Think Before You Radiocarbon

Recently I organised a few days’ excavation that didn’t turn up the kind of stuff I was hoping for. Still, I brought some materials home that may serve to shed some light on what exactly it was we dug into. All those nondescript little pits, all those sooty hearths full of cracked stone — when were they made and used?

Enter radiocarbon. This dating method works on anything organic, that is, anything with carbon in it. Running one sample costs about $500, so you have multiple reasons to be smart about which samples you send to the lab. I thought my thinking about this might interest you, Dear Reader.

We dug 175 sunken features, but I don’t have 175 samples. Most features yielded no datable material (in these situations, usually charcoal, other charred plant remains or bone). Of those that did, many also contained modern junk identifying them as recent refuse pits. I don’t want to spend any money dating them.

I have about 20 samples, and I’ve decided to date five. That’s not too bad for a 1000 square meter trench that contained nothing that appears relevant to my research project. I’m of course only running samples of which I know exactly what stratigraphic context they came from. I’m avoiding a few iffy ones that may have been contaminated, such as one that was collected from the sieve instead of from the feature’s section. I’m trying to cover different kinds of feature (hearth, posthole, pit), though charcoal is of course most abundant in hearths. And I want to spread the dated features out across the trench in order to catch any horizontal variation.

Radiocarbon dates the moment when the tissue concerned stopped receiving carbon from the environment. For a leaf, this is the moment it stopped photosynthesising. For the soft tissues of animals (who, through the food chain, receive carbon taken from the air by plants), it’s a matter of months. For bones, a bit more than that. And for dentine, the interior of teeth, it’s years or decades.

Charcoal is tricky. Every year ring in a tree has a different radiocarbon date. Chop down an old oak, sample the centre of its trunk and some of its leaves, date both samples, and you’ll get a discrepancy of centuries. This is because once a year-ring has formed, it ceases to receive carbon from the living parts of the tree. Therefore, I’ve left my selected charcoal samples to a wood anatomist. Often he can select young twigs or pieces of bark, with a low intrinsic age. Second best, he can select bits of charcoal from a tree species with a short maximum lifespan. Oaks live for centuries, but alders and aspens mostly don’t, so they’re better. Cereal grains and other seeds are excellent, no intrinsic age at all.

On the samples go to the radiocarbon lab, where they’re cleaned, processed and transformed into a graphite coating on little brass plugs. Then some of the graphite is oxidised to CO2 and sent into a particle accelerator. Heavy unstable carbon-14 lands on one detector, lighter and stable carbon-12 on another, and the ratio between them shows how long time has passed since that tissue stopped receiving carbon.

Or it would, if the availability of carbon-14 in the atmosphere were constant. Which it is not. But that can be compensated for.

Back to the tree rings. In warm years, all the trees lay down thick rings, and conversely in cold years. Therefore, I can convert the ring thicknesses of a tree I just cut down into a series of figures, like “thick – thick – medium – thin – thick – medium – thick” etc. With large enough a piece of wood (>50 year rings), I get a unique sequence of thickness estimates that has never occurred before or later in tree history. Armed with my graph (or, actually these days, my database and stat software), I then seek out an old building and saw off a chunk of wood that provides me with a similar dataset. When I find the part where the two datasets overlap (“thick – thick – medium – thin – thick” etc.), I end up with a combined dataset that reaches way back before the original tree I cut down had started to grow. And this process kan be, and has been, repeated until we have tree-ring datasets that extend thousands of years into the past.

We know exactly what year each of those thousands of tree rings formed. This allows us to cross-check their radiocarbon dates, which provides us with a calibration curve. This curve allows us to translate from apparent radiocarbon dates to actual calendar dates. Anybody can to this at home, the best Oxonian software and most recent datasets are freeware.

Without calibration, you miss your actual date by a thousand years in many parts of the past. Around the introduction of agriculture to Scandyland, c. 4000 BC, for instance, the error is a full millennium.

The need for calibration also means that the precision of radiocarbon is not uniform across the millennia. In the 7th century AD, the calibration curve has a shape that allows highly precise dating, e.g. AD 625+-15 years. In other eras, e.g. about 4000 BC sad to tell, the calibration curve forms plateaux where a range of different radiocarbon figures map onto the same calendar date after calibration, e.g. 3950 BC. This means that the entire Scandy process of neolithisation gets compressed, looking like it happened in 3950 BC. There simply aren’t any possible radiocarbon values that will point, after calibration, at 4050 BC or 3850 BC.

Now, what about those five samples of mine? I hope to get them dated in December once some funding paperwork has gone through. I expect them to land in the interval 500 BC to AD 400, because that’s the usual date of this kind of site with loads of pointless pits and hearths. However, I wouldn’t be surprised if I got one date at about AD 1900 and another one in the 6th Millennium BC, because those are the periods from which the dig actually produced any artefacts. I’ll get back to you on the subject when I know.

Questions?

Update 17 October: More fresh radiocarbon goodness in Swedish from Åsa of Ting & Tankar.

[More blog entries about , ; , .]

Comments

  1. #1 abb3w
    October 15, 2008

    Given the “cdesign proponentsists” crowd propensity for ignorant blather, a more extended set of remarks on what factors shape the calibration curve might be of interest.

  2. #2 Bob O'H
    October 16, 2008

    3950 BC? That’s perilously close to 4004 BC, you know.

  3. #3 Martin R
    October 16, 2008

    Abb3w, I don’t know much about that issue beyond the fact that carbon-14 is produced in the upper atmosphere when high-energy radiation from the sun hits carbon dioxide molecules. If I understand correctly, the calibration curve is largely shaped by long-term processes going on in the sun.

    Bob, indeed, but the authors of the Old Testament lived in an area where agriculture started several millennia before that date.

  4. #4 magetoo
    October 16, 2008

    That was a good explanation of the process of radiocarbon dating; how it’s done in practice. I’ve known the theory bit, but then we don’t live in physicsland with spherical cows grazing on the fields, so a reality check is nice.

    Are there any other radiometric(?) techniques that are used in archaeology? Other isotopes, other techniques entirely? I get the feeling that someone must have thought of something, even if it isn’t as neat and practical as C-14.

  5. #5 Martin R
    October 16, 2008

    Thanks! Check out OSL, TL and tephrochronology. Mostly, though, radiocarbon is what we use.

  6. #6 tenine
    October 16, 2008

    I’m surprised you just mention AMS dating. If you have enough carbon, the good ol’ radiometric approach works well enough not to make much difference and costs less.

  7. #7 ArchAsa
    October 16, 2008

    Good rundown on the headache of dating.
    Another serious problem for us dealing with the Stone Age, apart from calibration, is the trouble with samples containing marine remains. The C14 date is partly dependent upon the ratio of C13 isotopes, and that is much higher in the oceans. So anyone living off marine animals, as well as food containing fish or seal for instance, will appear to be anything from 100-400 years too old.

    This is a potential problem with dating charred crusts on pottery for instance.

    Happily, your Iron Age farmers should not have been feeding their dogs too much fish, nor were they cultivating maize (wich weirdly enough causes the same dating problem as marine life forms…). One less problem to contend with.

    Good luck.

  8. #8 Martin R
    October 16, 2008

    Tenine, a larger sample size greatly increases the risk of contamination and makes it much harder to keep the intrinsic age down (or at least known).

    Åsa, not to mention the weird effect seen in lakes sitting in cavities in limestone: living fish there give dates tens of thousands of years in the past!

  9. #9 tenine
    October 17, 2008

    Feeding corn to dogs went on in eastern North America and analyzing dog burials for Carbon isotopes is one way to get at whether prehistoric people had adopted corn agriculture.

    And not just lakes have the old carbon problem. Bivalves from the Tennessee River are not used in dating because the river flows through limestone and dolostone bedrock and its carbon content is “older” than the period of human occupation.

  10. #10 Martin R
    October 17, 2008

    That corn effect is just bizarre. How can a plant differentiate between different carbon isotopes!?

  11. #11 mgr
    October 17, 2008

    Difference in plant material uptake of C14 or C13 are due to the process by which they fix carbon dioxide. Corn, I beleive employs the Crassic Acid Cycle (C-4).

    Mike

  12. #12 megan
    October 22, 2008

    oh wow, limestone lakes, hadn’t thought about that.

    according to an article i found in American Antiquity, there’s a lengthy discussion about different plants, their carbon absorption, and the impact on radiocarbon dating in

    Brownman, D. L. 1981 – Isotopic Discrimination and Correction Factors in radiocarbon dating. IN ‘Advances in Archaeological Method & Theory’, edited vol, Mike Schiffer ed. p 241-295.

The site is undergoing maintenance presently. Commenting has been disabled. Please check back later!