Resolving the bulk delta N-15 values of ancient human and animal bone collagen via compound-specific nitrogen isotope analysis of constituent amino acids

Stable nitrogen isotope analysis is a fundamental tool in assessing dietary preferences and trophic positions within contemporary and ancient ecosystems. In order to assess more fully the dietary contributions to human tissue isotope values, a greater understanding of the complex biochemical and physiological factors which underpin bulk collagen delta N-15 values is necessary. Determinations of delta N-15 values of the individual amino acids which constitute bone collagen are necessary to unravel these relationships, since different amino acids display different delta N-15 values according to their biosynthetic origins. A range of collagen isolates from archaeological faunal and human bone (n = 12 and 11, respectively), representing a spectrum of terrestrial and marine protein origins and diets, were selected from coastal and near-coastal sites at the south-western tip of Africa. The collagens were hydrolysed and delta N-15 values of their constituent amino acids determined as N-acetylmethyl esters (NACME) via gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). The analytical approach employed accounts for 56% of bone collagen nitrogen. Reconstruction of bulk bone collagen delta N-15 values reveals a 2 parts per thousand offset from bulk collagen delta N-15 values which is attributable to the delta N-15 value of the amino acids which cannot currently be determined by GC-C-IRMS, notably arginine which comprises 53% of the nitrogen unaccounted for (23% of the total nitrogen). The delta N-15 values of individual amino acids provide insights into both the contributions of various amino acids to the bulk delta N-15 value of collagen and the factors influencing trophic position and the nitrogen source at the base of the food web. The similarity in the delta N-15 values of alanine, glutamate, proline and hydroxyproline reflects the common origin of their amino groups from glutamate. The depletion in the delta N-15 value of threonine with increasing trophic level indicates a fundamental difference between the biosynthetic pathway of threonine and the other amino acids. The delta N-15 value of phenylalanine does not change significantly with trophic level, reflecting its conservative nature as an essential amino acid, and thus represents the isotopic composition of the nitrogen at the base of the food web. Delta N-15(Glu-Phe) values in particular are shown to reflect trophic level nitrogen sources within a food web. In relation to the reconstruction of ancient human diet the contribution of marine and terrestrial protein are strongly reflected in Delta N-15(Glu-Phe), values. Differences in nitrogen metabolism are also shown to have an influence upon individual amino acid delta N-15 values with Delta N-15(Glu-Phe) values emphasising differences between the different physiological adaptations. The latter is demonstrated in tortoises, which can excrete nitrogen in the form of uric acid and urea and display negative Delta N-15(Glu-Phe) values whereas those for marine and terrestrial mammals are positive. The findings amplify the potential advantages of compound-specific nitrogen isotope analysis in the study of nitrogen flow within food webs and in the reconstruction of past human diets. (C) 2009 Elsevier Ltd. All rights reserved.