ACCURACY AND PRECISION OF RADIOCARBON DATES: GEOCHEMISTRY AND CHRONOLOGIC INTERPRETATION OF MEASUREMENTS ON PLEISTOCENE VERTEBRATES

The assessment of any radiocarbon data set involves a methodical examination of a number of factors that can contribute to the accuracy and precision of age estimates. Factors that can potentially affect age determinations of vertebrate fossils include the laboratory chemical pretreatment procedures applied to obtain the organic fraction and the methods used to measure the sample and calculate the precision of the measurement. For example, a variety of analytical techniques applied to samples from a mammoth discovered buried in upland silts near Lindsay in eastern Montana have provided a set of radiocarbon determinations ranging from about 9,500 to 12,500 radiocarbon years before present (RCYBP). Different chemical pretreatment procedures, ranging from application of HCl and NaOH to ultrafiltration, were used to remove possible exogenic organics incorporated into the bone, resulting in the isolation of different components of the organic (or total acid insoluble) fraction. Bone samples from the Lindsay mammoth were dated by six laboratories, using beta decay for samples measured in the 1960s and 1970s (Washington State University, Teledyn Isotopes, University of Saskatchewan) and direct ion counting using accelerator mass spectrometry (ams) for samples measured since the 1990s (Beta Analytic, Stafford Research Laboratories, NSF-Arizona AMS Facility). The subset of ams determinations are more precise but range from about 11,500 to 12,500 RCYBP. If the radiocarbon determinations on the bone samples that are older than 12,000 RCYBP provide the most accurate age estimates, then the conclusion would be that the Lindsay mammoth is older than the Younger Dryas climate event and also older than Clovis artifact assemblages. The variability in the radiocarbon results derived from multiple samples of the Lindsay mammoth appear to be the result of differences in laboratory processing and data analysis techniques.