Guide to Radiocarbon Sampling

This pocket guide is intended to assist researchers collecting ¹⁴C samples in the field, lab, or museum to make the best decisions to achieve their research goals. In the moment of choosing your samples you often face physical, intellectual and logistical challenges: weather extremes; time crunches; complex stratigraphic observations; and the inevitable critical find at 3pm on the last day of the field season. This guide is a resource to help clarify your decision process in those moments, and to help you do good science.

The advice and recommendations that follow are suggested best practices. The more you apply these ideas the greater the confidence you will have in your results. Whenever you have the opportunity to improve the quality of your sample – even incrementally – do it. Do your best given your experience, your budget, the conditions, and the tools at hand. Do not allow the perfect to become the enemy of the good.

Sampling in the excavation context is guided by an understanding of depositional processes and interpretation of natural and cultural stratigraphy. Collecting samples opportunistically during excavation is a good idea, but targeted sampling when the entire context is fully understood is critical. When you collect a sample, always define the question that will be answered by getting a ¹⁴C date. Write this down explicitly in your notes when you collect the sample. It may seem obvious that charcoal from a fire pit tells you the age of the pit, but it also may indicate the age of the floor it is built on, the age of associated artifacts, and constrain the age of another stratum that overlays it. Write it all down.

Target abrupt changes in stratigraphy or lithology, which indicate changes in external conditions. Sample directly above and below such breaks to determine whether the break also represents a hiatus of deposition. Sample all discrete features. Take more samples than you expect or can afford to analyze.

Typically, the ¹⁴C age of a dated sample is indirectly related to an event of interest. When dating charcoal from a hearth, we want to know when the hearth was made and the timing of associated artifacts and behaviors. What we date is the death of the plant from which the charcoal was derived, which can predate the hearth by decades or centuries in some cases. To reduce the effects of old wood and in-built ages, short-lived samples are recommended where possible.

• Single year growth: seeds; grains; nutshells; antlers; pinecones; inflorescences; leaves; twigs; grasses; outer wood; outer lip of mollusk shells; insects.
• <10 years: most animal bone (incl. brief lifespan and collagen turnover), smaller branches; certain mollusks.
• 10+ years: inner wood of trees; collagen in dentine and the petrous portion of the temporal; bulk soil organics.
• Avoid multiple or mixed samples

Aluminum Foil

Most kitchen foil is coated to help it separate from the roll, and this likely contains some form of carbon. There is a legend that only either the shiny side or the dull side has this coating; in fact, it is on both sides. Being food grade, though, the coating is water soluble and will be removed by standard pretreatments. Uncoated chemistry grade foil is available as well.

Fold your foil, don't crush it!

Consolidants and Adhesives

If a bone or artifact must be consolidated in the field before removing it, use the minimum required and record where specifically it was applied so it can be avoided or removed when later sampled in the lab. Always note the possible presence of a consolidant when submitting the material to a ¹⁴C lab. Likewise, always assume any material held in museum collections has something applied to it.

Whether working with field samples back in the lab or collecting samples in a museum setting, certain tools and materials are always useful to have on hand.

Cutting bone with a Dremel should be done in a hood with safety glasses and a particle mask if necessary. Bone dust is messy and not good to inhale.

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