Contents lists available at ScienceDirect Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep Best practices for calibrating and reporting stable isotope measurements in archaeology Paul Szpak a,⁎ , Jessica Z. Metcalfe b , Rebecca A. Macdonald b a Department of Anthropology, Trent University, 1600 West Bank Drive, Peterborough, ON K9L 0G2, Canada b Department of Anthropology, The University of British Columbia, Vancouver, BC V6T 1Z1, Canada ARTICLE INFO Keywords: Isotopes δ-Values Calibration Standards Precision Accuracy Analytical uncertainty ABSTRACT The use of isotopic measurements in archaeological research has increased rapidly over the past ~ 25 years, owing largely to the proliferation of the instruments required to produce these measurements relatively quickly and cheaply. Unfortunately, the understanding of how to adequately calibrate and report these isotopic data has not kept pace. We surveyed nearly 500 archaeological research papers published within the past 25 years that presented original isotopic data. We found that, generally, the majority of studies do not provide adequate information regarding how isotopic measurements were calibrated, nor how analytical uncertainty (precision and accuracy) was assessed. We review and present recommendations for data analysis, calibration, and reporting to aid archaeological researchers who use isotopic measurements and practices. We present a simple method for quantifying standard analytical uncertainty using data that would be provided by most laboratories. 1. Introduction Isotope ratio mass spectrometry (IRMS) is used widely in archae- ological 1 studies to address a variety of questions. Beginning in the late 1990s, the direct interfacing of rapid and automated combustion techniques (e.g., elemental analyzers connected via continuous-flow to IRMS systems) for analyzing bulk organic materials decreased analytical costs and dramatically increased the number of analyses that could feasibly be performed in a given study. Prior to that time relatively few isotopic studies had been conducted in archaeology, and each study produced at most a few dozen measurements. In recent years, an abundance of studies has been conducted, producing thou- sands of measurements (Fig. 1). Given the now widespread availability of technology to produce isotopic measurements quickly and cheaply, it is important to examine how these measurements are being reported. This is particularly important in archaeology as the researchers primarily responsible for disseminating the results in publications are often not directly involved in obtaining the raw measurements and transforming them into calibrated δ-(delta) values. Moreover, results obtained from commercial laboratories may lack the relevant details or be difficult to interpret with respect to analytical uncertainty, particu- larly for scholars with a limited understanding of isotope ratio mass spectrometry. A decade ago, Jardine and Cunjak (2005) commented on the increase in laboratories providing isotopic measurements and recognized the potential of a widening knowledge gap between IRMS operators and ecologists disseminating these data. We have noticed a similarly widening knowledge gap in archaeology, particularly as it relates to the reporting of analytical methods and uncertainty. While a number of studies have attempted to examine within- and among- laboratory variation in isotopic measurements, the emphasis has been on sample preparation specifically (e.g., Guiry et al., 2016; Jørkov et al., 2007; Sealy et al., 2014), or more generally on measurements produced by different laboratories (e.g., Pestle et al., 2014). Little attention has been paid to the effects of data calibration or the quantification of measurement accuracy, precision, and overall uncer- tainty. The purpose of this paper was fourfold. First, we sought to evaluate the reporting of stable carbon and nitrogen isotopic measurements and their associated uncertainties in the archaeological literature. To do this, we performed a review of relevant literature, focusing on data reporting, calibration methods and quality control (accuracy and precision). The results of this survey suggested that a review of methods and strategies for reporting isotopic data would be useful to archae- ologists utilizing IRMS in their research. As such, the second purpose of the paper was to review data reporting and quality control methods and present them in a manner accessible to researchers who are reporting isotopic measurements but not generating the measurements them- selves. Third, on the basis of our literature survey and review of http://dx.doi.org/10.1016/j.jasrep.2017.05.007 Received 31 October 2016; Received in revised form 24 April 2017; Accepted 1 May 2017 ⁎ Corresponding author. E-mail address: paulszpak@trentu.ca (P. Szpak). 1 For the purposes of this discussion, we use the terms ‘archaeological’ and ‘archaeology’ as catch-alls for isotopic studies in both archaeology and physical anthropology. Journal of Archaeological Science: Reports 13 (2017) 609–616 2352-409X/ © 2017 Elsevier Ltd. All rights reserved. MARK