Molecular characterization of charcoal to identify adsorbed SOM and assess the effectiveness of common SOM-removing pretreatments prior to radiocarbon dating

A significant part of our knowledge on Holocene landscape development and associated human history in northwestern Europe is based on radiocarbon dating of charcoal originating from podzols. However, these soils are characterized by leaching of soil organic matter (SOM) that may adsorb to older charcoal, resulting in incorrect charcoal radiocarbon ages. Several pretreatments, such as an acid-base-acid (ABA) sequence for Holocene-aged samples and a harsher acid-base-oxidation (ABOx) sequence for older samples, have been developed to remove the adsorbed fractions. While commonly used, evidence for the effectiveness of these pretreatments thus far has been circumstantial, by assessing the impact of these pretreatments on the radiocarbon age. In this study, we performed a direct molecular assessment of the presence, adsorption, and removal of SOM-compounds by pyrolysis-GC-MS based characterization of charcoal fragments and SOM obtained from a podzol located in the Laarder Wasmeren area, the Netherlands. In addition, the removal efficiency of adsorbed SOM by four different pretreatments was analysed. Molecular evidence for significant adsorption of SOM was found and the four pretreatments showed different efficiencies in removal thereof as evidenced by the residual charcoal composition. ABOx resulted in the most simplified Py-GC-MS chromatogram and thus comes closest to the isolation of recalcitrant carbon. The most recalcitrant organic fraction is likely to have the best interpretable age, since the highest decontamination can be expected for this fraction. Hence, it is advisable to use ABOx as standard pretreatment for the removal of adsorbed SOM fractions from charcoal, provided that the sample size is large enough