Food residues help date ancient pottery
Sherds—pottery fragments—are among the most abundant artifacts from archaeological sites, with the earliest pots dating back more than 10 000 years. To determine roughly when the pottery was made and used, archaeologists look to where the sherds are found relative to other, more easily datable objects. In principle, radiocarbon dating could assign a more precise date to those fragments. However, the clay that was dug from the ground to make the pot is much older than the pot itself, and that renders carbon dating problematic. Now researchers in Richard Evershed’s lab at the University of Bristol in the UK have developed a robust, alternative method: They isolate and then apply radiocarbon dating to the fatty acids adsorbed by the pottery during its period of use. Working at the Bristol Radiocarbon Accelerator Mass Spectrometer facility, the researchers used their technique to date pottery from late Stone Age settlements in Europe and Saharan Africa.
This pottery bowl was recovered in southwest England alongside the Sweet Track causeway, which was constructed around 3800 BCE.
E. Casanova et al., Nature 580, 506 (2020)
Efforts to isolate fatty-acid residues from ancient pottery for dating have been thwarted by contamination during sample preparation and separation. After chemically separating animal fats from the clay matrix in a collection of sherds, Evershed and colleagues purified two commonly occurring fatty acids using preparative gas chromatography, in which vaporized compounds are separated and trapped. The researchers performed high-field NMR spectroscopy on the compounds to identify impurities that might affect radiocarbon dating. Avoiding contamination through strict cleaning procedures and the use of a solventless trap they invented, the researchers measured the carbon-14 content of both isolated compounds and determined their radiocarbon ages, with a margin of error of 20 years. Agreement of the dates based on the two different fatty-acid compounds provided a means for internal verification of the results. The researchers confirmed their method by dating samples whose ages had already been well determined by other means—for example, from dendrochronology or studies of surrounding organic matter, such as bones and seeds.
Compound-specific radiocarbon dating could become increasingly important as archaeologists seek to construct generation-by-generation life histories at prehistoric sites. (E. Casanova et al., Nature 580, 506, 2020 .)