Abstract
The acid-base-acid (ABA) procedure is a common chemical pretreatment used on most organic matter samples. Although this pretreatment is straightforward, there is no consensus among labs about the optimum strength of the chemicals, or even the temperature, to be used for digestions. Comparisons between 14C results obtained by samples undergoing ABA against other wet oxidations, such as acid-base oxidation followed by stepped-combustion (ABOX-SC) or α-cellulose on wood, have sometimes suggested that ABA does not always remove all contaminating carbon. In addition, if not all ABA protocols are the same, could it be said that one procedure is better than others or comparable to wet-oxidation pretreatments, if rigorously applied? To determine whether 14C-free samples showed any fraction modern carbon (FmC) deviations due to pretreatment strategies, 3 experiments were carried out. The first compared 14C results for wood samples at or near the limit of 14C dating, pretreated with a standard ABA protocol used at UC Irvine's Keck Carbon Cycle Accelerator Mass Spectrometry (KCCAMS) facility, with those obtained from ABA- and ABOX-SC-prepared aliquots of the same samples performed at Australian National University (ANU) in 2001. The second experiment subjected wood samples, ranging from ∼12 ka BP to 14C-free, to 5 selected published ABA pretreatments. Third, we evaluated whether the ABA protocol needs a higher-strength final acid step (particularly important for removing CO2 absorption when samples are submerged in alkaline solutions). We are able to show that rigorous ABA treatments exist that can provide robust, reproducible results for many wood samples that are known to be >>50 ka BP, implying that a lack of control over the procedure's outcome might be sample-related, rather than due simply to inadequacies in the chemical pretreatment.
Highlights
For radiocarbon accelerator mass spectrometry (AMS) dating to obtain accurate ages, samples must undergo chemical pretreatments capable of removing exogenous carbon (C) incorporated on or within the material, during in situ burial or laboratory handling
Comparisons between 14C results obtained by samples undergoing ABA against other wet oxidations, such as acid-base oxidation followed by stepped-combustion (ABOX-stepped-combustion procedure (SC)) or -cellulose on wood, have sometimes suggested that ABA does not always remove all contaminating carbon
The first compared 14C results for wood samples at or near the limit of 14C dating, pretreated with a standard ABA protocol used at UC Irvine’s Keck Carbon Cycle Accelerator Mass Spectrometry (KCCAMS) facility, with those obtained from ABA- and ABOX-SC-prepared aliquots of the same samples performed at Australian National University (ANU) in 2001
Summary
For radiocarbon accelerator mass spectrometry (AMS) dating to obtain accurate ages, samples must undergo chemical pretreatments capable of removing exogenous carbon (C) incorporated on or within the material, during in situ burial or laboratory handling. A common pretreatment for many organic matter samples that most laboratories worldwide have used for decades is the well-known ABA (acid-base-acid) procedure During this pretreatment, samples are initially washed using weak acids and bases in order to remove secondary carbon-containing species, such as carbonates and humic or fulvic acids that might have accumulated on the sample over time. Comparisons between 14C age results obtained by organic samples (mostly charcoal and wood) undergoing ABA versus other wet oxidations, such as the acid-base-oxidation (ABOX) followed by stepped-combustion procedure (SC) or alpha-cellulose, have sometimes suggested that ABA does not always remove all contaminating C (Santos et al 2001; Higham et al 2009 and references therein).
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