Abstract
An established method of estimating the trophic level of an organism is through stable isotope analysis of its tissues and those of its diet. This method has been used in archaeology to reconstruct past human diet from the stable nitrogen isotope (δ15N) values of human and herbivore bone collagen. However, this approach, using the 15N-enrichment of human bone collagen δ15N values over associated herbivore bone collagen δ15N values to predict the relative importance of animal protein, relies on the assumptions that: (i) the δ15N values of plants consumed by humans and herbivores are identical, and (ii) the 15N-enrichment between diet and consumer is consistent. Bone collagen amino acid δ15N values have the potential to tackle these uncertainties, as they constrain the factors influencing bone collagen δ15N values. In this study, the δ15N values of glutamic acid and phenylalanine in human and herbivore bone collagen isolates from Neolithic sites in Germany, Greece and Turkey were determined by gas chromatography-combustion-isotope ratio mass spectrometry. The fraction of animal protein in total dietary protein consumed by the humans was estimated by: (i) comparing bulk human and herbivore collagen δ15N values, (ii) comparing bulk human and herbivore collagen and ancient charred cereal grain δ15N values, (iii) comparing human bone collagen δ15NGlutamic acid and δ15NPhenylalanine values, and (iv) comparing δ15NGlutamic acid values of human and herbivore bone collagen and estimated δ15NGlutamic acid values of ancient charred cereal grains. Where determined cereal grain δ15N values are higher than estimated herbivore forage values, estimates of animal protein consumption are significantly lower, emphasising the importance of the plant nitrogen contribution to human bone collagen. This study also highlights the need for further investigation into: (i) the Δ15NConsumer-Diet values of glutamic acid and phenylalanine in terrestrial ecosystems, and (ii) Δ15NGlutamic acid-Phenylalanine values of common plant foods in order to improve the accuracy and more widespread applicability of amino acid-based methods for palaeodietary reconstruction.
Highlights
Equations have been developed to estimate the trophic level of humans and fauna in aquatic, C3-plant-based and C4-plant-based ecosystems based on the fact that the d15N values of the amino acids glutamic acid (Glu) and phenylalanine (Phe) increase to different extents with trophic level (8 and 0.4‰ respectively; Chikaraishi et al, 2011, 2010, 2009, 2007; Steffan et al, 2013)
The benefit of this approach is that the d15N values of Glu (d15NGlu) and Phe (d15NPhe) provide an internal trophic level indicator, precluding the need to rely upon the bone collagen d15N values of preserved fauna, whose tissues may not have contributed to the human diet
We have used the d15N values of bone collagen, bone collagen amino acids, plant protein and plant protein amino acids in four different palaeodietary models to calculate the proportion of animal protein in human diet at three different archaeological sites
Summary
Equations have been developed to estimate the trophic level of humans and fauna in aquatic, C3-plant-based and C4-plant-based ecosystems based on the fact that the d15N values of the amino acids glutamic acid (Glu) and phenylalanine (Phe) increase to different extents with trophic level (8 and 0.4‰ respectively; Chikaraishi et al, 2011, 2010, 2009, 2007; Steffan et al, 2013) The benefit of this approach is that the d15N values of Glu (d15NGlu) and Phe (d15NPhe) provide an internal trophic level indicator, precluding the need to rely upon the bone collagen d15N values of preserved fauna, whose tissues may not have contributed to the human diet
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