction Much of chronology construction in archaeological science depends on radiocarbon dating. The quality of the chronology depends fundamentally on the quality of the measurements made in its support. Therefore, it is crucial that the quality of the measurements be assured ) Corresponding author. Tel.: C ; fax: C address: marian@stats.gla.ac.uk (E.M. Scott). as part of the overall quality assurance of the science underpinning the chronology. Measurement quality assurance as implemented by the laboratory has a number of components, including the use of in-house reference materials, measurement of international standards, development and implementation of detailed procedural documentation and regular participation in laboratory inter-comparisons. This latter aspect of laboratory quality assurance provides an independent check on laboratory performance, verifying both laboratory accuracy and precision /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.jas

2 1210 E.M. Scott et al. / Journal of Archaeological Science 31 (2004) 1209e1213 During the past 15e20 years there have been several, large inter-comparison studies. In these first inter-comparisons, one of the key questions was of comparability amongst laboratories [3,5,6]. The inter-comparisons found evidence of general agreement amongst laboratories, although there were aberrant results (and laboratories), and that there were no significant differences amongst the laboratory types. However, the sample sizes used were optimized for radiometric laboratories and typically only one or two known-age materials were used. Thus, after a 5 year interval, the 14 C community has undertaken a further inter-comparison, with attention being paid to sample size issues and also to accuracy relative to the master 14 C calibration. 2. The Fourth International Radiocarbon Inter-comparison (FIRI) The fundamental aims and objectives of FIRI [1] reflect a continuing commitment to the issues of accuracy and precision in basic 14 C research and can be simply summarized: demonstration of the comparability of routine analyses of both accelerator mass spectrometry (AMS) and radiometric laboratories; quantification of the extent of and sources of any variation; investigation of the effects of sample size, pretreatment and precision requirements on the results. 3. Samples As in previous inter-comparisons [3], FIRI has focussed on the use of natural materials, and also implemented a two-stage design, with a set of core samples and a set of optional samples [1,7]. Over 130 laboratories world-wide were invited to indicate their willingness to participate by completing an initial questionnaire indicating the types and amounts of samples which they would require. Over 120 laboratories responded positively. The study design was then finalized and involved both AMS and radiometric laboratories in the measurement of a series of natural routinely dated samples spanning the 14 C activity range. It was also identified as important that an archive of material be created and held for future reference. Given the success in recruiting participating laboratories, the quantities of material which were required were considerable and varied between 10 and 20 kg depending on any pretreatment process which the material would undergo. It was also identified in the design that the activities (ages) of some of the samples should be known. This meant in reality that dendro-chronologically dated wood samples were essential. Furthermore, a set of samples which Table 1 Geographical distribution of participating laboratories Broad geographical description Europe (EU) 35 Europe (non-eu) 15 North America and Canada 13 South America 2 Asia and the Far East 15 Australia and New Zealand 4 could be used for AMS laboratories only (e.g. bone and textile) was also required. 4. Laboratory participation By the deadline for results submission of December 2000, 92 sets of results had been received. This represented a completion rate of 75%, which is extremely successful and exceeds that recorded in the previous inter-comparison (TIRI). The broad geographical distribution of the participating laboratories is shown in Table 1. The number and types of laboratories are summarized in Table 2. Although there were 92 sets of results, several laboratories operate independent systems, thus the total number of submitted results exceeds the number of participating laboratories. In addition, eight laboratories submitted results for AMS, through target preparation and then measurement in a remote facility. Of particular note here is that more than 80% of operational 14 C AMS facilities returned results. 5. Samples identified, their provenance and activity 5.1. Dendro-calibrated wood (D, F, H and I) Number of laboratories Bulk samples of precisely defined growth increments of oak and pine were obtained from the premier dendrodating laboratories in the Queen s University of Belfast and the University of Hohenheim. These samples had the immediate advantage that they could be correlated directly with material that had been used to define the master 14 C calibration curve. Table 2 Distribution of types of participating laboratory Laboratory type Liquid scintillation counting (LSC) 42 Gas proportional counting (GPC) 19 Accelerator mass spectrometry (AMS) 17 Target feeder for AMS 8 Direct absorption and LSC 4 Number participating

3 E.M. Scott et al. / Journal of Archaeological Science 31 (2004) 1209e Old wood (A, B) A previous collaboration, organized via the IAEA, involved the use of a bulk sample of alleged infinitely old (O50,000 year) Kauri wood from New Zealand. However, a significant number of the laboratories reported finite 14 C ages that equated with the age being significantly younger than that originally proposed. A newly recovered replacement log from the same site was included in the current study. Samples of this age are extremely useful for testing the influence of an individual laboratory s choice of background standard on the derived 14 C age Peat (E) A bulk sample of peat was recovered from a rapidly accumulated horizon within a late glacial sedimentary sequence exposed in coastal cliffs at St. Bee s head, Cumbria Biogenic marine sediment (C) A sufficient mass of this material, in the form of a turbidite, was available from a previous NERC funded inter-comparison exercise. This material had been archived in air-sealed containers Grain (G, J) Several tens of kilograms of mashed barley, representing grain grown during 1998, and so with enhanced 14 C activity were provided by the Glengoyne Distillery. This type of material had been used successfully in previous inter-calibration studies Other materials A further dendro-chronologically dated wood sample was also obtained from the Cambridge Department of Quaternary Science. This sample comprised 60 rings and was dated to the 19th century. A wood sample (approximately 10 kg) from the Scythian burial site Dogee Barrow completed the samples for radiometric dating. Textiles (cloth and leather samples) and three samples of mammoth tusk were made available for AMS dating. 6. Pre-treatment, testing and selection of the inter-calibration standards It was important that the inter-calibration should attempt to assess not only the technical adequacy of radiocarbon measurements per se but also recognize the effect, if any, of selective pre-treatment (decontamination) procedures that are undertaken by individual laboratories. Consequently, all samples were homogenized and for selected samples (one wood sample and peat), defined organic components (cellulose and humic acid, respectively) were extracted and purified prior to their issue. Representative aliquots were then sampled at random from bulk storage for homogeneity testing. This comprehensive suite of 14 C measurements confirmed the compositional integrity of all available standards [2,8]. It was decided that the final inter-calibration protocol should require each participant to contribute a minimum of ten analyses based on seven specified samples (including three duplicates) with results to be returned, in standard reporting format, within 12 months of their dispatch. The materials comprising the suite of standards issued to all participants are listed in Table 3. Other bulk samples that had been collected, together Table 3 FIRI sample catalogue Sample code Sample name Provenance and activity Provider Core samples A and B Kauri wood New Zealand (near background) A. Hogg, IAEA C Marine turbidite From Madeira Abyssal Plain (O3 half lives) TIRI D and F Belfast pine (whole Dendro-dated wood from master chronology M. Baillie wood) E St. Bee s Head peat Cumbria (approximately 2 half-lives) D. Harkness, M. Walker G and J Barley mash Modern sample (1998) Glengoyne Distillery H German oak Dendro-dated wood from master chronology M. Spurk I Optional samples Belfast pine (cellulose component) Dendro-dated wood from master chronology M. Baillie Cambridge wood Dendro-dated wood (19th century) R. Switsur Mammoth tusks From Pechora River, Russia Kh. Arslanov, S. Gulliksen Dogee wood Scythian burial mound G. Zaitseva Textiles Scythian burial mounds and Egyptian grave G. Zaitseva, M. van Strydonck

4 1212 E.M. Scott et al. / Journal of Archaeological Science 31 (2004) 1209e1213 with the excess from preparation of the inter-calibration suite, have been archived because of their potential future value as routine quality assurance standards. The optional samples were distributed on request upon completion of the main inter-comparison. 7. Results While detailed statistical analyses and comprehensive notes of the formal analysis have been published in the scientific literature (Radiocarbon, 2003 [8]), it is pertinent to summarize some of the major features from FIRI, in particular those issues which are likely to be of fundamental interest to the wider (multi-disciplinary) community of user scientists who look to benefit from routine radiocarbon measurements. In that context we focus on measures of accuracy and variation using the known-age dendro-dated samples (D, F, H and I). Preliminary analysis used to screen the results for any widely discrepant values and to assess the broad comparability of the results indicated that some outliers were apparent for all samples (although in small numbers) and were associated typically with a small number of individual laboratories. These laboratories were contacted and asked to investigate any potential causes. In most instances there is information within the design of the programme to allow the individual laboratories to source their respective problem(s). For example, in many instances attention is drawn to the importance of defining, monitoring and recording appropriate background and/or modern (zero aged) reference activities in routine dating work. Furthermore, where gross problems were evident, these were almost inevitably associated with more recently established facilities that are dependent on liquid scintillation counting procedures. Initial consensus values were then evaluated for each sample with the outliers omitted with the final consensus values being evaluated using a similar procedure to that described in Rozanski et al. [5]. The results for the dendro-dated wood samples are now discussed. The results summarizing the spread in the results for the four dendro-dated wood samples are shown in Table 4. The inter-quartile range (IQR) in years BP is the range within which 50% of the data lies and it is clear that the IQR is broadly comparable for the four samples. The full range of the data for each sample is Table 5 Dendro-dated wood samples summary Sample substantially larger but also broadly comparable over the four samples. Further analysis of the dendro-dated woods then focussed on quantification of the overall accuracy, relative to the master calibration curve (INTCAL) [4]. This showed very good agreement between the estimated consensus values and the 14 C results for the corresponding samples on the master calibration curve as summarized in Table 5. Next, the results for each sample were analyzed to explore the sources of variation, focusing on three factors: the background and modern standards used by each laboratory and the laboratory type. This analysis indicated that for these samples, neither the modern standard nor background used was a significant factor in explaining the variation in the results. For all known age wood samples, no significant difference was found between the AMS and radiometric laboratories (with the exception of sample F). This result is most likely to have occurred by chance. 8. Conclusions Dendro-date (BC) 14 C age from INTCAL 1998 (BP) D and F 3200e I 3299e H 313e Consensus value (BP) This project has demonstrated concordant results for the vast majority of the laboratories that took part and provided assistance and advice to those that produced results beyond the limits of recognized statistical variability. In addition, at the end of FIRI, a significant and invaluable archive of 14 C reference materials has now been created. For the 14 C user community, such an exercise provides an assurance of the on-going initiative by the 14 C community to ensure that their results are of the highest quality. 14 C dating remains a key tool for the archaeologist and assuring the quality of the measurement remains an essential laboratory function. 14 C inter-comparisons, like FIRI, are, and will continue to be, an important part of laboratory quality assurance procedures, providing an independent check on measurement capabilities. Table 4 Range of ages quoted for dendro-dated wood samples Sample IQR (years) Range (years) D and F H I Consensus value (years BP) Acknowledgements This work has been supported by NERC (GR9/03389) and the European Commission (SMT4-CT ). We also wish to express our gratitude to the sample providers, Mike Baillie, Marco Spurk, Roy Switsur, Glengoyne Distilleries, Ganna Zaitseva, Kh. Arslanov,

5 E.M. Scott et al. / Journal of Archaeological Science 31 (2004) 1209e Steinar Gulliksen, Mark van Strydonck, John Thomson, Israel Carmi and Alan Hogg. References [1] C. Bryant, I. Carmi, G. Cook, S. Gulliksen, D. Harkness, J. Heinemeier, E. McGee, P. Naysmith, G. Possnert, M. Scott, J. van der Plicht, M. van Strydonck, Sample requirements and design of a inter-laboratory trial for radiocarbon laboratories, NIM (B) 172 (2000) 355e358. [2] C. Bryant, I. Carmi, G. Cook, S. Gulliksen, D. Harkness, J. Heinemeier, E. McGee, P. Naysmith, G. Possnert, M. Scott, H. van der Plicht, M. van Strydonck, Is comparability of 14 C dates an issue? A status report on the Fourth International Radiocarbon Intercomparison, Radiocarbon 43 (2A) (2001) 321e325. [3] S. Gulliksen, E.M. Scott, TIRI report, Radiocarbon 37 (2) (1995) 820e821. [4] M. Stuiver, P.J. Reimer, E. Bard, J.W. Beck, G.S. Burr, K.A. Hughen, B. Kromer, G. McCormac, J. van der Plicht, M. Spurk, INTCAL 98 radiocarbon age calibration, 24,000e0 cal BP, Radiocarbon 40 (3) (1998) 1041e1083. [5] K. Rozanski, W. Stichler, R. Gonfiantini, E.M. Scott, R.P. Beukens, B. Kromer, J. van der Plicht, The IAEA 14 C intercomparison exercise 1990, Radiocarbon 34 (3) (1992) 506e519. [6] E.M. Scott, T.C. Aitchison, D.D. Harkness, G.T. Cook, M.S. Baxter, An overview of all three stages of the international radiocarbon intercomparison, Radiocarbon 32 (2) (1990) 303e319. [7] E.M. Scott, D.D. Harkness, G.T. Cook, Analytical protocol and quality assurance for 14 C analyses: proposal for a further intercomparison, Radiocarbon 39 (3) (1997) 347e351. [8] E.M. Scott, C. Bryant, I. Carmi, G.T. Cook, S. Gulliksen, D.D. Harkness, J. Heinemeier, E. McGee, P. Naysmith, G. Possnert, J. van der Plicht, M. van Strydonck, Pre-testing and homogeneity results for samples used in the Fourth International Radiocarbon Intercomparison (FIRI), TIRI and FIRI special volume, Radiocarbon 45 (2) (2003).