Stable isotope analysis of the protein extracted from human and animal bones provides direct information on their lifetime diets. It is increasingly used in many areas of archaeology, and was first applied to reconstruct past human diets in the late ‘70s (Vogel and van der Merwe 1977; DeNiro and Epstein 1978; van der Merwe et al. 1981; van der Merwe 1982; Richards and van Klinken 1997). The stable isotopes measured are of the elements carbon and nitrogen. The isotopic ratio of two isotopes of carbon (12 and 13) is reported as a δ13C value, while the ratio of two isotopes of nitrogen (15 and 14) is given as a δ15N value. As foods consumed by humans and animals are broken down in the body, carbon and nitrogen atoms from the food are redeposited in body tissues. There are fractionation effects in this process, therefore, body tissues have different δ13C and δ15N values than those of the foods eaten. For bone collagen, the δ13C and δ15N values indicate the sources of dietary protein eaten over many years. The δ13C values of foods are fairly well known, as are the magnitudes of fractionation effects in the body. It is therefore possible to measure the δ13C and δ15N values of archaeological human and animal bone collagen and infer what foods were eaten.
Stable isotopes in past human diets
Stable isotope analysis has made five major contributions to the reconstruction of past human diet:
- The distinction of C3 and C4 plants . The major differences in carbon isotope values in plants occur between plants that use the C3 vs. the C4. Photosynthetic C3 plants include temperate grasses, all trees and shrubs, all fruits and nuts and cultivated roots and tubers. C4 plants on the other hand are predominantly tropical grasses including some important domesticates such as maize, millet and rice (Keegan 1989). C4 plants have heavier δ13C than do C3 plants.
- The distinction of nitrogen-fixing and non-fixing plants . Because the δ15N value of atmospheric nitrogen is approximately 0 ‰ and the δ15N of nitrogen in soils can be much higher, plants which fix atmospheric nitrogen (i.e. legumes) often have lower δ15N values than plants which do not fix nitrogen (Katzenberg 1992).
- The distinction of marine and terrestrial foodstuffs . The main carbon source for marine plants and animals (dissolved CO 2) has a δ13C value approximately 0 ‰ whereas CO2 in the atmosphere, the main source of carbon terrestrial plants and animals, has a δ13C value of -7 ‰. Therefore, mammals that consume only marine protein have δ13C values approximately 7 per mil heavier than those that consume only terrestrial protein. Similarly, δ15N values of marine organisms are typically higher than those for terrestrial ones, with some values ranging above 20 % (Katzenberg 1992).
- The distinction between freshwater and marine foodstuffs . The δ13C values of aquatic plants (from lakes and rivers) and the fish that feed on them may vary widely reflecting the contribution of carbon or nitrogen sources respectively (Schwartz and Schoeninger 1991). Often, the δ13C values are more terrestrial-like than marine plants and fish, yet have similarly high δ15N values.
- The effect of weaning and breast-feeding in past populations . Breast-fed children have δ15N values approximately three parts per thousand higher than their mothers (Fogel et. al. 1989). This is due to the so-called “trophic level effect” (Schoeninger and DeNiro 1984) where consumer bone collagen δ15N is enriched by 2-4 parts per thousand over average dietary protein. By measuring the δ15N values of a bone with a high turnover rate such as a rib it is possible to observe generally when weaning occurred in a past population (Richards et. al. 2002).
Application of stable isotopes analysis in the MH Argolid project
Samples of bone collagen have been taken from human ribs of all populations included in the Middle Helladic Argolid project as well as of animal bones in order to have a control over environmental and climatic changes during the case study period. The results from Lerna and Aspis have been processed; the final reports for both sites have been submitted. The analysis of samples from Asine is currently under way; the report will be submitted in early 2008. The results will be presented at the One-Day Conference on 19 December 2007.
We also plan to take samples from the Argos ‘tumuli’, the Prehistoric Cemetery at Mycenae and Midea within this year.
The aims of the stable isotope analysis in the MH Argolid project are:
- To define broad dietary patterns of the case study populations and explore whether there are variations between communities and different environmental zones (e.g. coastal/inland).
- To investigate possible differences of the isotopic signals between population subgroups, e.g. age and sex groups, within the same cemetery.
- To explore the effect of weaning and breast-feeding on the population of MH Argolid.
The analysis is undertaken by Mike P. Richards (Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig). The results are processed by Sevi Triantaphyllou (Lerna, Aspis) and Anne Ingvarsson-Sundström (Asine).
To view the References, click here
