Carbon isotope fractionation during low temperature carbonization of foxtail and common millets

Abstract

Stable carbon isotopes of organic matter and fossilized plant remains can be used to effectively reconstruct local palaeoclimate changes, especially from plants using a single photosynthetic mode. The charred grains of foxtail and common millet are chemically stable in the environment and have been preserved widely and continuously throughout the Holocene in North China. The charred remains of these species are ideal materials for reconstructing the palaeoclimate based on δ13C of foxtail and common millets heated to temperatures up to around 250°C. This study reports δ13C values of modern millets carbonized at different temperatures. The results indicate that there are no significant changes in δ13C of intact and charred samples of foxtail millet (⩽0.46‰) and common millet (⩽0.49‰) for temperatures below 300°C. The δ13C of charred foxtail millet formed at 250°C were 0.2‰ higher in δ13C than the source samples. In contrast, the δ13C of charred common millet formed at 250°C were 0.2‰ lighter in δ13C than the source samples. The δ13C values of grains were determined in part by the carbon content (i.e., starches, lignins and lipids) and the variable thermal tolerances of these compounds to heating. However, the observed 13C carbonization associated with fractionation of only 0.2‰ in grains is much less than the natural variation typically found in wood. We therefore suggest that δ13C measured in carbonized grains can serve as an effective indicator for paleoenvironmental and archaeological reconstructions.