Dietary diversity of Bronze-Iron Age populations of Kazakhstan quantitatively estimated through the compound-specific nitrogen analysis of amino acids

Abstract

The identification of which food sources were important to the diets of Central Asian populations during the Bronze and Iron Age has been heavily debated over the past decade. Stable isotope studies of human bulk collagen have often reported elevated nitrogen isotopic compositions (δ15N) that were more than one step higher in the food chain than consumed terrestrial animals, suggesting the additional contribution of fish to human diets. On the other hand, both fish and C4 plants, possibly millets, could have influenced the elevated carbon isotopic compositions (δ13C) in human bone collagen. Here we report the results of the first compound-specific nitrogen isotopic analysis of amino acids conducted on human and faunal remains from Bronze and Iron Age sites of Kazakhstan. In this paper, we examine the possible role of millet in leading to high δ15N values of human bone collagen, in addition to fish. Typically, high δ15N bulk values in humans, are linked to the consumption of particular resources such as fish and C4 plants, or extreme carnivorous diets. However, the δ15N of individual amino acids of the populations of the Kazakh steppe demonstrate trophic positions lower than what we expect in bulk collagen irrespective of δ15N and δ13C. Besides, hardly any evidence of fish consumption is identified by also the δ15N of phenylalanine. The high δ15N in human bulk collagen which obscure reconstruction of prehistoric diets may be coming from complex and diverse factors such as aridity or other dietary factors beyond fish consumption.