The impact on archaeology of radiocarbon dating by accelerator mass spectrometry

Abstract

Radiocarbon dating by accelerator mass spectrometry (AMS) differs fundamentally from conventional ¹⁴ C dating because it is based on direct determination of the ratio of ¹⁴ C : ¹² C atoms rather than on counting the radioactivity of ¹⁴ C. It is therefore possible to measure much lower levels of ¹⁴ C in a sample much more rapidly than the conventional technique allows. Consequently, minimum sample size is reduced approximately 1000-fold (from ca . 1 g to ca . 1 mg) and the datable time span of the method can, theoretically, be doubled (from ca . 40 ka to ca . 80 ka). As yet, extension of the time span has not been achieved, because of the effects of sample contamination, but the great reduction in sample size is already having a major impact on archaeology by extending the range of organic remains that can be dated, and, especially, by allowing the archaeologist and the radiocarbon chemist to adopt more selective sampling strategies. This greater selectivity, in the field and the laboratory, is the most important archaeological attribute of AMS ¹⁴ C dating. It allows on-site chronological consistency to be tested by multiple sampling; archaeological materials to be dated that contain too little C, or are too rare or valuable, to be dated by the conventional method; and the validity of a date to be tested by isolating and independently dating particular fractions in chemically complex samples. AMS laboratories have only been processing archaeological samples since 1982, but already several, notably those at Oxford, Toronto, and Tucson, Arizona, have made substantial contributions to archaeological dating. The Oxford laboratory has, since 1983, processed ca. 1200 samples and published over 500 archaeological dates. Particular attention is therefore paid in this paper to the archaeological significance of the dates obtained at Oxford. The ams 14C technique can contribute to archaeological dating in two complementary ways: (i) by testing prevailing assumptions about the antiquity of indirectly dated objects and materials, i.e. verification or falsification dating; and (ii) by dating new or existing archaeological sequences in greater detail than can be achieved by the conventional ¹⁴ C technique, i.e. the building of new and more detailed chronologies. In this paper, recent archaeological applications of the new technique are reviewed under these two headings: verification dating applied to the origin and spread of anatomically modern humans in Europe and the Americas, to putative evidence for early (pre-Neolithic) agriculture in Israel and Egypt, and to the dating of rare Palaeolithic and later artefacts; and the building of new and more-detailed chronologies illustrated by reference to Upper Palaeolithic sequences in Europe, Mesolithic—Neolithic sequences in Southwest Asia, and Neolithic-Bronze Age chronologies in Britain. It is concluded that the development and application of the ams technique represents a revolution in ¹⁴ C dating that will have a profound impact on many aspects of archaeological research.