AMS Carbon Dating of Charcoal or Wood

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• Charcoal and wood are two of the most widely tested samples for radiocarbon dating.
• These materials' true age can be affected by the old wood effect.
• Contaminants of charcoal and wood samples can be artificial or natural.
• There are several physical pretreatment methods for wood and charcoal.
• Chemical pretreatment of these samples involve alternate acid and alkali washes.

Charcoal and wood are two of the most widely used materials for accelerator mass spectrometry (AMS) radiocarbon dating. AMS labs prefer to carbon date charcoal and wood because these materials do not need complex pretreatment.

Willard Libby, the pioneer of radiocarbon dating, identified charcoal to be the most reliable material to carbon date.

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Beta Analytic provides radiometric and AMS radiocarbon dating of wood and charcoal.
Sample size required for Charcoal: 10-50 grams (AMS), 20 grams (radiometric).
Sample size required for Wood: 20-50 grams (AMS), 50 grams (radiometric)
Recommended container: Ziplock Bags

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Time-Width and the Old Wood Problem

The time-width of an organism refers to its total growth and exchange period with the biosphere. The time-width affects the way radiocarbon age is converted into calendar age for a sample. A wood’s time-width depends on the number of tree rings taken for radiocarbon dating. Fragments of charcoal, however, may have a time width that can’t be quantified.

One of the main assumptions of radiocarbon dating is that the organism’s time of death is also the time it ceased carbon exchange with the biosphere. If this is not the case, such as in wood, the radiocarbon age of the organism at death is not zero.

When radiocarbon dating a piece of wood or charcoal, the event dated is the growth of the tree ring. Trees grow by the addition of rings, and these rings stop exchanging carbon with the biosphere once they are laid down. Thus, the radiocarbon age of a single tree’s heartwood and sapwood will not be the same with the innermost heartwood being significantly older than the sapwood.

Any charcoal or wood sample that is carbon dated will have an apparent age, which may result in errors of up to hundreds of years unless short-lived tree species or twigs are selected for radiocarbon dating.

A sample’s radiocarbon age can tell us when the organism was alive and not when the material from that organism was used. The “old wood” problem must be taken into account to avoid wrong conclusions when linking artifacts to event and context.

Delayed use and reuse are processes that also contribute to the “old wood” problem. Charcoal or wood could have been seasoned prior to the actual use of the timber that provided the sample that has been radiocarbon dated. Hardwoods that are very resilient against decay could have been reused in other structures in later years.

The effects of these depositional processes may not be quantifiable but should not be overlooked because the carbon 14 dating results might turn out to be too old for the context being dated.

Charcoal or Wood Sample Contamination

The carbon-containing materials surrounding a wood or a charcoal sample when it was still buried as well as those used during its collection and preservation might have already altered its carbon 14 content. Any material that adds to the carbon content to a sample is considered a contaminant.

Natural contaminants to wood and charcoal are those introduced in the post-depositional environment like humic and fulvic acids in soil. These are acids produced by the microbial degradation of plant and animal tissues. Rootlet intrusions also introduce modern carbon on wood and charcoal samples. Limestone is also another possible contaminant depending on the excavation site.

Artificial contaminants to wood and charcoal samples are those introduced by negligence or unawareness of the people collecting and processing the samples. Artificial contaminants include ash from tobacco, hair and fibers, paper from packing material, oil, grease, and even glue.

Effect of Contamination on Carbon Dating Results

The effect of contamination on wood or charcoal samples subjected to AMS radiocarbon dating depends on the type of contaminant, degree of contamination, and the relative age of the samples and the contaminant.

If limestone has not been removed prior to AMS radiocarbon dating, the results will be considerably older than the wood or charcoal’s true age because limestone, being geological in origin, will be much older than any archaeological samples.

Humic and fulvic acids may attach to surfaces of wood and charcoal and exchange carbon in a process called adsorption. This occurrence can make the sample’s radiocarbon age too young or too old depending on the age of the organism that produced the organic acids. Penetration of roots on the charcoal or wood samples also introduce modern carbon into them.

In general, infinite-age contaminants add considerable number of years to the true age of a charcoal or wood sample while modern carbon make any sample significantly younger.

In order to get accurate results, AMS labs perform pretreatment on all wood and charcoal samples before subjecting them to radiocarbon dating.

Physical Pretreatment of Wood or Charcoal

The removal of contamination without the use of chemicals falls under physical pretreatment. Physical pretreatment done on wood or charcoal in AMS labs involves the removal of plant rootlets using tweezers, cleaning by scraping off the surface with a scalpel, and reduction of sample size.

AMS labs use a hammer or chisel to splinter wood, which can also be pulverized into sawdust in mills. An AMS lab analyst crushes charcoal samples in a petri dish or with a mortar and pestle. Size reduction is done to increase the surface area of the sample for succeeding pretreatment.

Chemical Pretreatment of Wood or Charcoal

Chemical pretreatment is done before AMS carbon 14 dating to ensure that all possible contaminants have been removed before the analysis.

Different AMS labs may have slight variations in their procedures for the chemical pretreatment of charcoal or wood, but most often than not, they use the same chemicals. Chemical concentrations, temperatures, exposure times, and number of repetitions greatly depend on the condition and nature of the sample submitted.

Analysts of AMS labs wash wood and charcoal samples with hot hydrochloric acid (HCl) to eliminate carbonates followed by an alkali like sodium hydroxide (NaOH) to remove the remaining organic acids. The last step is a final acid rinse to neutralize the alkali prior to sample drying.

There are also cases when the wood or charcoal samples are only acid washed. This is done when the carbon source is soluble in an alkali. The carbon dating result, however, reflects total organic content.

For wood samples that are either too old or too contaminated, AMS lab analysts add a cellulose-extraction step after the acid-alkali-acid treatment. Cellulose extraction is done by immersing the sample in a sodium chlorite (NaClO2) solution under controlled pH and temperature.