Preservation of stable isotopic composition in charred grains: Implications for paleoenvironmental and archeological research

Abstract

The charring process can preserve archaeobotanical remains, providing valuable insights into past climates, agricultural practices, and plant growth conditions. However, the impact of charring on stable isotopes, especially at temperatures above 300°C, remains poorly understood. To investigate this, wheat (einkorn, emmer) and millet grains were experimentally charred at 300°C, 450°C, and 550°C for 15 min to 48 h under aerobic and anaerobic conditions. Consequently, the stable isotopic ratios of carbon (¹³C/¹²C; δ¹³C), nitrogen (¹⁵N/¹⁴N; δ¹⁵N), and oxygen (¹⁸O/¹⁶O; δ¹⁸O) were determined in fully charred grains with a blackened pericarp, visually comparable to archeological remains. Our results indicate that δ¹³C values are well preserved in charred grains, showing minimal shifts (<0.55‰ for wheat and <0.2‰ for millet), while δ¹⁵N values retained species-specific differences, with moderate variations (<1.31‰ for wheat and <0.68‰ for millet) influenced by charring conditions. In contrast, δ¹⁸O values showed high variability in experimentally charred grains, limiting their reliability for interpretative use. These findings underscore the potential of carbon and, to a lesser extent, nitrogen isotopes in charred grains, for reconstructing ancient growth conditions and improving the understanding of formation processes in archeological research.