How ¹⁴C dates on wood charcoal increase precision when dating colonization: The examples of Iceland and Polynesia

Archaeological chronologies use many radiocarbon (¹⁴C) dates, some of which may be misleading. Strict ‘chronometric hygiene’ protocols, which aim to enhance the overall accuracy and precision of ¹⁴C datasets by removing all potentially problematic samples, mean that so few dates remain in some locations that accurate chronologies cannot be established. ¹⁴C dates on charcoal can be affected by an ‘old-wood’ effect, and so they are often removed from analyses, despite >40,000 being available worldwide, representing > $25 million. We show that when a Bayesian chronological model is used, which incorporates an Outlier Model specific to wood charcoal, the ¹⁴C dataset of Iceland's Viking Age settlement agrees well with ice core-dated tephrochronology and written sources. Greatest accuracy comes from an even temporal distribution of ¹⁴C dates and more dates lead to greater precision (<20 years). This shows how charcoal-based ¹⁴C chronologies can pinpoint the transformational human settlement of islands in the Atlantic, Oceania, and elsewhere.