A novel marine dietary indicator utilising compound-specific bone collagen amino acid δ13C values of ancient humans

The application of bone collagen stable carbon and nitrogen isotope analysis to human palaeodietary reconstruction in tropical or arid regions is limited by two factors: (i) the overlap in C4 and high marine protein (HMP) consumer bulk collagen δ13C values, and (ii) the unpredictability of bulk collagen δ15N values in regions of extreme aridity (<400 mm of rain per annum). Hence, the identification of HMP consumption among archaeological human populations can be problematic. In an endeavour to identify a more precise marine palaeodietary indicator, a range of collagen samples from archaeological faunal and human bone (n=14 and 26, respectively), representing a spectrum of C3, C4 and HMP diets, were selected from coastal and near-coastal sites in the Western Cape, South Africa. Samples were subjected to compound-specific stable carbon isotope analysis of their constituent amino acids as trifluoroacetyl-isopropyl (TFA-IP) esters via gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). While human C4 and HMP consumers were indistinguishable with respect to bulk collagen δ13C values (−10.9±3.7‰ and −11.7±1.5‰, respectively) they were shown to be readily distinguished based on Δ13CGlycine-Phenylalanine values (+4.0±1.6‰ and +12.0±1.9‰, respectively). The relationship between HMP consumption and elevated Δ13CGlycine-Phenylalanine values was verified by: (i) the similarly elevated values exhibited by marine species when compared to terrestrial faunal species (+12.5±0.9‰ and +3.2±4.2‰, respectively), and (ii) the strong correlation observed between human Δ13CGlycine-Phenylalanine and bulk collagen δ15N values (R2=0.83, p<0.001; n=26), the latter being a well-documented marine dietary indicator. It was concluded that Δ13CGlycine-Phenylalanine values offer considerable potential as indicators of HMP consumption and a valuable substitute for bone collagen δ15N values in arid regions where bulk δ15N values are unpredictable.