Short-term modifications in the chemical structure of wood charcoals: Implications for anthracological investigations

Archaeological wood charcoals offer a unique window to study paleoclimates through their ¹³C isotope composition (δ¹³C) provided that (i) past heating temperatures are assessed to correct δ¹³C modifications related to carbonisation, and (ii) they have not been modified by post-depositional processes occurring in soil after their formation. Our goal was to assess how post-depositional processes can modify (i) the determination of past heating temperatures by Raman thermometry and (ii) the δ¹³C signature of charcoals, notably through the occurrence of exogenous organic matter (OM). To this end, short-term post-depositional processes were simulated on oak and pine charcoals − produced at 400 and 600 °C − incubated in vermicompost for 6 months. While almost all the studied charcoals showed no evidence of the occurrence of exogenous OM, pine charcoal produced at 400 °C appeared to be subjected to an organic coating with incubation time. This organic coating may have led to a decrease in the H_D/H_G ratio, a proxy of carbonisation temperatures obtained from Raman spectra. In contrast, the H_D/H_G ratio increased in oak charcoal produced at 400 and 600 °C at certain incubation times. Although the processes behind these modifications remain unclear, this investigation highlights that the Raman thermometer may be biased when assessing the heating temperatures of archaeological charcoals. In addition, subtle modifications in the chemistry of pine charcoal produced at 400 °C were enough to yield an increase in δ¹³C by up to 0.3 ‰ at the short-term scale of our incubation experiments. This study suggests that long-term experiments are necessary to assess the effect of post-depositional processes on the chemical structure of charcoals and to provide a valuable framework to minimize potential biases in the use of Raman thermometry and of the δ¹³C signature.