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

Abstract

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\u202f°C, 450\u202f°C and 550\u202f°C from radiocarbon-free wood, and exposed to environmental contamination for 1–3\u202fyrs. The results show a >94% reduction in carbon contamination for the 450\u202f°C and 550\u202f°C charcoal samples occurred using A-Ox/hypy, but this treatment was less effective for the 350\u202f°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 (