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Radiocarbon Dating of “Old” Charcoal Using a Wet Oxidation, Stepped-Combustion Procedure

Published online by Cambridge University Press:  18 July 2016

M I Bird ,
L K Ayliffe ,
L K Fifield ,
C S M Turney ,
R G Cresswell ,
T T Barrows  and
B David
M I Bird
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia
L K Ayliffe
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia
L K Fifield
Affiliation:
Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra A.C.T. 0200, Australia
C S M Turney
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia
R G Cresswell
Affiliation:
Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra A.C.T. 0200, Australia
T T Barrows
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra A.C.T. 0200, Australia
B David
Affiliation:
Geography and Environmental Science, Monash University, Clayton, Victoria, 3168 Australia
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Abstract

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We present results that validate a new wet oxidation, stepped-combustion procedure for dating “old” charcoal samples. An acid–base–wet oxidation (ABOX) pretreatment procedure has been developed that is used in place of the conventional acid-base-acid (ABA) pretreatment. Combustions and graphitizations 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 the line at any stage of the procedure. Combustions are performed at 3 temperatures (330 °, 630 ° and 850 °) with a graphite target 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 confidence can be placed on the final age. The pretreatment, combustion, graphitization, and measurement blank for the procedure, based on the analysis of a “radiocarbon-dead” graphite, is 0.5 ± 0.5 μg C (1σ, n=14), equivalent to 0.04 ± 0.02 pMC or an “age” of approximately 60 ka for a 1 mg graphite target. Analyses of a “radiocarbon-dead” natural charcoal after ABOX pretreatment and stepped combustion suggest that the total blank (including contamination not removed by pretreatment) may be higher than for graphite, ranging up to 0.10 ± 0.02 pMC. Additional experiments confirm good agreement with accepted values for the international low-14C “New Kauri” standard (0.16–0.25 pMC). They also confirm excellent reproducibility, with 3 separate dates on different aliquots of a charcoal sample from Ngarrabullgan Cave (Queensland, Australia) ranging from 35.2 to 35.5 ka 14C BP. It is 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 used to produce reliable 14C dates on charcoal up to at least 50 ka.

Type
Articles
Information
Radiocarbon , Volume 41 , Issue 2 , 1999 , pp. 127 - 140
Copyright
Copyright © The American Journal of Science 

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