Radiocarbon dating of "old" charcoal using a wet oxidation, stepped-combustion procedure

We present results that validate a new wet oxidation, stepped-combustion pr ocedure for dating "old" charcoal samples. An acid-base-wet oxidation (ABOX ) pretreatment procedure has been developed that is used in place of the co nventional acid-base-acid (ABA) pretreatment. Combustions and graphitizatio ns are performed in a vacuum line that is insulated from the atmosphere by a second backing vacuum to eliminate the risk of atmospheric leakage into t he line at any stage of the procedure. Combustions are performed at 3 tempe ratures (330 degrees C, 630 degrees C and 850 degrees C) with a graphite ta rget produced from the CO2 evolved during each combustion step. In this way , the removal of any contamination can be monitored, and a high degree of c onfidence can be placed on the final age. The pretreatment, combustion, gra phitization, and measurement blank for the procedure, based on the analysis of a "radiocarbon-dead" graphite, is 0.5 +/- 0.5 mu g C (1 sigma, n=14), e quivalent to 0.04 +/- 0.02 pMC or an "age" of approximately 60 ka for a 1 m g graphite target. Analyses of a "radiocarbon-dead" natural charcoal after ABOX pretreatment and stepped combustion suggest that the total blank (incl uding contamination not removed by pretreatment) may be higher than for gra phite, ranging up to 0.10 rt 0.02 pMC. Additional experiments confirm good agreement with accepted values for the international low-C-14 "New Kauri" s tandard (0.16-0.25 pMC). They also confirm excellent reproducibility, with 3 separate dates on different aliquots of a charcoal sample from Ngarrabull gan Cave (Queensland, Australia) ranging from 35.2 to 35.5 ka C-14 BP. It i s also demonstrated that the ABOX pretreatment, in conjunction with the new vacuum line described here, is able to remove contamination not removed by the conventional ABA pretreatment, suggesting that the technique can be us ed to produce reliable C-14 dates on charcoal up to at least 50 ka.