Disentangling the multi-phase alteration of fossil charcoals: A cautionary tale from the Isle of Mull, Scotland

Fossil charcoal material offers quantitative insights into prehistoric fire behaviour and ecosystem disturbance through applications of geothermometry. Such methods rely on the analysis of chemical changes within the carbonaceous microstructure of charcoal – corresponding to temperature of formation, or ‘pyrolysis intensity’. Of the techniques commonly applied, Raman spectroscopy has offered insights into the relationships between thermal, structural, and chemical aspects of organic carbon under thermal maturation. This includes broader considerations of energy flux in fire events. The reliance upon material chemistry, however, suggests a susceptibility to additional chemical and thermal alterations that have otherwise been underexplored. Our Raman spectroscopic investigation of palaeocharcoal from the Isle of Mull (Scotland), entrained within a lahar deposit that has undergone intrusion, reveals geothermometric discrepancies and spectral features consistent with microstructural modification. These include a pronounced blueshift of the G-band, carbonyl bonding, prominent defect functions, and a significant reduction in G-band width. The disparity in presentation, however, suggests a more complex origin. We attribute these spectral features to a co-occurrence of oxidative weathering and progressive maturation during intrusion emplacement. Considerable questions are therefore raised as to the accuracy of existing studies in organic carbon geothermometry, particularly in the study of palaeofire. This potentially limits our understanding of fire intensification under climatic warming – past, present, and future. • Fossil charcoals show evidence of thermal and oxidative alteration in tandem. • Deviations in typical Raman spectral geometry reveal sources of alteration. • Igneous intrusion presents a natural source of thermal alteration in charcoal. • The alteration of organic carbon is not dependent upon thermometric history. • Context remains vital in the study of organic carbon via Raman spectroscopy.