Stable Carbon And Nitrogen Isotope Analyses On Modern Foxtail Millet And Broomcom Millet Of The Loess Plateau And The Isotope Fractions During Carbonization

This study took66samples of modern foxtail millet and19samples of modern broomcorn millet from the Loess Plateau and its neighboring regions, did stable carbon and nitrogen isotopes analyses on grains and leaves. In the mean time, the grains were charred through different methods to observe the might happening carbon and nitrogen fractionations during the processes. In this study, the following conclusions were obtained:(1) The mean δ13C values for the grain samples of foxtail millet and broomcorn millet on the Loess Plateau are-12.26±0.06‰and-12.76±0.13%‰(σ), respectively. Foxtail millets have relatively positive values. The mean813C values for the leave samples of foxtail millet is-13.06±0.06‰(σ), while for the broomcorn millet, that value is-13.71±0.11‰(σ), the same trend to that of grains. Also, for both millets, leaves have much negative813C values compared to grains. The mean δ15N values for the grain samples of foxtail millet and broomcorn millet are1.14±0.25‰(σ) and1.32±0.39‰(σ), respectively. Leaves have much negative δ15N values compared to grains. The previous believed that the stable isotope results of animals eat the same plants with human beings can stand for the whole population may be wrong, as the animals may eat the different organs of plants those have different isotope signals.(2) After heating for28h in the oven on the metal rack at200℃, there is no significant change of the δ13C and δ15N values for foxtai millet grain samples. After heating for8h in the oven on the base metal plate at200℃, neither the mean value of δ13C or δI5N for foxtail millet changed. Namely, the charred foxtail millet formed under low temperature preserve the original δ13C signal and can be used for palaeoenvironmental reconstruction. Both of the δ13C and δ15N values for broomcorn millet changed after heating at200℃. After combusting for2h in the muffle furnace at600℃, the mean δ13C values for both millets significantly changed.(3) Exclude the impact of environmental factors on δ13C values,δ13C values between different varieties of foxtail millet samples have maximum difference of1.32‰, the variety should be concerned when studying δ13C values of plants. Foxtail millet grain samples of the same species in different grow conditions make maximum δ13C value difference of0.96‰. Both the environmental factors and genetic diversity determine the δ13C variety of millets.(4) There are some sort of linear relationships between δ13C values and δ15N values of the millets, which might be used to assist identifying foxtail millet and broomcorn millet.(5) When precipitation decline, the δ13C values of the grain samples for foxtail millet turn negative, while the δ15N values turn positive. Spatially, from southeast to northwest of the Loess Plateau, the δ13C values of the foxtail millet grains increase while the δ15N values decrease. Similar law is not seen among the broomcorn millet.