Should archaeologists care about 14C inter-comparisons? Why? A summary report on SIRI

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)

The fifth international radiocarbon intercomparison (VIRI): An assessment of laboratory performance in stage 3

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 zscores, 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

Why do we need 14C inter-comparisons?: The Glasgow 14C inter-comparison series, a reflection over 30 years

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

Extraction and measurement of cosmogenic in situ 14C from quartz

Unlike 14C that is produced in the upper atmosphere by the 14N(n,p)14C reaction, in situ 14C is produced within minerals at the earth’s surface by a number of spallation reactions including 17O(n,α)14C, 16O(n,2pn)14C and 14N(n,p)14C (Gosse & Phillips, 2001). A range of cosmic-ray produced radionuclides including 10Be, 26Al and 36Cl, which are formed in surface minerals, are now used to establish ages for formerly un-dateable deposits, however, their long half-lives render them insensitive to recent events and rapidly eroding deposits. Pure quartz (SiO2) is an ideal mineral for in situ 14C dating due to its lack of cleavage in the mineral grains, ensuring resistance to contamination by atmospheric 14C. This resistance to weathering under surface conditions, coupled with the relatively short half-life of 5730 years, provides a unique cosmogenic nuclide tool for the measurement of rapid erosion rates (>10-3 cm yr-1) and events occurring over the past 25,000 yr (Lal, 1991). Furthermore, recent advances in 14C dating by AMS have provided the opportunity to measure the very small quantities of carbon that can be extracted from quartz. The vacuum system that I have designed and built to extract carbon from quartz is based on that used at the University of Arizona (Lifton 1997), which uses resistance heating of samples to a temperature of approximately 1100ºC in the presence of lithium metaborate (LiBO2) to fuse the quartz. In the presence of O2, any carbon present is released and oxidised to CO2, which is subsequently cryogenically trapped and graphitised for AMS measurement. In previous work (Naysmith et al., 2004) it has been shown that the extraction system produced a stable blank value but when running Lifton’s PP-4 standard sample, the system generated larger volumes of CO2 but only half the number of carbon atoms compared to Lifton. In this study, new data for CO2 blank values, system blank values and new PP-4 data will be presented. The original vacuum system has been modified to try and reduce the volume of CO2 produced from each combustion. Further improvements in the cleaning and handling of the quartz sleeves before they were used in the extraction process were implemented in an attempt to reduce the contamination associated with the combustion stage of the process. The CO2 purification has been improved and results show that realistic volumes of CO2 are being generated from quartz samples. A new-shielded quartz sample has been obtained from a depth of greater than 250 m. The results from this show it to have a very low 14C atom content. A new sample of PP-4 quartz was obtained from the University of Arizona and the results (in 14C atoms g-1 SiO2) agree with values published by Lifton (Miller et al., 2006) for this sample. The data from both these samples are included in this study

Neural circuitry of acoustic startle habituation and prepulse inhibition in the context of sex steroid hormones using innovative silent functional MRI and electromyography techniques

This project aimed to explore the neural basis of acoustic startle reflex (ASR) plasticity, namely startle habituation and prepulse inhibition (PPI), and the influence of sex steroid hormones on startle habituation, PPI, and their associated neural substrates. Startle habituation and PPI are two forms of startle plasticity which result in the attenuation of the startle reflex response; however, these two processes reflect different attentional and sensorimotor processing. Moreover, sex differences have been observed in both startle habituation and PPI, with animal and human studies illustrating more rapid habituation and more PPI in males, compared to females. Sex differences in startle modulation can be explored through sex steroid hormones, namely oestrogens, progesterone, and androgens, in the brain, of which these hormone levels will naturally fluctuate, such as during puberty, menstrual cycle, menopause and andropause. In addition, pharmaceutical intervention can affect endogenous sex steroid hormone levels, such as hormonal contraception use or hormonal replacement therapy (HRT). Consequently, the observed sex differences in startle habituation and PPI, may reflect the contribution of sex steroid hormones on its underlying neural circuitry.Neural circuitry underpinning the primary acoustic startle pathway and ASR plasticity has stemmed from animal models which are acoustic-focused. Functional neuroimaging studies in male-only and mixed-sex human studies have corroborated these findings, illustrating cortico-striatal-pallido-thalamic (CSPT) circuitry in PPI, and subcortical, cortical and brainstem activity during startle habituation. Yet existing human functional magnetic resonance imaging (fMRI) studies of ASR, startle habituation and PPI have been challenging due to the loud scanner noise caused by rapid switching of gradient coils in conventional fMRI sequences. In this context, Looping Star, a multi-echo, zero echo time (ZTE), near-silent pulse sequence offers a novel approach to fMRI research. This will encourage the use of auditory startle paradigms in functional neuroimaging studies of startle habituation and PPI, allowing for better translation from animal models of the neurofunctional basis of these forms of ASR plasticity.This project aimed to demonstrate the applicability of Looping Star with simultaneous electromyography (EMG) to an auditory experimental paradigm to study startle habituation and PPI. Before exploring sex steroid hormone differences on PPI and startle habituation neural circuitry, the project aimed to map the neural correlates of startle habituation and PPI in a healthy mixed-sex adult sample. To explore sex differences observed in PPI and startle habituation, the influence of biological sex and hormonal contraception was investigated. Our goal was to explore whether there were differences in neural activity during startle habituation and PPI between sexes, and between non-hormonal contraceptive users and hormonal contraceptive users.To aid in the development of the fMRI study, the published EMG study was conducted to systematically explore the effect of task and population parameters on prepulse-induced startle modulation. This would ensure a reliable and valid paradigm was developed for future imaging and drug research. Stimulus onset asynchrony (SOA), and task order were shown to affect prepulse-induced modulation (PPI and prepulse facilitation, PPF). Sex differences in PPI were observed, with males showing more PPI than females. A follow-up of the influence of hormonal contraception on PPI also showed greater PPI in males, compared to females not on hormonal contraception. Consequently, task and population parameters should be considered in the design of future research.The simultaneous EMG-silent fMRI study was designed based on the EMG study. The auditory startle paradigm consisted of acoustic startle probes to elicit ASR, and repetition of this stimulus was expected to produce habituation effects. PPI trials used SOA based on the EMG study to elicit PPI, but PPF was not examined in the fMRI study as more investigative work was required for robust PPF research. Startle habituation and PPI were analysed separately, and to derive stronger inferences about brain-behaviour correlations, EMG-assessed measures of startle habituation (regression slope) or PPI (percentage) were modelled at the group level as a covariate. In addition, raw ASR amplitude was modelled at the individual level for parametric modulation to better characterise changes in neural activity with expected decrease in ASR during startle habituation and PPI. Hypothesised PPI neural circuitry were derived from the published systematic review of functional neuroimaging research on PPI and PPF. Functional neuroimaging studies of startle habituation studies are few.Neural activity in thalamic (right), striatal, and insula regions of interest, which decreased in line with startle habituation, was observed across the whole group. Brainstem and thalamic (left) activity were also observed and decreased with more startle habituation, but this was not significant after corrections for multiple comparisons. There were no group differences in startle habituation. Startle habituation neural activity was consistent across sexes and hormonal contraception use, suggesting consistent neural recruitment in our sample. PPI, on the other hand, showed group differences in neural activity, as males and females not on hormonal contraception showed significantly more globus pallidus activity than females on hormonal contraception during PPI trials with SOA 60 ms, compared to pulse-only trials. This finding was not replicated on trials with SOA 120 ms, and this may indicate sensitivity of PPI to SOA when illustrating group differences. In line with the EMG study, PPI scores differed with males and females on hormonal contraception showing greater PPI than females not on hormonal contraception, but these differences were not shown in neural activity. This may result from differences in exploring PPI in a laboratory vs MRI environment.This project sheds light on sensory information processing mechanisms in healthy populations and can be used to inform clinical studies investigating these processes in clinical disorders with aberrant PPI and startle habituation, such as Parkinson’s disease, Huntington’s disease, and schizophrenia. In addition, sex differences associated with these disorders can be examined in line with the current findings

A new database program installed at the SUERC radiocarbon laboratory

The SUERC Radiocarbon Dating Laboratory has recently replaced its spreadsheet-based record keeping with a new database program, custom designed to help laboratory staff manage the high throughput of nearly 5000 cathodes in the past year. The system can accept data from a variety of sources in addition to manual entry; experimental results can be uploaded from spreadsheets, while integration with graphitisation lines means that graphite yields are automatically recorded. The system is able to pass radiocarbon results directly to OxCal 4 for calibration, with the resulting plots incorporated into the dating certificates issued to submitters. There are also benefits to submitters, with electronic sample submission both eliminating transcription errors and speeding up the logging-in process which keeps turnaround times down. For bone samples, data on collagen yields are now stored electronically and are more readily obtainable from the laboratory. The new SUERC Radiocarbon Dating Laboratory database will make a significant contribution to maintaining the high quality of results produced by the laboratory, aiding staff in tracking sample progress and monitoring quality assurance (QA) samples going through the laboratory, eliminating transcription errors and making communication easier between laboratory staff and sample submitters

Is there a fifth international radiocarbon intercomparison (VIRI)?

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 C-14 community has undertaken a widescale, far-reaching, and evolving program of inter- comparisons, to the benefit of laboratories and users alike. The benefit to the users is, however, indirect, since the C-14 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 C-14 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

Makers of Modern Thought; or, Five Hundred Years' Struggle (1200 A. D. to 1699 A. D.) Between Science, Ignorance, and Superstition

n/

Sources of anthropogenic C-14 to the North Sea

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 C-14 distribution around the UK coast, the pattern of dilution with increasing distance from Sellafield appeared to be perturbed by anomalously high C-14 activities in marine biota in the coastal environment of northeast England. This present study was undertaken during 1998 and 1999 to determine whether this C-14 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 Tc-99 between Sellafield and various points on the UK coast to determine likely Sellafield- derived C-14 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 C-14 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 C-14 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 C-14 enrichments; therefore, further research is required to ascertain the source of this C-14