Theoretical and observed C/N ratios in human bone collagen

The composition of the diet of ancient humans is commonly estimated by measuring the δ13C and δ15N ratios of collagen extracted from human bone. Because bone has been subjected to diagenesis during burial it is possible that its isotopic composition has been altered. DeNiro (1985) showed that if collagen has an atomic C/N ratio outside the interval 2.9 to 3.6, then it may show aberrant isotopic values. Here we show that the theoretical ideal C/N ratio of human collagen is at the midpoint of that interval (3.243). We also note that most measured ratios lie very close to that value. Deviations from that value due either to alteration of collagen or admixture of contaminant organic molecules are likely to shift the δ values away from their initial values. Values of C/N higher than 3.243 are likely to be due to contamination with humic matter while lower values might be due to selective loss of amino acids.