A Pre-treatment Routine for Correcting Age-Offsets on AMS Radiocarbon Dated Ceramic Residue

Kansas State University, Manhattan KS

Investigators

Abstract

Dr. Donna C. Roper, from Kansas State University, and Dr. Linda Scott Cummings, from PaleoResearch Institute, will collaborate on a study designed to develop a pre-treatment protocol for correcting age-offsets on AMS radiocarbon dated ceramic residue. Since its introduction in the mid-twentieth century, radiocarbon dating has been vital to archaeologists as well as biologists, geologists and scientists conducting basic and applied research in other fields. The method has seen considerable advancement in the ensuing decades, enabling development of evermore accurate and precise timelines. Capable of producing high precision dates, the AMS technique permits assay of small materials, such as individual corn kernel fragments or seeds that are short-lived and contemporaneous with their context, thus yielding ages with higher accuracy than ever before. Also, the range of organic materials believed to be appropriate for archaeological dating is broadened. Charred food burned onto ceramic vessels during cooking, often termed ceramic residue, is one of the materials now commonly selected for dating and increasingly used in North America, Europe, and Asia. As its use increases, ceramic residue ages are often noted to be older than dates on other short-lived material from the same context, and thus are not accurate. Continuing to assume the accuracy of these dates and integrating them into timelines degrades chronologies precisely at the time the discipline is able to produce timelines of moderately high resolution and enable archaeologists to work toward viable narratives of historical process. Two previous approaches to addressing the problem have included determining chemical composition as a predictor of which samples likely are affected, and developing correction factors for ages. Neither is proving capable of satisfactorily addressing the problem; indeed, recent conclusions regarding correction factors are that age offsets are too variable for any single local, or regional correction factor to be accurate. The study team for this project proposes a third approach that includes a pre-treatment protocol to remove and discard compounds responsible for the anomalous dates. As a result, dates may be obtained on a fraction of residue shown to produce dates contemporaneous with associated short-lived materials. Limited trials have produced encouraging results toward developing a ceramic residue treatment protocol for general use. AMS dates on residue and on context materials known to produce accurate ages will be paired to test the efficacy of the developing method. Tests will consist of dating multiple fractions, including removed compounds, from a few residues to obtain information concerning the old carbon content. Samples will be drawn from late prehistoric individual dwellings in the North American Great Plains that had use-lives on the order of a decade to a decade-and-a-half. Testing and vetting methods aims toward development of a pre-treatment protocol capable of obtaining accurate dates on ceramic residue anywhere in the world, as residues are fast becoming a significant part of the radiocarbon dating record. Furthermore, this method will lead to a future effort involving chemical separation of compounds containing old carbon in or adhering to other materials, opening the door to application of this protocol to additional samples so they may produce accurate and concordant radiocarbon dates.