Improved pretreatment method for the isolation and decontamination of pyrogenic carbon for radiocarbon dating using hydrogen pyrolysis

Orr, Theresa J., Wurster, Christopher M., Levchenko, Vladimir, Ascough, Philippa L., and Bird, Michael I. (2021) Improved pretreatment method for the isolation and decontamination of pyrogenic carbon for radiocarbon dating using hydrogen pyrolysis. Quaternary Geochronology, 61. 101124.

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Abstract

Pyrogenic carbon (charcoal, black carbon, elemental carbon) is one of the most common materials used for radiocarbon dating of terrestrial samples. However, exogenous carbon contamination can compromise the accuracy of radiocarbon ages. This study presents the results of two chemical pretreatments prior to hydrogen pyrolysis (hypy) as improved protocols for the isolation and decontamination of pyrogenic carbon, i) a simple acid-oxidation step (A-Ox/hypy) and ii) acid-base-acid (ABA/hypy). The A-Ox/hypy protocol uses HNO3 and H2O2, while ABA/hypy uses HCl and NaOH. Both pretreatments remove labile and inorganic carbon before hypy, decreasing the potential for in situ production of pyrogenic carbon during the hypy reaction. The effectiveness of each protocol was directly measured on charcoal artificially produced at 350 ◦C, 450 ◦C and 550 ◦C from radiocarbon-free wood, and exposed to environmental contamination for 1–3 yrs. The results show a >94% reduction in carbon contamination for the 450 ◦C and 550 ◦C charcoal samples occurred using A-Ox/hypy, but this treatment was less effective for the 350 ◦C charcoal. A >99% reduction in carbon contamination in all charcoal samples examined occurred using ABA/hypy. The A-Ox/hypy protocol was further tested on cave guano sediments, which had previously reported erroneous dates following simple organic solvent extraction followed by ABA pretreatment. Effective decontamination was achieved using A-Ox/hypy on the guano, which corrected a radiocarbon age reversal. Overall, ABA/hypy effectively decontaminated the charcoals and was a more efficient pretreatment for charcoal than A-Ox/hypy, however resulting in larger sample mass loss. Therefore, ABA/hypy is the recommended protocol for older (>30,000 14C yr BP) charcoal or sediment samples, or where date accuracy is imperative, while A-Ox/hypy represents an improved protocol for the quick and cost-effective measurement of younger samples (<30,000 14C yr BP) when sample size is of concern.

Item ID: 67134
Item Type: Article (Research - C1)
ISSN: 1871-1014
Keywords: Radiocarbon dating; ABA; Hydrogen pyrolysis; Pretreatment; Charcoal
Copyright Information: © 2020 Elsevier B.V. All rights reserved.
Funders: Australian Research Council (ARC), ARC Centre of Excellence for Biodiversity and Heritage (ARC), AINSE, Natural Environment Research Council (UK) (NERC)
Projects and Grants: ARC FL140100044, ARC CE170100015, AINSE grant ALNGRA10061, AINSE grant ALNGRA12019, NERC NE/F017456/1
Date Deposited: 11 Mar 2021 23:55
FoR Codes: 43 HISTORY, HERITAGE AND ARCHAEOLOGY > 4301 Archaeology > 430101 Archaeological science @ 50%
37 EARTH SCIENCES > 3703 Geochemistry > 370303 Isotope geochemistry @ 50%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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