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Radiocarbon (14C) dating is routinely used, yet occasionally, issues still arise surrounding laboratory offsets, and unexpected and unexplained variability. Quality assurance and quality control have long been recognized as important in addressing the two issues of comparability (or bias, accuracy) and uncertainty or variability (or precision) of measurements both within and between laboratories (Long and Kalin 1990). The 14C community and the wider user communities have supported interlaboratory comparisons as one of several strands to ensure the quality of measurements (Scott et al. 2018). The nature of the intercomparisons has evolved as the laboratory characteristics have changed s. The next intercomparison is currently being planned to take place in 2019–2020. The focus of our work in designing intercomparisons is to (1) assist laboratories by contributing to their QA/QC processes, (2) supplement and enhance our suite of reference materials that are available to laboratories, (3) provide consensus 14C values with associated (small) uncertainties for performance checking, and (4) provide estimates of laboratory offsets and error multipliers which can inform subsequent modeling and laboratory improvements.

Over the past 30 years, the format of the radiocarbon (14C) intercomparison studies has changed, however, the selection of sample types used in these studies has remained constant—namely, natural and routinely dated materials that could subsequently be used as in-house reference materials. One such material is peat which has been used 12 times, starting with the ICS in 1988. Peat from Iceland (TIRI), Ellanmore (TIRI), Letham Moss (ICS, VIRI, and SIRI), and St Bees, UK (FIRI and VIRI) have been used, as well as a near-background peat from Siberia. In the main, these peat samples have been provided as the humic acid fraction, with the main advantage being that the humic acid is extracted in solution and then precipitated (the solution phase providing the homogenisation) which is a key requirement for a reference material. In this paper, we will revisit the peat results and explore their findings. In addition, for the last 8 years, the Letham Moss sample has been used in the SUERC 14C laboratory as an in-house standard or reference material. This has resulted in several thousand measurements. Such a rich data set is explored to illustrate the benefits arising from the intercomparison program.

Each of the laboratory intercomparisons (from ICS onwards) has included wood samples, many of them dendrochronologically dated. In the early years, as a result of the majority of laboratories being radiometric, these samples were typically blocks of 20–40 rings, but more recently (SIRI), they have been single ring samples. The sample ages have spanned background through to modern. In some intercomparisons, we have examined different wood pretreatment effects, in others the focus has been on background samples. In this paper, we illustrate what we have learned from these extensive intercomparisons involving wood samples and how the results contribute to the global IntCal effort.

Radiocarbon measurement is a well-established, routinely used, yet complex series of inter-linked procedures. The degree of sample pre-treatment varies considerably depending on the material, the methods of processing pre-treated material vary across laboratories and the detection of 14C at low levels remains challenging. As in any complex measurement process, the questions of quality assurance and quality control become paramount, both internally, i.e. within a laboratory and externally, across laboratories. The issue of comparability of measurements (and thus bias, accuracy and precision of measurement) from the diverse laboratories is one that has been the focus of considerable attention for some time, both within the 14C community and the wider user communities. In the early years of the technique when there was only a small number of laboratories in existence, inter-comparisons would function on an ad hoc basis, usually involving small numbers of laboratories (e.g. Otlet et al, 1980). However, as more laboratories were set-up and the detection methods were further developed (e.g. new AMS facilities), the need for more systematic work was recognised. The international efforts to create a global calibration curve also requires the use of data generated by different laboratories at different times, so that evidence of laboratory offsets is needed to inform curve formation. As a result of these factors, but also as part of general good laboratory practice, including laboratory benchmarking and quality assurance, the 14C community has undertaken a wide-scale, far-reaching and evolving programme of global inter-comparisons, to the benefit of laboratories and users alike. This paper looks at some of that history and considers what has been achieved in the past 30 years.

Radiocarbon (14C) wiggle-match dating is a technique with a substantial potential to improve the precision of dating timbers in situations where dendrochronology is not tenable. However, one of the key reasons why obtaining a dendrochronological determination might be difficult is the short-lived nature of timbers on a range of archaeological sites, something that also affects the efficiency of the wiggle-match dating technique. Combined with the potential for high expense that the technique presents, it is paramount that wiggle-match dating research design has a good empirical basis. To this end we dated 50 consecutive, individual rings from a timber that grew during the Hallstatt radiocarbon calibration plateau (ca. 750–400 cal BC) in southwest Scotland. The results indicate that (1) the precision and accuracy of wiggle-match dates carried out on short-lived sequences during the Hallstatt plateau may suffer due to insufficient resolution of the calibration data, (2) sampling time-frames roughly equivalent to the underpinning calibration data are recommended (for the period in question this means decadal blocks), and (3) short-lived sequences are at risk of losing accuracy if the actual past trend of radiocarbon diverges from the mean of the radiocarbon calibration curve.

Bone is frequently dated in archaeological studies and, especially for very old bones (more than 40,000 years old), it is critical to have an accurate and precise measure of the material-specific background value and its associated uncertainty. The SUERC Radiocarbon Laboratory has obtained a mammoth bone as a background bone standard and an appropriate number are now routinely prepared and measured in each AMS batch, resulting in the accumulation of a large number of background bone results over a two-year period. Additionally, information on which of the two accelerator mass spectrometry (AMS) instruments was used to make the radiocarbon (14C) measurements, and which sample pretreatment method (modified Longin or modified ultrafiltration) was used to extract and purify the collagen, is recorded for each sample. These data have been used to estimate the laboratory bone background (to be subtracted from each unknown bone sample prepared in the laboratory) and its associated analytical uncertainty. The statistical analysis of the bone results has made use of a linear mixed effects model to examine the variation, and to apportion the overall variation between and within batches on both AMS instruments, and the different pretreatment methods used at SUERC.

The SUERC Radiocarbon Laboratory employs a one-step “background subtraction” method when calculating 14C ages. An interglacial wood (VIRI Sample K) is employed as the non-bone organic background standard, while a mammoth bone (LQH12) from Latton Quarry is used as the bone background standard. Results over several years demonstrate that the bone background is consistently around a factor of two higher and more variable than the wood background. As a result, the uncertainty on routine bone measurements is higher than for other sample types. This study investigates the factors that may contribute to the difference in F14C values and the higher variability. Preparations of collagen using modified Longin or ultrafiltration methods show no significant difference, nor does eliminating the collagen dissolution step. Two bone samples of known infinite age with respect to radiocarbon are compared and again no significant difference is observed. Finally, the quantity and age of the organic matter in the water used during the pretreatment is investigated and it is shown that there is insufficient organic matter in the reverse osmosis water to influence background values significantly. The attention is now on determining if incomplete demineralization could lead to contaminants being retained by the phosphate in the hydroxyapatite.

Radiocarbon (14C) dating is used widely in many projects as a basis for the creation and testing of chronological constructs. 14C measurements are by their nature complex and the degree of sample pretreatment varies considerably depending on the material. Within the United Kingdom and Europe, there are a number of well-established laboratories and increasingly, archaeologists are not just commissioning new dates, but also using statistical modelling of assemblages of dates, perhaps measured in different laboratories, to provide formal date estimates for their sites. The issue of comparability of measurements (and thus bias, accuracy and precision of measurement) from the diverse laboratories is one which has been the focus of some attention both within the 14C community and the wider user communities for some time. As a result of this but also as part of laboratory benchmarking and quality assurance, the 14C community has undertaken a wide-scale, far-reaching, and evolving program of intercomparisons, to the benefit of laboratories and users alike. This paper summarizes the most recent exercise, the Sixth International Radiocarbon Intercomparison (SIRI).

This paper describes all the major procedures adopted by the Scottish Universities Environmental Research Centre (SUERC) Radiocarbon Dating Laboratory. This includes sample pretreatment, graphite production, accelerator mass spectrometry (AMS) measurement, associated stable isotope measurements, data handling, and age calculations, but with the main emphasis being on the chemical pretreatment methods. All of the above enable the laboratory to provide a complete analytical service comprising advice on sample selection, preparation and analysis of samples, and Bayesian analysis of resulting 14C (and other) data. This applies to both our research and commercial activities. The pretreatment methods that we mainly focus on are used to remove contaminant carbon from a range of sample types or to isolate a particular chemical fraction from a sample prior to combustion/hydrolysis, graphitization, and subsequent AMS 14C measurement. The methods described are for bone (collagen extraction, with and without ultrafiltration), cremated bone, tooth enamel, charcoal, grain, carbon residues, shell, wood (including alpha-cellulose isolation), peat, sediments, textiles, fuel/biofuel, and forensic samples.

A 30-year-old Japanese cedar (Cryptomeria japonica), collected from Iwaki, Fukushima in 2014, was analyzed for the long-lived radionuclide (14)C. Values of Δ(14)C varied from 211.7‰ in 1984 to 16.9‰ in 2013. The temporal Δ(14)C variation can be described as an exponential decline, indistinguishable from the general Northern Hemisphere Zone 2 (NH Zone 2) values in the atmosphere, until at least 1994. Values of Δ(14)C for 1999 and 2004 are slightly depleted compared with NH Zone 2 values, while from 1999 to 2013 the data suggest a clear depletion with a 2-8 ppmV additional CO2 contribution from a (14)C-free (i.e. fossil carbon) source. This change coincides with local traffic increases since two nearby expressways were opened in the 1990's. In addition, the small but visible (14)C pulse observed in the 2011 tree-ring might be caused by release from the damaged reactors during the Fukushima nuclear accident.

(14)C analysis of flue gas by accelerator mass spectrometry (AMS) and liquid scintillation counting (LSC) were used to determine the biomass fraction of mixed waste at an operational energy-from-waste (EfW) plant. Results were converted to bioenergy (% total) using mathematical algorithms and assessed against existing industry methodologies which involve manual sorting and selective dissolution (SD) of feedstock. Simultaneous determinations using flue gas showed excellent agreement: 44.8 ± 2.7% for AMS and 44.6 ± 12.3% for LSC. Comparable bioenergy results were obtained using a feedstock manual sort procedure (41.4%), whilst a procedure based on selective dissolution of representative waste material is reported as 75.5% (no errors quoted). (14)C techniques present significant advantages in data acquisition, precision and reliability for both electricity generator and industry regulator.

Radiocarbon (14C) has been measured in single tree ring samples collected from the southwest of the Fukushima Dai-ichi Nuclear Power Plant. Our data indicate south-westwards dispersion of radiocarbon and the highest 14C activity observed so far in the local environment during the 2011 accident. The abnormally high 14C activity in the late wood of 2011 ring may imply an unknown source of radiocarbon nearby after the accident. The influence of 14C shrank from 30 km during normal reactor operation to 14 km for the accident in the northwest of FDNPP, but remains unclear in the southwest.

Japanese cedar leaves from Iwaki, Fukushima were analyzed for carbon, cesium and iodine isotopic compositions before and after the 2011 nuclear accident. The Δ14C values reflect ambient atmospheric 14C concentrations during the year the leaves were sampled/defoliated, and also previous year(s). The elevated 129I and 134,137Cs concentrations are attributed to direct exposure to the radioactive fallout for the pre-fallout-expended leaves and to internal translocation from older parts of the tree for post-fallout-expended leaves. 134Cs/137Cs and 129I/137Cs activity ratios suggest insignificant isotopic and elemental fractionation during translocation. However, fractionation between radioiodine and radiocesium is significant during transportation from the source.

In this paper, we describe improvements to the in situ cosmogenic radiocarbon extraction system at SUERC made since 2004, highlighting the factors that potentially control the reduction of analytical variability. We also present new results on system blanks and of measurements of in situ14C in shielded quartz and a surface quartz sample used at the University of Arizona as an in situ14C standard (PP-4). The SUERC in situ14C extraction system was built in 2001 and is based on a combustion technique following the design of the extraction system at the University of Arizona. Our preliminary results suggest that the continuous running of the extraction system and the monitoring of gas collecting time and of the temperature of the cryogenic traps used in the gas cleaning steps are key to maintaining low and stable system blanks. Our latest average system blank is 2.02 ± 0.23 x 10514C atoms. This is consistent with those recently published by the University of Arizona and ETH in situ14C labs. Measurements of in situ14C concentrations in sample PP-4 yield an average of 3.82 ± 0.23 x 105 atoms g–1 quartz, again consistent with published values.

The Fifth International Radiocarbon Intercomparison (VIRI) continues the tradition of the TIRI (third) and FIRI (fourth) (Scott 2003) intercomparisons and operates in addition to any within-laboratory quality assurance measures as an independent check on laboratory procedures. VIRI is a phased intercomparison; results for the first phase, which employed grain samples, were reported in Scott et al. (2007). The second phase, involving bone samples, is reported here. The third and final phase, which includes samples of peat, wood, and shell, has also been completed and a companion paper appears in these proceedings.Five bone samples were made available and included Sample E: mammoth bone (>5 half-lives); Sample F: horse bone (from Siberia, excavated in 2001; and Samples H and I: whale bones (approximately 2 half-lives). Sample G (human bone) was accessible only to accelerator mass spectrometry (AMS) laboratories because of the limited amount of sample available. More than 40 laboratories participated in Phase 2 and consensus values for the ages were as follows: Sample E = 39,305 14C yr BP (standard deviation [1 σ = 121 yr); Sample F = 2513 yr BP (1 σ = 5 yr); Sample G = 969 yr BP (1 σ = 5 yr); Sample H = 9528 yr BP (1 σ = 7 yr); and Sample I = 8331 yr BP (1 σ = 6 yr). Sample G had previously been dated by 4 laboratories and a weighted mean of 934 ± 12 yr BP had been quoted. Sample I had previously been dated at 8335 ± 25 yr BP and Sample H had been dated at 9565 ± 130 yr BP. Results for Sample H and Sample I are in good agreement with the previous results; Sample G results, however, give a value that is significantly older than the previously reported results.

The new SUERC AMS Laboratory was described at AMS-9. Since then there have been technological developments, added and improved analysis capability, the formation of additional complementary groups, the purchase of another instrument, and many samples measured. Be, C, Al, Cl, Ca and I-AMS are established with measured species changing weekly. Full-terminal potential running is now routine and 5 MV is proving sufficient for all species. Ion detection and evaluation has also been improved by the use of even thinner detector windows as necessary and a more powerful on-line data analysis system. Routine radiocarbon measurement is to 4‰ reproducibility at middling current and graphite samples of as little as 100 μg C can be accommodated. Even smaller samples can be measured as CO2 using He carrier in the gas capable ion source. Two new sample preparation laboratories at SUERC are the NERC Cosmogenic Isotope Analysis Facility and the SUERC/University of Glasgow Centre for Geosciences Cosmogenic Nuclide Laboratory. In particular these benefit from competitive Be-AMS and Cl-AMS performance.

It is now almost 10 yr since radiocarbon dating of cremated bone was first developed using the small carbonate component contained within the hydroxyapatite-based inorganic fraction. Currently, a significant number of 14C laboratories date cremated bone as part of their routine dating service. As a general investigation of cremated bone dating since this initial development, a small, cremated bone intercomparison study took place in 2005, involving 6 laboratories. Six cremated bone samples (including 2 sets of duplicates), with ages spanning approximately 1500–2800 BP, were sent to the laboratories. The results, which showed relatively good agreement amongst the laboratories and between the duplicate samples, are discussed in detail.

All measurement is subject to error, which creates uncertainty. Every time that an analytical radiocarbon measurement is repeated under identical conditions on an identical sample (even if this were possible), a different result is obtained. However, laboratories typically make only 1 measurement on a sample, but they are still able to provide an estimate of the analytical uncertainty that reflects the range of values (or the spread) in results that would have been obtained were the measurement to be repeated many times under identical conditions. For a single measured 14C age, the commonly quoted error is based on counting statistics and is used to determine the uncertainty associated with the 14C age. The quoted error will include components due to other laboratory corrections and is assumed to represent the spread we would see were we able to repeat the measurement many times.Accuracy and precision in 14C dating are much desired properties. Accuracy of the measurement refers to the deviation (difference) of the measured value from the true value (or sometimes expected or consensus value), while precision refers to the variation (expected or observed) in a series of replicate measurements. Quality assurance and experimental assessment of these properties occupy much laboratory time through measurement of standards (primary and secondary), reference materials, and participation in interlaboratory trials. This paper introduces some of the most important terms commonly used in 14C dating and explains, through some simple examples, their interpretation.

The Fifth International Radiocarbon Intercomparison (VIRI) continues the tradition of the TIRI (third) and FIRI (fourth) intercomparisons (Scott 2003) and operates as an independent check on laboratory procedures in addition to any within-laboratory procedures for quality assurance. VIRI is a 4-yr project, with the first suite of samples (grain) sent out in September 2004 and the second suite (bone) sent out in December 2005. Further stages will include samples of peat, wood, and shell with a range of ages.The 4 grain samples included 2 samples (A and C) of barley mash (20 g for radiometric analysis and 2 g for AMS), a grain (barley) byproduct from the manufacture of Glengoyne malt whiskey. The 2 remaining charred grain samples (B and D) were from excavations at Beth Saida and Tel Hadar, respectively (10 g for radiometric analysis and 4 seeds for AMS) and were provided by Elisabetta Boaretto of the Weizmann Institute. Consensus values for samples A and C are 109.2 (standard deviation [1 σ] = 2.73) and 110.6 pMC (1 σ = 2.48), and 2805 (1 σ = 162.7) and 2835 BP (1 σ = 190.8) for samples B and D, respectively. Sample A is a new sample that was collected in 2001, while sample C was used in the FIRI trial as samples G & J (consensus value 110.7 pMC) and was collected in 1998. The expected ages (on archaeological grounds) of samples B and D are 2800 BP and 2850–2900 BP, respectively. The second suite of samples comprises bone, ranging in age from Medieval to “close to background,” and was distributed in December 2005. Samples for both radiometric and AMS laboratories include E: mammoth bone (>5 half-lives); F: horse bone (from Siberia, excavated in 2001); and H, I: whalebone. Finally, sample G (human bone) was only for AMS laboratories. Some of the issues related to using bone in a laboratory intercomparison will be discussed.

A 50-year-old Japanese cedar (Cryptomeria japonica) from Okuma, ∼1 km southwest of the Fukushima Dai-ichi Nuclear Power Plant, was cored and each annual ring was analysed for (14)C. The (14)C specific activity values varied from 330.4 Bq kg(-1) C in the tree ring formed in 1971 to 231.2 Bq kg(-1) C in the 2014 ring. During the periods 1971-1976 and 2011-2014, the (14)C specific activities are indistinguishable from the ambient background values. However, compared with the ambient atmospheric levels, the (14)C specific activities between 1977 and 2010 are significantly elevated, clearly indicating (14)C discharges from the reactors during their normal operations. In addition, the specific activities are positively correlated with the annual electricity generation values. The excess (14)C specific activities were36 Bq kg(-1) C, corresponding to an additional annual effective dose of2 μSv via the food ingestion pathway in the study location. The primary wind direction is east-southeast/southeast with a frequency of ∼30%, in comparison to ∼20% frequency for the direction of the site under study (north-northeast/northeast). This would tend to indicate a similar magnitude of additional effective dose and consequently no significant radiological impact of atmospheric (14)C discharges from the FDNPP during the entire period of normal operations. Additionally, no (14)C pulse in activity can be observed in the year 2011 ring. This might be caused by a limited (14)C release from the damaged reactors during the accident or that the prevailing wind during the short period of release (11th-25th March 2011) was not in the direction of Okuma.

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 C-14 community. As a result, the C-14 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 2001. The information arising from the studies is important for the appropriate interpretation of the scientific evidence provided by C-14 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 C-14 dates. (C) 2004 Elsevier Ltd. All rights reserved.

Prior to 1984, the reported marine 14C discharges from Sellafield were estimates: 0.2 TBq per annum from 1952 to 1969 and 1 TBq per annum until 1984 when measurements commenced. The relationship between the net excess 14C activity in annually collected Nori (Porphyra umbilicalis) seaweed samples and the annual discharges (estimated and measured) implies that the discharges were not as constant as the estimates. Based on the relationship between post-1984 measured discharges and the excess 14C in the seaweed, two simple empirical models were used to re-calculate the discharges between 1967 and 1984. Gamma-spectrometry measurements on the seaweed also indicate that Porphyra is a sensitive indicator of changes in discharge of other radionuclides, brought about by the introduction of new waste clean-up technologies within Sellafield.

The Sellafield nuclear fuel reprocessing plant on the northwest coast of England is the largest source of anthropogenic radiocarbon to the UK coastal environment. In a mid-1990s study of 14C distribution around the UK coast, the pattern of dilution with increasing distance from Sellafield appeared to be perturbed by anomalously high 14C activities in marine biota in the coastal environment of northeast England. This present study was undertaken during 1998 and 1999 to determine whether this 14C enhancement was due to Sellafield or the nuclear power plants on the east coast. Seawater, seaweed (Fucus sp.), and mussel (Mytilus edulis) samples that were collected from the vicinity of the Torness and Hartlepool advanced gas-cooled reactor (AGR) nuclear power stations were all enhanced above the contemporary regional background activity derived from natural production and atmospheric nuclear weapons testing. We used previously published dilution factors and transfer times for 99Tc between Sellafield and various points on the UK coast to determine likely Sellafield-derived 14C contributions to the activities at the nuclear power plant sites. The results suggest that the activities observed at Torness, which are only marginally enhanced above the natural background activity, are possibly due to discharges from Sellafield; however, the significant 14C enhancements at Hartlepool are not Sellafield-derived. Furthermore, since both reactors have the same fundamental design, the low activities at the Torness AGR imply that the activities at Hartlepool are not from the AGR, suggesting that there is an input of 14C to the marine environment in the vicinity of Hartlepool which is probably non-nuclear-power related. However, there is no other authorized site in the area that could account for the observed 14C enrichments; therefore, further research is required to ascertain the source of this 14C.

A new National Electrostatic Corporation (NEC) 5MV accelerator mass spectrometer became operational at the Scottish Universities Environmental Research Centre (SUERC) in July 2002. It has 2 Cs sputter negative ion sources: a 134-sample source (S1) for the routine measurement of all species, and a hybrid source (S2) with 40 spaces for radiocarbon measurements with either graphite or CO2 samples. A number of performance tests on graphite samples have been carried out on both sources. A precison of better than 0.3% is feasible for modern samples on a routine basis. The 14C background of the machine and the graphite preparation process blank are 0.04 ± 0.01 and 0.16 ± 0.05 pMC, respectively, indicating that 14C dating back to ~50 kyr BP is possible. The normalized 14C values for a series of reference materials agree well with the IAEA, TIRI, and FIRI consensus values. Routine measurement of 14C has been underway since May 2003. Preliminary results of performance tests on the CO2 gas ion source are also reported.

Radiocarbon is produced within minerals at the earth's surface (in situ production) by a number of spallation reactions. Its relatively short half-life of 5730 yr provides us with a unique cosmogenic nuclide tool for the measurement of rapid erosion rates (>10−3 cm yr−1) and events occurring over the past 25 kyr. At SUERC, we have designed and built a vacuum system to extract 14C from quartz which is based on a system developed at the University of Arizona. This system uses resistance heating of samples to a temperature of approximately 1100° in the presence of lithium metaborate (LiBO2) to dissolve the quartz and liberate any carbon present. During extraction, the carbon is oxidized to CO2 in an O2 atmosphere so that it may be collected cryogenically. The CO2 is subsequently purified and converted to graphite for accelerator mass spectrometry (AMS) measurement. One of the biggest problems in measuring in situ 14C is establishing a low and reproducible system blank and efficient extraction of the in situ 14C component. Here, we present initial data for 14C-free CO2, derived from geological carbonate and added to the vacuum system to determine the system blank. Shielded quartz samples (which should be 14C free) and a surface quartz sample routinely analyzed at the University of Arizona were also analyzed at SUERC, and the data compared with values derived from the University of Arizona system.

Radiocarbon laboratories undertake rigorous programmes of internal quality control (QC) and overall quality assurance (QA). In a laboratory “inter-comparison” samples of the same age are dated at different laboratories using a range of techniques and the results are then compared. The authors summarise the results of the fourth of these scientific audits.

Interlaboratory comparisons have been widely used in applied radiocarbon science. These are an important part of ongoing quality assurance (QA) programmes, which are vital to the appropriate interpretation of the evidence provided by the C-14 record in Quaternary applications (including climate change and environmental reconstruction). International comparisons of laboratory performance are an essential component of the quality assurance process in radiocarbon dating. If the user community is to have confidence in radiocarbon results, it needs to be assured that laboratories world wide are producing measurements that are reliable and in accordance with 'good practice'. The findings from the most recent (completed in 2001) and extensive (more than 90 participating laboratories) radiocarbon intercomparison (FIRI) are reported here. This study was designed (i) to assess comparability, or otherwise, of the results from different laboratories and (ii) to quantify the extent and possible causes of any interlaboratory variation. The results demonstrate that there are no significant differences amongst the main measurement techniques (gas proportional counting, liquid scintillation counting and accelerator mass spectrometry (AMS)) but there is evidence of small laboratory offsets relative to known age samples for some laboratories. There is also evidence in some cases of underestimation of measurement precision. Approximately 10% of all results were classified as extreme (outliers) and these results were generated by 14% of the laboratories. Overall, the evidence supports the fact that radiocarbon laboratories are generally accurate and precise but that, notwithstanding internal QA procedures, some problems still occur, which can best be detected by participation in independent intercomparisons such as FIRI, where the results allow individual laboratories to assess their performance and to take remedial measures where necessary. The results from FIRI are significant in that they show a broad measure of agreement between measurements made in different laboratories on a wide range of materials and they also demonstrate no statistically significant difference between measurements made by radiometric or AMS techniques. Copyright (C) 2002 John Wiley Sons, Ltd.

For more than 15 years, the radiocarbon community has participated in a series of laboratory intercomparisons in response to the issue of comparability of measurements as perceived within the wider user communities (Scott et al. 1990; Rozanski et al. 1992; Gulliksen and Scott 1995; Scott et al. 1997). In this report, we provide an update on the current 14C laboratory intercomparison and reflect on future issues linked to the laboratory intercomparison program, not least those resulting from a significant growth in the number of accelerator mass spectrometry (AMS) facilities providing routine dating of small samples (milligram size).

Since the early 1950s, the Sellafield nuclear fuel reprocessing plant in Northwest England has released radiocarbon into the Irish Sea in a mainly inorganic form as part of its authorized liquid effluent discharge. In contrast to the trend in which the activities of most radionuclides in the Sellafield liquid effluent have decreased substantially, 14C discharges have increased since 1994–95. This has largely been due to a policy change favoring marine discharges over atmospheric discharges. 14C is radiologically important due to its long half life, mobility in the environment, and propensity for entering the food chain. Current models for radionuclide dispersal in the Irish Sea are based on a reversible equilibrium distribution coefficient (kd), an approach which has been shown to be inadequate for 14C. Development of predictive models for the fate of Sellafield-derived 14C requires a thorough understanding of the biogeochemical fluxes between different carbon reservoirs and the processes controlling the net flux of 14C out of the Irish Sea, through the North Channel. In this study, both an empirical and a halving time approach indicate that close to 100% of the 14C that is discharged from Sellafield is dispersed beyond the Irish Sea on a time-scale of months in the form of DIC, with little transfer to the PIC, POC, and DOC fractions, indicating that the “dilute and disperse” mechanism is operating satisfactorily. This is consistent with previous research that indicated little transfer of 14C to Irish Sea sediments. While significant 14C enhancements have been observed in the biota of the Irish Sea, this observation is not necessarily in conflict with either of the above as the total biomass has to be taken into account in any calculations of 14C retention within the Irish Sea.

We operate a new NEC 250 kV single-stage accelerator mass spectrometer (SSAMS) next to our established 5 MV tandem. This permits good comparison of 14C-AMS and challenges SSAMS performance. Initial SSAMS ion-optical deficiencies have been addressed by shimming the injection magnet and 3‰ 14C/13C measurement with background limited by sample chemistry is routine. Higher-precision analysis is also achievable. A problematic measurement interference remains, however. The small spectrometer was procured to add measurement capacity, and for experiments with positive ions, but is now the preferred instrument for both convenience and measurement quality.

Radiocarbon age offsets between foraminifera and bulk carbonate samples were investigated in a sediment core from the western Rockall Plateau in the North East Atlantic. Within the surface mixed layer (SML), foraminifera (forams) ages were significantly younger than those of the bulk sediment at corresponding depths. Below the SML, no significant offset was observed between the two, which is in contrast to previous studies. The absence of an offset below the SML may be the result of advective biological mixing dominating over diffusive mixing in the SML, or the foram size fraction being below the threshold at which size selective mixing becomes active. Within the SML, the sampling strategy may bias the collection of forams towards the youngest specimens at each depth.

An on-going inter-comparison programme which is focused on assessing and establishing consensus protocols to be applied in the identification, selection and sub-sampling of materials for subsequent C-14 analysis is described. The outcome of the programme will provide a detailed quantification of the uncertainties associated with C-14 measurements including the issues of accuracy and precision. Such projects have become recognised as a fundamental aspect of continuing laboratory quality assurance schemes, providing a mechanism for the harmonisation of measurements and for demonstrating the traceability of results.The design of this study and its rationale are described. In summary, a suite of core samples has been defined which will be made available to both AMS and radiometric laboratories. These core materials are representative of routinely dated material and their ages span the full range of the applied C-14 time-scale. Two of the samples are of wood from the German and Irish dendrochronologies, thus providing a direct connection to the master dendrochronological calibration curve. Further samples link this new inter-comparison to past studies.Sample size and precision have been identified as being of paramount importance in defining dating confidence, and so several core samples have been identified for more in-depth study of these practical issues. In addition to the core samples, optional samples have been identified and prepared specifically for either AMS and/or radiometric laboratories. For AMS laboratories, these include bone, textile, leather and parchment samples. Participation in the study requires a commitment to a minimum of 10 core analyses, with results to be returned within a year. (C) 2000 Elsevier Science B.V. All rights reserved.

Prior to this study, almost no up-to-date information was available on the ‘background’ level of 14C present in the water and biota of the UK coastal marine environment. The weighted mean 14C activity derived from the lowest activities of dissolved inorganic carbon (DIC) and biota for sites which are remote from potential sources is 247·6±1·0 Bq kg-1 carbon. This is proposed as the best estimate of the natural/weapons testing ‘background’ for 1995 and should be subtracted from the activity derived for any sample to establish the excess due to UK anthropogenic inputs. 14C activities in the DIC component of seawater and a range of marine biota are significantly enhanced above the expected ‘background’ value in the environment around the British Nuclear Fuels plc reprocessing plant at Sellafield, Cumbria, NW England and Amersham International plc, Cardiff, Wales. The enrichments around Sellafield are largely confined to the NE Irish Sea while those at Cardiff are confined to the Severn Estuary. The dose from 14C to the Sellafield seafood-consuming critical group (

Radiocarbon is an important constituent of the low level, liquid, radioactive effluent discharged from the Sellafield nuclear fuel reprocessing plant in northwest England, but despite the fact that it gives the highest collective dose commitment of all the nuclides in the waste, its behavior in the Irish Sea is poorly defined. There is therefore a clear requirement for an improved understanding of 14C behavior in the Irish Sea, to assist with dose evaluation modeling and definition of the mixing and accumulation characteristics of the sediment in this area. In this context, results are presented here for a temporal study of 14C activities in four geochemical fractions of seawater and in a sediment core from the vicinity of the Sellafield effluent outfall. Clear 14C enrichments in the dissolved inorganic carbon (DIC) and particulate organic carbon (POC) components of seawater were observed, with temporal trends in activity that were related to variations in the Sellafield discharge. Smaller, but nevertheless detectable, enrichments were also observed for particulate inorganic carbon (PIC) and dissolved organic carbon (DOC) in the seawater. The distribution of 137Cs and 241Am revealed that the sediment core could be classified into three zones in which the intensity of mixing decreased discontinuously with depth. Bulk carbonate 14C analyses of the core demonstrated the presence of glacial or pre-glacial carbonate in the system, but failed to show any evidence of contaminant 14C input or provide information on sediment accumulation processes. In contrast, analysis of bulk organic matter from the sediment provided clear evidence of the recent perturbation of a well mixed system by input of younger material, consistent with the recent input of contaminant 14C from Sellafield and possibly weapons testing fallout.

The British Nuclear Fuels plc reprocessing plant at Sellafield in Cumbria, Northwest England, discharges between 1 and 3 TBq of 14C per annum as low-level waste via a pipeline into the Eastern Irish Sea. Our results demonstrate 14C activities in excess of the current ambient level (i.e., ca. 260 Bq kg−1 carbon) in a range of both biotic and abiotic samples with evidence that a significant percentage of the discharges are in the form of dissolved inorganic carbon (DIC). There also appears to be a significant seasonal influence on the uptake of the DIC by the phytoplankton, most likely linked to their bloom periods. Results from different aspects of the research are integrated to provide an opportunity to consider the overall distribution and behavior of 14C in the Eastern Irish Sea. We also discuss the results in terms of existing sediment chronologies and sedimentation rates and consider the possible consequences of phytoplankton blooms on the cycling of the other radionuclides.

The issue of comparability of measurements (and thus bias, accuracy, and precision of measurement) from diverse laboratories is one which has been the focus of some attention both within the radiocarbon community and the wider user communities. As a result, the 14C community has undertaken a widescale, far-reaching, and evolving program of intercomparisons, to the benefit of laboratories and users alike. The benefit to the users is, however, indirect, since the 14C intercomparisons have not been used to generate “league tables” of performance, but rather to allow individual laboratories to check procedures and modify them as required.The historical progression of 14C laboratory intercomparisons from the Third (TIRI, completed in 1995, Gulliksen and Scott 1995) and Fourth (FIRI, completed in 2000, Scott 2003; Boaretto et al. 2000; Bryant et al. 2002) suggests that a Fifth (VIRI) should also be expected. We describe the plans for VIRI.

Determination of the radiocarbon (14C) content of airborne particulate matter yields insight into the proportion of the carbonaceous material derived from fossil and contemporary carbon sources. Daily samples of PM2.5 were collected by high-volume sampler at an urban background site in Birmingham, UK, and the fraction of 14C in both the total carbon, and in the organic and elemental carbon fractions, determined by two-stage combustion to CO2, graphitisation and quantification by accelerator mass spectrometry. OC and EC content was also determined by Sunset Analyzer. The mean fraction contemporary TC in the PM2.5 samples was 0.50 (range 0.27–0.66, n = 26). There was no seasonality to the data, but there was a positive trend between fraction contemporary TC and magnitude of SOC/TC ratio and for the high values of these two parameters to be associated with air-mass back trajectories arriving in Birmingham from over land. Using a five-compartment mass balance model on fraction contemporary carbon in OC and EC, the following average source apportionment for the TC in these PM2.5 samples was derived: 27% fossil EC; 20% fossil OC; 2% biomass EC; 10% biomass OC; and 41% biogenic OC. The latter category will comprise, in addition to BVOC-derived SOC, other non-combustion contemporary carbon sources such as biological particles, vegetative detritus, humic material and tyre wear. The proportion of total PM2.5 at this location estimated to derive from BVOC-derived secondary organic aerosol was 9–29%. The findings from this work are consistent with those from elsewhere in Europe and support the conclusion of a significant and ubiquitous contribution from non-fossil biogenic sources to the carbon in terrestrial aerosol.

Many of the Loch Tay crannogs were built in the Early Iron Age and so calibration of the radiocarbon ages produces very broad calendar age ranges due to the well-documented Hallstatt plateau in the calibration curve. However, the large oak timbers that were used in the construction of some of the crannogs potentially provide a means of improving the precision of the dating through subdividing them into decadal or subdecadal increments, dating them to high precision and wiggle-matching the resulting data to the master 14C calibration curve. We obtained a sample from 1 oak timber from Oakbank Crannog comprising 70 rings (Sample OB06 WMS 1, T103) including sapwood that was complete to the bark edge. The timber is situated on the northeast edge of the main living area of the crannog and as a large and strong oak pile would have been a useful support in more than 1 phase of occupation and may be related to the earliest construction phase of the site. This was sectioned into 5-yr increments and dated to a precision of approximately ±8–16 14C yr (1 σ). The wiggle-match predicts that the last ring dated was formed around 500 BC (maximum range of 520–465 BC) and should be taken as indicative of the likely time of construction of Oakbank Crannog. This is a considerable improvement on the estimates based on single 14C ages made on oak samples, which typically encompassed the period from around 800–400 BC.

Proficiency testing is a widely used, international procedure common within the analytical chemistry community. A proficiency trial (which VIRI is) often follows a standard protocol, including analysis that is typically based on z-scores, with one key quantity, σp. From a laboratory intercomparison (sometimes called a proficiency trial), we hope to gain an assessment of accuracy (in this case, from dendro-dated samples), laboratory precision (from any duplicate samples), and generally, an overall measure of performance, including measurement variability and hence realistic estimates of uncertainty. In addition, given our stated aim of creating an archive of reference materials, we also gain a determination of consensus values for new reference materials.VIRI samples have been chosen to deliver these objectives and the sample ages included in the different stages, by design, spanned modern to background. With regard to pretreatment, some samples required intensive pretreatment (e.g. bone), while others required none (e.g. cellulose and humic acid). Sample size was not optimized, and indeed some samples were provided solely for accelerator mass spectrometry (AMS) measurement. In this sense, VIRI presented a more challenging exercise than previous intercomparisons, since by its design in stages, one can explore improvements (or deteriorations) over time in laboratory performance. At each stage, more than 50 laboratories have participated, with an increasing demographic shift towards more AMS and fewer radiometric laboratories.

C-14 measurement uses a number of standards and reference materials with different properties. Historically the absolute calibration of C-14 measurement was tied to 1890 wood, through the 'primary' standard of NBS-OxI (produced by the National Bureau of Standards, now NIST - National Institute of Standards and technology) subsequently replaced by NBS-OxII. These are both internationally calibrated and certified materials, whose C-14 activities are known absolutely.A second tier of materials, often called secondary standards or reference materials, and including internationally recognised materials such as ANU-sucrose (now also IAEA-C6), Chinese - sucrose and the IAEA C1-C6 series, augmented by additional oxalic acid samples are also used routinely. The activity of these materials has been estimated from large numbers of measurements made by many laboratories. Recently, further natural materials from the Third and Fourth International Radiocarbon Inter-comparisons (TIRI and FIRI) have been added to this list. The activities of these standards and reference materials span both the applied C-14 age range and the chemical composition range of typical samples, but this is not achieved uniformly and there is a continuing need for reference materials for laboratory quality control and measurement-traceability purposes.In this paper, we review the development of C-14 Standards and reference materials and consider the future requirements for such materials within the C-14 AMS community. (C) 2004 Elsevier B.V. All rights reserved.

The results of this study indicate that scintillation counters employing burst-counting circuitry are capable of producing accurate age measurements. Replicate analyses confirm the validity of the minimum error of 50-60 years quoted on routine age measurements carried out at this laboratory.

The dune of Oitavos, the underlying paleosol, and Helix sp. gastropod shells found within the paleosol were dated using a combination of radiocarbon and blue optically stimulated luminescence (OSL). The organic component of the paleosol produced a significantly older age (∼20,000 cal BP) than the OSL age measurement (∼15,000 yr), while 14C age measurements on the inorganic component and the gastropods produced ages of ∼35,000 yr and ∼34,000 yr, respectively. Rare-earth element analyses provide evidence that the gastropods incorporate geological carbonate, making them an unreliable indicator of the age of the paleosol. We propose that the 14C age of the small organic component of the paleosol is also likely to be unreliable due to incorporation of residual material. The OSL age measurement of the upper paleosol (∼15,000 yr) is consistent with the age for the base of the dune (∼14,500 yr). The younger OSL age for the top of the dune (∼12,000 yr) suggests that it was built up by at least 2 sand pulses or that there was a remobilization of material at the top during its evolution, prior to consolidation.