Written by Rachel Wood, Associate Professor, Oxford Radiocarbon Accelerator Unit, School of Archaeology, University of Oxford.

Radiocarbon dating is routinely used to work out the age of archaeological and palaeontological sites, and often pops up in news articles and TV dramas. But some substantial problems remain. One of these is the calibration process, which allows us to convert the ratio of ¹⁴C (“radiocarbon”) and ¹²C (the common stable form of carbon) to an age estimate. This is particularly challenging when we are trying to date marine shells or any animal that has eaten food from the marine system. This means that it can be difficult for us to get an accurate age for a sample that should be straightforward – for example, the skeleton of Medieval person or a Mesolithic dog.

The Oxford Radiocarbon Accelerator Unit is starting a project looking for samples of marine shells to help resolve this problem (Fig. 1). Natural history collections in museums are key to its success, and we would be very grateful to hear if anyone has a collection of pre-1950 marine bivalve shells.

The Problem

At school, we are taught that radiocarbon dating works because radiocarbon decays radioactively at a known rate. By comparing the amount of ¹⁴C to a stable form of carbon called ¹²C, we can work out the age of a sample (Fig. 2). This is true, but only partially so because the starting ¹⁴C:¹²C varies. To get around this problem, we need to calibrate radiocarbon dates. Most ¹⁴C is produced in the upper atmosphere, and is incorporated into plants by photosynthesis and then passed through the food chain. This means, that if someone is eating terrestrial food – they will have a similar ¹⁴C:¹²C ratio in their bodies as in the atmosphere.