Variability of the stable carbon isotope ratio in modern and archaeological millets: evidence from northern China

Stable carbon isotopic analyses of human skeletal remains may provide fundamental evidence for human dietary reconstruction and subsistence strategies. Millet is closely associated with the emergence and development of agriculture-based societies in northern China. Although often overlooked, baseline values of millet seeds are essential for using stable isotope analysis to understand past human and animal diets. Here, we report spatial and temporal variations in the δ13C values of millets by analyzing modern samples, including seeds and leaves, as well as archaeological samples. The δ13C values of modern foxtail millet seeds range from −13.9 to −11.3‰, with a mean value of −12.3 ± 0.5‰ (1σ, n = 66), while δ13C values for modern common millet seeds vary between −14.3 and −12.0‰, with a mean value of −12.8 ± 0.6‰ (1σ, n = 19). There is an approximately 1‰ temporal change in δ13C for millet grains. Leaves have lower δ13C values than grains, implying that eaters living on different tissues of the same plant could show different isotopic values. These background δ13C values must be considered when reconstructing the dietary history of a millet-based society.