Ecology and Evolution by Thomas Larsen, Yusuke Yokoyama, and Ricardo Fernandes. ... radiometric counting and AMS, have advantages and disadvantages ... most cases adequate for the less demanding applications of ecological studies.
SUPPLEMENTARY INFORMATION “Radiocarbon in ecology: Insights and perspectives from aquatic and terrestrial studies” in Methods in Ecology and Evolution by Thomas Larsen, Yusuke Yokoyama, and Ricardo Fernandes. Radiocarbon nomenclature and analysis
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Radiocarbon Age (years before 1950 AD) Figure S1. Decay curve for 14C showing the activity at a half-life of 5730 years relative to per mill depletion (14C) and percentage of modern. Radiocarbon nomenclature Radiocarbon measurements were originally performed with proportional counters or liquid scintillators detecting the radioactive decay or activity of 14C. However, the majority of 14C measurements are now performed with accelerator mass spectrometers (AMS) that count the number of 14C atoms relative to total carbon atoms in the sample expressed as the ratio of 14C relative to 13C and 12C atoms. Regardless of the technique in question, the measured activities or ratios can be converted into radiocarbon age (years before present – see Fig. S1) by calibrating against the absolute radiocarbon standard (Aabs) and accounting for isotope fractionation of the specific sample, a procedure called normalization (Stuiver & Polach 1977). Aabs corresponds to the hypothetical specific activity of atmospheric carbon of the year 1950 making the assumption that it was free from human perturbations. Like 13C, 14C is subject to isotopic fractionation and the normalisation procedure assumes that 14C/12C fractionation is approximately twice that of 13C/12C. The normalisation procedure uses a fixed reference 13C value of -25.0 ‰ to account for the fact that some materials are 13C depleted (such as photosynthetic materials) or enriched (such as inorganic carbon) relative to the contemporary 13C value of the atmosphere. For AMS measurements, two sample types are analysed along the natural samples to correct for background noise and ensure standardized numbers across labs: 1) blank samples devoid of 14C such as coal, limestone or some other geologic 1
SUPPLEMENTARY INFORMATION material and 2) a universally recognized standard such as Oxalic Acid II from the National Institute of Standards and Technology (NIST)(Currie 2004). Analytical methods Both 14C measurement methods, radiometric counting and AMS, have advantages and disadvantages (Aitken 1990). An advantage of AMS is that 1 milligram of carbon or even less is needed as opposed to a few grams of carbon for radiometric counting. Thus, sample sizes can be several orders of magnitude smaller (milligrams vs grams) with AMS than radiometric measurements. The disadvantage of the small sample size is that rigorous protocols are needed to eliminate contaminants. However, measurement times are typically much faster with AMS than radiometric counting, which can take days. Although AMS measurements usually achieve higher precision and lower backgrounds than radiometric counting, the disadvantage is that AMS are costly to operate. Biological samples are introduced into AMS systems in graphitized form, a demanding and labour intensive pre-treatment protocol. Moreover, the most sensitive AMS machines operate under ultrahigh vacuum at 3 million volts. New developments such as automated graphitization systems attached to an Elemental Analyser which can select types of gas injection into the graphitization system, direct injection of carbon dioxide (van Duijn et al. 2014) and improvements in instrument design (Suter et al. 2010; Synal et al. 2013) can help reducing analytical costs, analytical uncertainty and sample amounts. The recent commercialization of compact AMS systems with automated sample handling and liquid chromatographic interfacing are also leading to reduced analytical costs. While these compact AMS systems have lower measurement precision compared to the traditional AMS systems, they are in most cases adequate for the less demanding applications of ecological studies. New and emerging technologies based on radiocarbon laser-based detection are showing a lot of promise in terms of simplifying sample pre-treatment, and reducing costs and instrument footprint although technical hurdles still need to be overcome (Vuong et al. 2015; Galli et al. 2016).
References Aitken, M.J. (1990) Science-Based Dating in Archaeology. Taylor & Francis. Currie, L.A. (2004) The remarkable metrological history of radiocarbon dating II. Journal of Research of the National Institute of Standards and Technology, 109, 185-217. Galli, I., Bartalini, S., Ballerini, R., Barucci, M., Cancio, P., De Pas, M., Giusfredi, G., Mazzotti, D., Akikusa, N. & De Natale, P. (2016) Spectroscopic detection of radiocarbon dioxide at parts-perquadrillion sensitivity. Optica, 3, 385-388. Stuiver, M. & Polach, H.A. (1977) Discussion: Reporting of 14C data. Radiocarbon, 19, 355-363. Suter, M., Müller, A.M., Alfimov, V., Christl, M., Schulze-König, T., Kubik, P.W., Synal, H.A., Vockenhuber, C. & Wacker, L. (2010) Are Compact AMS Facilities a Competitive Alternative to Larger Tandem Accelerators? Radiocarbon, 52, 319-330. Synal, H.A., Schulze-König, T., Seiler, M., Suter, M. & Wacker, L. (2013) Mass spectrometric detection of radiocarbon for dating applications. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 294, 349-352. van Duijn, E., Sandman, H., Grossouw, D., Mocking, J.A.J., Coulier, L. & Vaes, W.H.J. (2014) Automated Combustion Accelerator Mass Spectrometry for the Analysis of Biomedical Samples in the Low Attomole Range. Analytical Chemistry, 86, 7635-7641. Vuong, L.T., Song, Q., Lee, H.J., Roffel, A.F., Shin, S.-H., Shin, Y.G. & Dueker, S.R. (2015) Opportunities in low-level radiocarbon microtracing: applications and new technology. Future Science OA, 2.
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