Prof Christopher Ramsey

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.

Abstract:

If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical meth- ods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14C calibration curve. In most dating applications, however, we have larger numbers of 14C measurements and we wish to relate those to events in the past. Bayesian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal software (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used "uniform phase" models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run sim- ulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing pos- sible problems with statistical models of this kind. © 2009 by the Arizona Board of Regents on behalf of the University of Arizona.

Abstract:

While an interhemispheric offset in atmospheric radiocarbon levels from AD 1950-950 is now well established, its existence earlier in the Holocene is less clear, with some studies reporting globally uniform 14C levels while others finding Southern Hemisphere samples older by a few decades. In this paper, we present a method for wiggle-matching Southern Hemisphere data sets against Northern Hemisphere curves, using the Bayesian calibration program OxCal 4.1 with the Reservoir Offset function accommodating a potential interhemispheric offset. The accuracy and robustness of this approach is confirmed by wiggle-matching known-calendar age sequences of the Southern Hemisphere calibration curve SHCal04 against the Northern Hemisphere curve IntCal04. We also show that 5 of 9 Holocene Southern Hemisphere data sets are capable of yielding reliable offset information. Those data sets that are accurate and precise show that interhemispheric offset levels in the Early Holocene are similar to modern levels, confirming SHCal04 as the curve of choice for calibrating Southern Hemisphere samples. © 2009 by the Arizona Board of Regents on behalf of the University of Arizona.