Impact of charring on cereal grain characteristics: linking prehistoric manuring practice to δ~(15)N signatures in archaeobotanical material

摘要

Systematic use of animal manure has been demonstrated to be detectable in the plant δ~(15)N value but evidence of manure affecting isotopic composition is mainly based on studies of fresh plant material. These findings can potentially be applied to archaeobotanical assemblages and thus provide information about prehistoric manuring practice. Prehistoric grains are generally found in a charred state of which the exact charring conditions are unknown but most likely often multifarious. In this study we examined the influence of grain weight and a range of charring conditions with regards duration, temperature, oxygen availability, and manuring. The charring was applied to emmer, spelt and naked barley with assessment of weight loss, N concentration and δ~(15)N. There were only small and non-systematic variations in δ~(15)N in relation to grain weight class. We also found that charring did not distort δ~(15)N in either a systematic or substantial way and conclude that manuring most likely will be detectable in archaeobotanical charred grains. As certain within-grain variability in δ~(15)N existed, especially in the intensively manured grains, the resolution of this kind of information should be carefully considered. However, despite attempts to deliberately tamper and distort the grain δ~(15)N signature, the changes observed in this study were too small to be of any consequences for the archaeobotanical applicability of the method. Thus the isotope method offers unique evidence about prehistoric manuring practice.