What do stable isotopes reveal about the diet of ancient populations?
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The analysis of stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) present in bone collagen allows for the study of the diet of ancient populations. These methods provide information on the resources consumed by individuals, their terrestrial or marine origin, and their relative position in the food web (food chain).
Isotopic analysis thus makes it possible to better understand the dietary habits of a group, but also to highlight potential differences related to social and economic practices. From the results obtained on several individuals, scientists can characterize the major dietary trends of a population and contribute to answering archaeological questions.
In summary:
- Stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) allows for the study of the diet of ancient populations.
- Isotopic signatures provide information on the resources consumed and the relative contribution of terrestrial and marine resources.
- Analysis of bone collagen requires checking its preservation state to ensure the reliability of the results.
- Comparing several individuals allows for the study of dietary differences within a population.
How to determine diet using stable isotopes of carbon and nitrogen?
Carbon-14 dating is not the only technique used in archaeometry. The analysis of stable carbon and nitrogen isotopes also provides valuable information about the lifestyle of ancient populations.
The carbon isotopic ratio (δ¹³C) notably allows for the distinction of different food sources. Plants primarily use two types of photosynthesis, known as C3 and C4, which exhibit different carbon isotopic signatures.
- C3 plants, such as wheat, rice, or trees, generally show more negative δ¹³C values.
- C4 plants, such as corn and sugarcane, generally show less negative δ¹³C values.
These differences are found in the food chain and help characterize the resources consumed by humans and animals. The nitrogen isotopic ratio (δ¹⁵N) provides complementary information. In particular, it allows us to study the relative position of individuals in the food web and to better understand the contribution of different protein sources to their diet.
By combining carbon and nitrogen analyses, it is thus possible to study the major dietary trends of an individual or a group, notably the relative contribution of terrestrial and marine resources.
Why check the preservation state of bone collagen?
The interpretation of isotopic results depends on the quality of the analyzed samples. Over time, bone collagen can degrade or be altered by its environment, which risks compromising the reliability of the measurements.
The quantification of carbon and nitrogen concentrations, as well as the analysis of the C/N ratio, allow for the evaluation of the quality of the extracted collagen. A C/N ratio between 2.9 and 3.6 is generally considered compatible with well-preserved collagen for isotopic analyses and radiocarbon dating.
These criteria must, however, be interpreted in conjunction with other conservation indicators and according to the nature of the sample. They help determine whether the collagen is sufficiently preserved to allow for reliable exploitation of the results.
What do the results of the isotopic analysis teach us?
Stable isotope analyses of carbon and nitrogen provide information on the dietary habits of ancient populations, notably:
| Categories of resources consumed | Relative contribution of certain food sources | Differences between individuals or groups |
| Terrestrial or marine resources and foods from different levels of the food web | Plant resources and animal proteins | Distinct dietary practices contributing to the study of social or economic inequalities |
The interpretation of the results depends on the archaeological context, the environment, and the resources available at the time studied. Isotopic signatures do not always make it possible to identify a precise food, but they help researchers test hypotheses and better understand ancient lifestyles.
Studying a sufficiently representative set of individuals is essential for characterizing the dietary trends of a population and avoiding the drawing of general conclusions from a few isolated results.
What equipment is used for stable isotope analysis at CIRAM?
The analysis of stable carbon and nitrogen isotopes relies on two complementary pieces of equipment: an elemental analyzer and an isotope ratio mass spectrometer (IRMS).
The elemental analyzer allows for the measurement of carbon and nitrogen concentrations in the sample after combustion. The resulting gases are then introduced into the IRMS, which measures the isotopic ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N). The results are expressed in per mille (‰), according to the δ notation used in isotopic geochemistry.
At CIRAM, these analyses are based on a vario ISOTOPE select elemental analyzer and an isoprime precisION isotope ratio mass spectrometer, from the brand ELEMENTAR. These instruments make it possible to characterize samples and produce the data necessary for the study of ancient diets.
CIRAM's expertise in isotopic analysis
Since 2005, CIRAM has supported archaeological studies with its expertise in dating and scientific analysis. Our teams interpret the results, taking into account the characteristics of the samples and research issues, and provide documented reports to help researchers utilize their data.
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You may be asking yourself these questions about the diet of ancient populations?
Analyzing a set of individuals allows for the comparison of dietary habits and the search for differences between groups. A sufficiently representative corpus helps to avoid generalizing results obtained from a single individual.
Not always. Stable isotopes primarily help identify dietary trends and the relative contributions of different sources. Interpretation must take into account the environmental context, available resources, and other archaeological data.
Collagen retains isotopic information about the proteins consumed by an individual. Its analysis therefore makes it possible to reconstruct certain characteristics of their diet, provided that its preservation is sufficient.
They notably analyze the stable isotopes of carbon and nitrogen present in bone collagen. The results make it possible to study broad dietary trends and the types of resources consumed.
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