The efficiency of charcoal decontamination for radiocarbon dating by three pre-treatments - ABOX, ABA and hypy

Here we report results of a direct inter-comparison of the ability of three radiocarbon pre-treatment techniques to remove radiocarbon contamination introduced by exposing known-radiocarbon-free charcoal made at three different temperatures (300, 400 and 500 degrees C) to environmental contamination on a rainforest floor for one to three years. The initial charcoal had no measureable radiocarbon, but C-14 activity increased after environmental exposure to similar to 1 pMC after one year (apparent age of similar to 40,000 yrs BP) and similar to 5 pMC after three years (apparent age of similar to 25,000 years). For the 400 and 500 degrees C samples, all techniques were able to reduce contamination by >90%. Acid base oxidation (ABOX) provided the most reliable decontamination, reducing the radiocarbon activity of the one year samples to background, and reducing the radiocarbon activity of the three year samples to 0.04 +/- 0.02 pMC or less (apparent age >56,900 yrs BP). The performance of ABOX was superior to that of both acid-base-acid (ABA) and hydrogen pyrolysis (hypy) treatments, with ABA performing better than hypy in most cases. No technique was able to fully remove decontamination from the 300 degrees C charcoal (although ABOX again removed the most contamination), likely due to the incompletely pyrolized nature of the charcoal which is dominated by aromatic clusters of small ring size. All the techniques rely on removing contaminant carbon faster than indigenous carbon and this condition is not met in the case of charcoal produced at temperatures below similar to 350 degrees C. While all pre-treatments are useful for routine sample processing, the results suggest that ABOX is the only technique that can provide reliable decontamination of charcoal of an age close to the dating limit of the radiocarbon dating technique. (c) 2014 Elsevier B.V. All rights reserved.