Chemical degradation of thermally altered silicified organic matter during acid maceration: a case study from the Lower Devonian Rhynie chert

Abstract The effect of standard acid maceration on organic matter (OM) from ancient silicified sediments remains undocumented. Early silicification favours preservation of organic moieties against thermal alteration over time. In this study, we investigated the effects of acid maceration on the structure of OM isolated from the Lower Devonian Rhynie chert. The structure of OM was investigated by combining Rock-Eval pyrolysis and Raman spectroscopy. Besides a loss of thermolabile organic matter owing to solvent extraction, Rock-Eval pyrolysis showed that standard acid maceration also causes a loss of C-H emissions at high pyrolysis temperature (> 500 °C). The standard acid maceration procedure was also associated with the disappearance of the D4 and D5 Raman spectrum shoulders assigned to C–H bonds in aliphatics and bitumens, respectively, entrapped in the macromolecular network. Taken together, Rock-Eval pyrolysis and Raman spectroscopy indicate that standard acid maceration can lead to the chemical degradation of syngenetic hydrocarbonaceous moieties of OM isolated from ancient silicified and thermally altered sediments. In sediments having experienced early silicification, which hampers bitumen migration and favours pyrobitumen formation, we suggest that novel in situ molecular analytical techniques are required to provide a thorough examination of the syngenetic molecular content independent of the soluble/insoluble operational definition.