Carbonised food crusts adhering to pottery are now routinely dated to provide 14C ages for the use of the pottery concerned.
Because
fish and other aquatic foods are usually depleted in 14C relative to
plants and animals in terrestrial food chains, however, such dates can
be misleadingly old. This problem must be addressed if we are to develop
realistic chronologies for the introduction and spread of pottery
across Eurasia (see Dating the spread of pottery among
hunter-gatherer-fisher communities in north-eastern Europe project). The
challenge is to identify the sources of carbon in food crusts – or at
least, to identify which food crusts are derived mainly from terrestrial
foods, and to correct the 14C ages of “fishy” food crusts for any
aquatic reservoir effects.
This
project is concerned with gauging the effectiveness of screening
methods that might be used to identify the potential for reservoir
effects in food crusts. Effective methods must be faster and more
economical than 14C dating, and use little or no material; we also need
to be able to test previously dated food crusts as well as screening
potential samples for future dating. Results so far indicate that the
light stable isotopes δ13C and δ15N are reasonably reliable indicators
of the contribution of aquatic foods to the carbon content of a food
crust, particularly at coastal sites. At inland sites, potential
reservoir effects in food crusts are much greater and more variable, and
the differences in δ13C and δ15N between terrestrial and aquatic foods
can be less marked, so a third isotopic system may be required to
identify fishy food crusts. There are several methodological challenges
(e.g. diagenesis, sample pretreatment, sample inhomogeneity) that
require further investigation, in part using modern experimental food
crusts.