Prof Christopher Ramsey

Abstract:

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. © 2010 by the Arizona Board of Regents on behalf of the University of Arizona.

Abstract:

Calibration is a core element of radiocarbon dating and is undergoing rapid development on a number of different fronts. This is most obvious in the area of 14C archives suitable for calibration purposes, which are now demonstrating much greater coherence over the earlier age range of the technique. Of particular significance to this end is the development of purely terrestrial archives such as those from the Lake Suigetsu sedimentary profile and Kauri tree rings from New Zealand, in addition to the groundwater records from speleothems. Equally important, however, is the development of statistical tools that can be used with, and help develop, such calibration data. In the context of sedimentary deposition, age-depth modeling provides a very useful way to analyze series of measurements from cores, with or without the presence of additional varve information. New methods are under development, making use of model averaging, that generate more robust age models. In addition, all calibration requires a coherent approach to outliers, for both single samples and where entire data sets might be offset relative to the calibration curve. This paper looks at current developments in these areas.

Abstract:

Subsamples of charcoal from a number of different excavation contexts at the early modern human (Homo sapiens) site of Niah Great Cave (Malaysia) were accelerator mass spectrometry (AMS) 14C dated. Samples were prepared using one of a number of different methods: untreated (control); acid-base-acid (ABA); and acid-base-wet oxidation with stepped combustion (ABOX-SC) after Bird et al. (1999). The results show that for material younger than ∼25 ka BP there is little difference between the two chemical pretreatment methods and the control. For charcoal beyond ∼25 ka BP, however, there are differences of up to 4000 a, with ABOX-SC ages being consistently older. This is argued to be a more effective pretreatment method for decontaminating charcoal samples prior to radiocarbon dating. For radiocarbon dating charcoals greater than ∼25 ka BP, the ABOX-SC pretreatment and combustion approach appears to be the most rigorous method for developing a robust chronological framework for tropical sequences and should be more widely applied in contexts where the material being dated is likely to be ancient. The new chronology developed for Niah Cave based on this technique suggests that the earliest human evidence dates back to at least 45 ka BP and may extend significantly earlier than this based on the recent discovery of lithics 50 cm below the earliest dated charcoal. Copyright © 2008 John Wiley & Sons, Ltd.