Assessing natural variation and the effects of charring, burial and pre-treatment on the stable carbon and nitrogen isotope values of archaeobotanical cereals and pulses

摘要

The aim of this study is to assess the potential of charred archaeobotanical cereal grain and pulse seed δ~(13)C and δ~(15)N values to provide evidence of crop growing conditions and as a potential component of palaeodietary studies. In order to reliably interpret archaeobotanical δ~(13)C and δ~(15)N values it is necessary to take into account the impact of charring, burial and laboratory pre-treatment procedures. We examine the effects of charring and burial on bulk δ~(13)C, δ~(15)N, %C, %N and C:N ratios in modern cereal and pulse material, and of cleaning by acid-base-acid (ABA) pre-treatment on modern and archaeobotanical charred material. Our study utilised bulk grain and seed samples to help account for within-ear/pod and between-plant variability in δ~(13)C and δ~(15)N values. Heating at relatively low temperatures and for prolonged times (230°C for up to 24h) is conducive to the formation of well preserved, undistorted charred cereal grain and pulse seed. Heating for 24h has a systematic and predictable effect on δ~(15)N values, with increases of around 1‰ on average in cereal grains and pulse seeds, and no consistent impact on δ~(13)C values. Increases in δ~(15)N are likely due to the loss of lighter 14N via N-containing volatiles. Burial (for up to 2 years) and ABA pre-treatment have no significant effects on δ~(13)C or δ~(15)N values. After pre-treatment, however, the %C and %N contents of the archaeobotanical material more closely resembles that of the modern charred grains and seeds, suggesting that archaeobotanical remains accumulate non-structural material during burial but retain their original carbon and nitrogen content. Therefore %C, %N contents and C:N ratios can provide useful criteria for assessing archaeobotanical preservation.