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

Abstract

Archaeological chronologies use many radiocarbon ( 14 C) dates, some of which may be misleading. Strict 'chronometric hygiene protocols', which aim to enhance the overall accuracy and precision of 14 C datasets by removing all potentially problematic samples, mean that so few dates remain in some locations that accurate chronologies cannot be established. 14 C 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 14 C dataset of

A model-based approach to the tempo of “collapse”: The case of Rapa Nui (Easter Island)

“…AD (68.2% HPD). The difference between our results and those recently published by Schmid et al (2018) is potentially explained by their use of several younger 14 C samples that are unrelated to colonization, some samples not derived from archaeological contexts (e.g., those samples from Mann et al (2008)), and their presentation of 68.2% HPD rather than 95.4% estimates. Given the available radiocarbon data, our results provide currently the most accurate, if somewhat less precise, colonization estimate for the island and add to a growing corpus of analyses suggesting initial Polynesian colonization of Rapa Nui between the late 12th and early 13th centuries AD.…”

“…We built two colonization models using 14 C samples from archaeological contexts: one with only short-lived plant remains (n ¼ 9), and a second with these nine short-lived samples and 19 unidentified charcoal samples. For the second model, we apply a Charcoal Outlier parameter to assess the influence of unidentified charcoal samples on the precision of our colonization estimate (Bronk Ramsey, 2009;Dee and Bronk Ramsey, 2014;Schmid et al, 2018).…”

Redating of the To’aga site (Ofu Island, Manu’a) and a revised chronology for the Lapita to Polynesian Plainware transition in Tonga and Samoa

“…( [29] pg 67) has suggested in such large-scale phase models ~280 dates will produce results of the highest precision, while the most accurate results are achieved where sampling density is uniformly distributed. Moreover, while many researchers question the usefulness of using 14 C dating especially across the "Radiocarbon Plateau" (ca.…”

“…dates. It has been demonstrated in Bayesian models that a small dataset has a greater detrimental impact on chronological resolution than the potential of unidentified charcoals to skew to older ages [29]. Outlier analysis specifically designed to account for this inbuilt age in charcoal is promising [5] [29], but requires additional constraints (e.g., dates on short-lived materials or stratigraphy) against which to anchor the model (see also [30] where the model allows a small number of charcoal samples with inbuilt age to be younger than the context they represent, as would be the case with intrusive material).…”

The importance of shell: Redating of the To’aga site (Ofu Island, Manu'a) and a revised chronology for the Lapita to Polynesian Plainware transition in Tonga and Sāmoa

“…Pacific based research has indicated that, except in the highest precision analyses, minor inbuilt age typically goes unnoticed, and the impact is therefore overlooked [39] [40]. Bayesian outlier analysis specifically designed to account for this inbuilt age in charcoal is promising [5] [41], but typically requires additional constraints (e.g., dates on short-lived materials or stratigraphy) against which to anchor the model (see also [42] where the model code allows a small number of charcoal samples with inbuilt age to be younger than the context they represent, as would be the case with intrusive material). Once outliers have been correctly coded it has been demonstrated that a small dataset has a greater detrimental impact on chronological resolution than the potential of unidentified charcoals to skew the model to older ages [41].…”