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

The aim of this study is to assess the potential of charred archaeobotanical cereal grain and pulse seed delta C-13 and delta N-15 values to provide evidence of crop growing conditions and as a potential component of palaeodietary studies. In order to reliably interpret archaeobotanical delta C-13 and delta N-15 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 delta C-13, delta N-15, %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 delta C-13 and delta N-15 values. Heating at relatively low temperatures and for prolonged times (230 degrees C for up to 24 h) is conducive to the formation of well preserved, undistorted charred cereal grain and pulse seed. Heating for 24 h has a systematic and predictable effect on delta N-15 values, with increases of around 1 parts per thousand on average in cereal grains and pulse seeds, and no consistent impact on delta C-13 values. Increases in delta N-15 are likely due to the loss of lighter N-14 via N-containing volatiles. Burial (for up to 2 years) and ABA pre-treatment have no significant effects on delta C-13 or delta N-15 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. (C) 2013 Elsevier Ltd. All rights reserved.