Charring Effects on Stable Carbon and Nitrogen Isotope Values on C4 Plants: Inferences for Archaeological Investigations

Abstract

Experimental studies demonstrated that charring affects stable isotope values of plant remains. Therefore, it is necessary to consider the impact of charring to reliably interpret δ13C and δ15N values in archaeobotanical remains before using this approach to reconstruct past water management, paleoclimatic changes, and infer paleodietary patterns. Research so far has focused mostly on C3 plants while the charring effect on C4 plants is less understood. This study explored the effects of charring on δ13C, δ15N, %C, %N, and C:N in grains of two C4 species, Sorghum bicolor (L.) Moench (NADP-ME) and Cenchrus americanus (L.) Morrone (heterotypic synonym Pennisetum glaucum (L.) R.Br.) (NAD-ME), grown under the same controlled environmental conditions (watering, light, atmospheric humidity). Sorghum and pearl millet grains were charred from 1 to 3 h at 200 to 300°C. Comparing first the uncharred grains, the results show that sorghum has lower δ15N and higher δ13C than pearl millet. This evidence is also recorded in the charred seeds. The charring experiments indicate that the temperature to which the grains are exposed has a higher impact than time on the preservation, mass loss, %C, %N, C:N, and δ13C and δ15N values. Every 50°C of increase resulted in significant increases of δ15N (+0.37‰) and of δ13C (+0.06‰). Increasing the duration of charring to 3 h resulted in no significant changes of δ15N (+0.17‰) and (δ13C) -0.04‰. The charring effects recorded between the uncharred and charred seeds span from -0.66‰ to 1.41‰ for δ15N with an average of +0.27‰, and from -0.81‰ to 0.17‰ with an average of -0.18‰ for δ13C. Considering the average values, this charring experiment shows that pearl millet is more affected by charring than sorghum; however, according to the standard variations, sorghum shows a greater variability charring effect than pearl millet. This study provides new information to correctly assessing the isotopic values obtained from ancient C4 crops, providing, for the first time, a charring offset specific for C4 plants. Furthermore, it suggests that NAD-ME and NADP-ME species present isotopic differences under the same growing conditions and this must be taken into account in analytical works on ancient C4 crops.

Keywords: CharringC4 plantsexperimental cultivationstable isotope analysisarchaeobotany