Alkali extraction of archaeological and geological charcoal: evidence for diagenetic degradation and formation of humic acids

摘要

Charcoal forms a crucial source of archaeological and palaeoenvironmental data, providing a record of\udcultural activities, past climatic conditions and a means of chronological control via radiocarbon (14C)\uddating. Key to this is the perceived resistance of charcoal to post-depositional alteration, however recent\udresearch has highlighted the possibility for alteration and degradation of charcoal in the environment. An\udimportant aspect of such diagenesis is the potential for addition of exogenous “humic acids’ (HAs), to\udaffect the accuracy of archaeological and palaeoenvironmental reconstructions based upon chemical\udanalyses of HA-containing charcoal. However the release of significant quantities of HA from apparently\udpristine charcoals raises the question whether some HA could be derived via diagenetic alteration of\udcharcoal itself. Here we address this question through comparison of freshly produced charcoal with\udsamples from archaeological and geological sites exposed to environmental conditions for millennia\udusing elemental (C/H/O) and isotopic (δ13C) measurements, Fourier Transform Infrared Spectroscopy\ud(FTIR) and proton Liquid-State Nuclear Magnetic Resonance (1H NMR). The results of analyses show that\udthe presence of highly carboxylated and aromatic alkali-extractable HA in charcoal from depositional\udenvironments can often be attributable to the effects of post-depositional processes, and that these\udsubstances can represent the products of post-depositional diagenetic alteration in charcoal.