Guide to Radiocarbon Sampling

This guide is intended to assist researchers collecting 14 C samples in the field, lab, or museum to make the best decisions to achieve their research goals with the best possible results. In the moment of choosing your samples you are often facing physical, intellectual, and logistical challenges: weather extremes; time crunches; complex stratigraphic observations; and the inevitable critical find at 3 p.m. on the last day of the field season. The purpose here is to provide a resource to help clarify your decision process in those moments, and to allow you to remain focused on doing good science.

The advice and recommendations that follow are not meant to say that if you fail to do one thing correctly you’ve ruined your 14 C sample irrevocably. 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.

Sampling from an 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 the getting a 14 C 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’s 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/can afford to analyze.

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

• Single year growth: seeds; grains; nutshells; pinecones; inflorescences; leaves; twigs; grasses; outer wood; outer lip of mollusk shells
• <10 years: most animal bone collagen (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’s 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 14 C lab.

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.