C. Bronk Ramsey

AMS RADIOCARBON DATING OF ANCIENT BONE USING ULTRAFILTRATION

The Oxford Radiocarbon Accelerator Unit (ORAU) has used an ultrafiltration protocol to further purify gelatin from archaeological bone since 2000. In this paper, the methodology is described, and it is shown that, in many instances, ultrafiltration successfully removes low molecular weight contaminants that less rigorous methods may not. These contaminants can sometimes be of a different radiocarbon age and, unless removed, may produce erroneous determinations, particularly when one is dating bones greater than 2 to 3 half-lives of 14C and the contaminants are of modern age. Results of the redating of bone of Late Middle and Early Upper Paleolithic age from the British Isles and Europe suggest that we may need to look again at the traditional chronology for these periods.

Rapid coupling between ice volume and polar temperature over the past 150,000 years.

Current global warming necessitates a detailed understanding of the relationships between climate and global ice volume. Highly resolved and continuous sea-level records are essential for quantifying ice-volume changes. However, an unbiased study of the timing of past ice-volume changes, relative to polar climate change, has so far been impossible because available sea-level records either were dated by using orbital tuning or ice-core timescales, or were discontinuous in time. Here we present an independent dating of a continuous, high-resolution sea-level record in millennial-scale detail throughout the past 150,000 years. We find that the timing of ice-volume fluctuations agrees well with that of variations in Antarctic climate and especially Greenland climate. Amplitudes of ice-volume fluctuations more closely match Antarctic (rather than Greenland) climate changes. Polar climate and ice-volume changes, and their rates of change, are found to covary within centennial response times. Finally, rates of sea-level rise reached at least 1.2 m per century during all major episodes of ice-volume reduction.

NotCal04; comparison/ calibration 14C records 26-50 cal kyr BP

The radiocarbon calibration curve IntCal04 extends back to 26 cal kyr BP. While several high-resolution records exist beyond this limit, these data sets exhibit discrepancies of up to several millennia. As a result, no calibration curve for the time range 26-50 cal kyr BP can be recommended as yet, but in this paper the IntCal04 working group compares the available data sets and offers a discussion of the information that they hold.

HYBRID ION SOURCES: RADIOCARBON MEASUREMENTS FROM MICROGRAM TO MILLIGRAM

Abstract Gas ion sources provide a very efficient way of dealing with very small samples and, by injecting samples in a stream of helium carrier gas, measurements can be obtained from as little as 1 μg carbon. This is particularly useful for measurements on high activity samples and for the study of sample contaminants in relation to high precision AMS measurements. Another benefit of gas based measurements is that they can be made in real time allowing a range of possible applications such as GC-AMS and LC-AMS which could make AMS more widely applicable in the life sciences. Graphite ion sources provide a more efficient use of accelerator time for the measurement of large samples because of the higher carbon currents attainable. The precisions attainable is this way are so good that re-evaluation of possible sources of systematic error (such as low level contamination and sample size effects) is essential. Hybrid ion sources capable of operating on either gas or graphite allow the advantages of both techniques to be exploited. With minor modifications to the gas ion source at Oxford we are now able to operate with both sample types. Currents from gas are typically 10–12 μA and those from graphite 40–50 μA while maximum currents from graphite exceed 300 μA. We discuss the implications of this for the future development of radiocarbon AMS facilities designed for a wide variety measurements and research.

Refinement of graphite target production at ORAU

Abstract The simple method of graphite target development, first presented at AMS-7 has been further developed and refined. Experiments have been performed to study the range of possible reaction conditions and the effect of these on the nature of the graphite generated. The results from these experiments have been used to make the method as robust as possible with a high success rate, a quick reaction time and very simple apparatus requirements. This paper covers the details of the experiments, the conclusions drawn from them, and the technique now employed for routine graphite sample preparation at ORAU.

Refining Background Corrections for Radiocarbon Dating of Bone Collagen at ORAU

During the laboratory pretreatment of samples for radiocarbon dating, small amounts of carbon may be added to a sample. Contamination can be incorporated at any stage: during chemical pretreatment, combustion to CO2, graphitization, or accelerator mass spectrometry (AMS) measurement. Such carbon contamination is often modern in age, and so can have an especially severe effect on samples older than ~25 ka BP. During the extraction of collagen from bone using the ultrafiltration protocol at the Oxford Radiocarbon Accelerator Unit (ORAU), small amounts of young carbon are added to the sample. Currently, this contamination is poorly characterized when less than 10 mg of collagen is extracted from a bone. Demand to date small collagen samples with 14C concentrations that approach the detection limit of AMS measurement has increased recently with the growing interest in, for example, directly dating Neanderthal remains and Upper Paleolithic bone artifacts. This paper aims to reduce the minimum collagen sample size required to produce a reliable date from 10 to 5 mg by re-examining the combustion background and subsequently the pretreatment background for bone. The average of 136 measurements of directly combusted nylon suggests that 0.0007 ± 0.001 mg of modern carbon is added to each sample, although the distribution is positively skewed. Regression analysis of the measurements of 52 collagen samples extracted from a bone of background age results in a background of just less than 50,000 BP for bone treated at ORAU.

The Middle to Upper Paleolithic transition: dating, stratigraphy, and isochronous markers.

Accurate and precise dating is vital to our understanding of the Middle to Upper Paleolithic transition. There are, however, a number of uncertainties in the chronologies currently available for this period. We attempt to examine these uncertainties by utilizing a number of recent developments in the field. These include: the precise dating of the Campanian Ignimbrite (CI) tephra by 40Ar/39Ar; the tracing of this tephra to a number of deposits that are radiocarbon dated; the publication of revised radiocarbon calibration data for the period, showing a much better convergence with other available data than during the recent IntCal comparison; and a layer-counted ice-core chronology extending beyond 40,000cal BP. Our data comparisons suggest that a reasonable overall convergence between calibrated radiocarbon ages and calendar dates is possible using the new curves. Additionally, we suggest that charcoal-based radiocarbon ages, as well as bone-based radiocarbon determinations, require cautious interpretation in this period. Potentially, these issues extend far beyond the sites in this study and should be of serious concern to archaeologists studying the Middle to Upper Paleolithic. We conclude by outlining a strategy for moving the science forward by a closer integration of archaeology, chronology, and stratigraphy.

Direct dating of pottery from its organic residues: new precision using compound-specific carbon isotopes

Techniques for identifying organic residues in pottery have been refined over the years by Professor Evershed and his colleagues. Here they address the problem of radiocarbon dating these residues by accelerator mass spectrometry (AMS) which in turn dates the use of the pot. Fatty acids from carcass and dairy products cooked in the pot were isolated from early Neolithic carinated bowls found at the Sweet Track, Somerset Levels, England, and then dated by AMS. The results were very consistent and gave an excellent match to the dendrochronological date of the trackway. The method has wide potential for the precise dating of pottery use on sites.

Dating Celtic art: a major radiocarbon dating programme of Iron Age and early Roman metalwork in Britain

This paper presents the first substantial set of radiocarbon determinations for the later Iron Age decorated metalwork known as Celtic art in Britain. Hitherto this material has been dated relatively on the basis of changes in decoration and form, which were then linked to materials with some absolute date. The latter process has tended to give relatively late dates, as most of the material with a firm date stems from the last century BC or after. This has meant that British Celtic art appears to be rather later than that on the Continent. Our results provide some tentative support for an earlier dating for at least some British material, more closely aligned to that on the Continent. Stead, building on earlier work, has developed a series of Stages or Styles for the decorations of Celtic art and we shall review these here. We also look at particular classes of artefacts and forms of deposition in the light of our new results. We conclude by drawing some contrasts between Bronze Age metalwork and that of Celtic art, reflecting on how far assumptions of a sequential series of changes, key to all forms of typology, are useful in the case of the latter.

Radiocarbon dating of single compounds isolated from pottery cooking vessel residues.

We have developed and demonstrated a practical methodology for dating specific compounds (and octadecanoic or stearic acid--C (sub 18:0) --in particular) from the lipid material surviving in archaeological cooking pots. Such compounds may be extracted from about 10 g of cooking potsherd, and, after derivatization, can be purified by gas chromatography. To obtain sufficient material for precise dating repetitive, accumulating, GC separation is necessary. Throughout the 6000-year period studied, and over a variety of site environments within England, dates on C (sub 18:0) show no apparent systematic error, but do have a greater variability than can be explained by the errors due to the separation chemistry and measurement process alone. This variability is as yet unexplained. Dates on C (sub 16:0) show greater variability and a systematic error of approximately 100-150 years too young, and it is possible that this is due to contamination from the burial environment. Further work should clarify this.