Prof Christopher Ramsey

Abstract:

Chemical and isotopic analyses have been made of pigment samples from two separate rock art sites in Argentina. The purpose of the study has been to establish the feasibility of extracting carbonaceous material from the samples which will permit reliable radiocarbon dates for the time of painting. The two sites, Catamarca and Rio Negro, present quite different problems. Most of the paper is concerned with Catamarca, and here we have shown that the paint pigments contain very little or no organic binder; but they do contain calcium oxalate derived from local cacti, and calcium carbonate derived probably from local plant ash. We describe a method to purify carbon extracted from the calcium oxalate, and present the dates obtained on both components. We show that, though rare, natural deposits containing both calcium oxalate and calcite do occur, but that they are very distinct in both 13C and 14C compositions; and we argue that they are very unlikely to contaminate the pigments to such an extent that the 14C dates are altered. For the Rio Negro site we show that the ground for the paint pigments contains carbon derived from fires burnt inside the cave, and discuss how analytical methods provide information to develop a strategy for extracting material, from both ground and pigment, for more reliable dating.

Abstract:

Statistical analysis is becoming much more widely used in conjunction with radiocarbon dating. In this paper I discuss the impact of Bayesian analysis (using computer programs such as OxCal) on archaeological research. In addition to simple analysis, the method has implications for the planning of dating projects and the assessment of the reliability of dates in their context.A new formalism for describing chronological models is introduced here: the Chronological Query Language (CQL), an extension of the model definitions found in the program OxCal.New methods of Bayesian analysis can be used to overcome some of the inherent biases in the uncertainty estimates of scientific dating methods. Most of these methods, including 14C, uranium series and thermoluminescence (TL), tend to favor some calendar dates over others. 14C calibration overcomes the problem where this is possible, but a Bayesian approach can be used more generally.

Abstract:

Autoradiography is widely and successfully used to image the distribution of radiolabelled tracer molecules in biological samples. The method is, however, limited in resolution and sensitivity, especially for 14C. Here we describe a new method for imaging 14C-labelled tracers in sections of biological tissue. A highly focused beam of gallium ions bombards the tissue, which is eroded (sputtered) into constituent atoms, molecules and secondary ions. The 14C ions are detected in the secondary beam by the most sensitive method available, namely accelerator mass spectrometry. The specimen is scanned pixel by pixel (1 x 2 microm), generating an image in a manner analogous to scanning electron microscopy. The method can thus be regarded as a specialized form of scanning secondary ion mass spectrometry (SIMS), referred to here as SIAMS (ref. 2). We have used SIAMS to localize the neurotransmitter gamma-aminobutyric acid (GABA) in thin sections of cerebral cortex, and show that it can generate 14C images that are much improved on 14C autoradiography. A scan takes 10-20 min and reveals individual axons, neurons and glial cells at high sensitivity. In principle, the resolution could be increased by up to tenfold, and the method could be extended to some other nuclides.