Charcoal reflectance measurements: implications for structural characterization and assessment of diagenetic alteration

Charcoal is a valuable source of archaeological and palaeoenvironmental proxy data. However growing evidence suggests that production conditions can strongly influence post-depositional alteration of charcoal. Consequently, both reconstruction of production temperature and understanding of the potential for diagenetic alteration are of great interest. Here, we use mean random reflectance (Romean) in conjunction with other chemical characterization methods to address these questions. Romean was obtained for a suite of modern analogue charcoal, produced under controlled conditions, and for a series of natural charcoal samples, obtained from archaeological and palaeoenvironmental deposits. Romean proves to be a robust measure to assess formation temperature for samples produced at 400 °C and above, even after exposure to highly oxidizing conditions. Romean is also useful for samples formed between 300 °C and 400 °C. However, if an assemblage of charcoals has been exposed to oxidizing conditions, lower temperature charcoals may be preferentially lost. It is apparent that charcoal produced at lower temperatures is more highly susceptible to chemical oxidation, and that there is a continuum in charcoal degradation potential, dependant upon fuel material and production conditions.