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

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 delta C-13 of foxtail and common millets heated to temperatures up to around 250 degrees C. This study reports delta C-13 values of modern millets carbonized at different temperatures. The results indicate that there are no significant changes in delta C-13 of intact and charred samples of foxtail millet (<= 0.46 parts per thousand) and common millet (<= 0.49 parts per thousand) for temperatures below 300 degrees C. The delta C-13 of charred foxtail millet formed at 250 degrees C were 0.2 parts per thousand higher in delta C-13 than the source samples. In contrast, the delta C-13 of charred common millet formed at 250 degrees C were 0.2 parts per thousand lighter in delta C-13 than the source samples. The delta C-13 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 C-13 carbonization associated with fractionation of only 0.2 parts per thousand in grains is much less than the natural variation typically found in wood. We therefore suggest that delta C-13 measured in carbonized grains can serve as an effective indicator for paleoenvironmental and archaeological reconstructions. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.