SPEED DATING!: ADVICE ON SAMPLING AND APPLICATIONS FOR RADIOCARBON DATING

Radiocarbon dating is used routinely in geologic, archaeologic, and paleontologic studies worldwide. The technique was developed in the late 1940s by Willard Libby at the University of Chicago, and has since transformed how research in these and other scientific disciplines is conducted. The advent of accelerator mass spectrometry in the late 1980s revolutionized radiocarbon dating, allowing for improved age determinations to be made on samples that are much smaller than what was previously required. Today, ages can be obtained for samples that weigh a milligram or less, which is both remarkable and potentially fraught with problems related to context and contamination. Our goal is to improve communication and understanding between field and laboratory scientists, so that the intended objective – obtaining a relevant, accurate, and precise date – can be achieved.

With regard to radiocarbon dating, the most important lesson that we can impart is this: no one has ever measured the age of anything. Ever. Instead, we measure the ratios of carbon isotopes and use the results to calculate an age. At least three criteria must be satisfied for an age to be reliable. First, the context of the sample must be known and understood. Second, the ¹⁴C activity of the sampled material (a plant, for example) must be in equilibrium with atmospheric ¹⁴C at the time it was alive. Finally, after burial, the sampled material must behave as a closed system with respect to carbon, such that contaminants are excluded and only the targeted carbon pool is analyzed.

Some types of samples (charcoal, plant macrofossils, and some terrestrial gastropod shells) meet these criteria better than others. In fact, we consider these materials to be “gold standards,” as their usage can result in exceptionally clear chronologic records if the stratigraphic context is well known. Other materials (bulk organics, humic acids, and aquatic gastropod shells) are less than ideal, and the resulting ages are often spurious. Thus, it is important to understand that radiocarbon dates are not equal in terms of their quality, which has significant implications when comparing or synthesizing ¹⁴C ages from various studies. The more field and laboratory scientists understand the potential pitfalls that the other may encounter, the better the results, and the resulting science, will be.