C:N value is one of the primary indications of the degree of preservation of bone protein (collagen) and/or contamination by soil organic substances. According to van Klinken (1999, 691), the characteristic %C values of intact collagens are 34.8±8.8 (1σ) wt %C; the nitrogen content of intact collagen is between 11 and 16 wt %. The recommended values for C:N atomic values are between 2.9 - 3.5 (calculated by dividing the percent of carbon by the percent of nitrogen in the bone and multiplying the number by 14/12, due to the different atomic weight of these elements). Sometimes excess salts remaining in the collagen after extraction make %C appear lower, however, this should not change the isotopic composition of the collagen. Higher values of %C may indicate the contamination from soil organic carbon. If C:N ratios are higher than 3.5 then the δ¹³C may point to the source of excess carbon, i.e contamination from humic acids in the soil will tend to produce more negative values (less than -24‰).

Interpretation of stable C and N isotope results

Carbon (δ¹³C) stable isotope analysis is used to estimate the proportions of so called C₃ (most plants) versus C₄ plants (maize, cane sugar, certain millets, chenopods, sorghums and amaranths) and to distinguish between the consumption of marine versus terrestrial organisms in an individual's diet. C₃ plants have δ¹³C values averaging about -26.5‰, while values for C₄ species average about -12.5‰. The bone collagen from herbivores that subsist only on C₃ grasses will give a value for δ¹³C of ca. -21.5‰. If the diet were based only on C₄ grasses then the value would be ca. -7.5‰. If a modern individual consumes both C₃ and C₄ species, the δ¹³C value for its collagen will lie somewhere between -21.5 and -7.5‰. The δ¹³C values of marine plants usually lie in an intermediate position between the values of terrestrial C₃ and C₄ plants, depending on their carbon sources. Therefore, marine fauna usually have carbon isotopic values that are less negative than those of terrestrial animals feeding on C₃-based foods and more negative than those feeding on C₄-based foods. Freshwater aquatic foodwebs appear to have C₃-like carbon isotopic compositions. A small (approximately 0.5-2‰) enrichment for carbon isotopic values occurs in each step of a foodchain. For example, if the δ¹³C value of bone collagen of a herbivore subsisting on C₃ plants provides a value of ca. -21.5‰, then the collagen of a human consuming the meat of that herbivore will give a δ¹³C value of ca. -20.5‰.

The nitrogen stable isotope (δ¹⁵N) analysis identifies the trophic level of an individual. The consumer collagen usually displays δ¹⁵N ratios, which are about 3-5‰ greater than its diet. This fractionation effect results in a ca. 3-5% increase in δ¹⁵N as an individual ascends each step of a foodchain. For example, the δ¹⁵N values of most modern plants are between 0 and 5‰. Therefore, the nitrogen isotopic values of populations that rely on terrestrial animal protein in their diet, would average from around 9‰. The nitrogen levels of a consumer will increase when relying more heavily on aquatic resources. The mixture of both terrestrial and marine (fish and mammals) food resources in the diet would typically reveal δ¹⁵N ratios of around 10‰; the highest δ¹⁵N ratios are usually expected from populations using aquatic sources as a main staple in the diet, because the foodchains in aquatic ecosystems are more extended than those of terrestrial ones. It should be noted that a number of non-dietary factors may influence the carbon and nitrogen isotopic ratios of individuals. These include climatic and canopy effects for δ¹³C ratios and climatic, physiological, crop manuring and nursing effects for δ¹⁵N ratios.

For more detailed information see http://en.wikipedia.org/wiki/Isotope_analysis.

Fig. 1. Simplified summary of stable carbon and nitrogen values for terrestrial and marine ecosystems (values are for flesh; to convert to bone collagen values, 5‰ should be added to δ13C values; δ15N values are the same for flesh and bone). Adapted from Schulting 1998.

Fig. 2. Examples of various human diets (after Schoeninger et al. 1983; Pollard 1993).