‘Vampirious gamechanger’ — photodegradation changes the interpretation of the origin of polycyclic aromatic hydrocarbons in geological samples

Despite minor sources being identified in phytoplankton and as a result of diagenetic processes, polycyclic aromatic hydrocarbons (PAHs) are well-established compounds, which are frequently used as combustion markers. The characteristic distributions of certain PAHs illustrated in several diagnostic plots are used to differentiate between pyrogenic and non-pyrogenic PAHs. These plots do not show the influence of photodegradation, which can hinder the proper interpretation of the origin of PAHs. Differences of up to three weeks in how PAHs resist photodegradation can change their distribution, leading to the coexistence of typically pyrogenic and non-pyrogenic indications. Despite its undoubted role, photodegradation was not discussed as a factor in modelling the distribution of PAHs within geological samples. The results of extensive molecular (GC–MS) and petrological surveys show that the analysed Early-Middle Jurassic (J 1 -J 2 ) and Early Cretaceous (Cr 1 ) strata contain broad (2- to 7-ring) PAHs distributions, suggesting their pyrogenic origin, which is accompanied by the domination of inertinite group macerals. J 1 -J 2 and Cr 1 strata are differentiated by the role of smoke-derived PAHs. LMW/Total ratio shows that smoke-derived PAHs are more common within the Cr 1 strata. Simultaneously, most Cr 1 strata are characterised by an unobvious distribution of 4-ring PAHs showing the coexistence of typically pyrogenic and non-pyrogenic values of the indicators used. The newly proposed fire temperature index shows that the average paleo-wildfire temperature was 273–425 °C with no differentiation between J 1 -J 2 and Cr 1 strata. This means that photodegradation is an important factor controlling 4-ring PAHs distribution and that unobvious PAHs distribution can be interpreted as a signal from a significant amount of photodegraded, smoke-derived PAHs. Conducted analyses suggest that 5–7 ring PAHs were not photodegraded in a significant way. • Geological polycyclic aromatic hydrocarbons (PAHs) were affected by photodegradation. • Photodegradation significantly altered the distribution of 4-ring PAHs. • Smoke-derived PAHs are more vulnerable to photodegradation processes. • An upgraded diagnostic plot defining the origin of 4-ring PAHs was proposed. • Wildfire temperature is the second main factor controlling 4-ring PAHs distribution.