Polycyclic aromatic hydrocarbons in Middle Jurassic sediments of the Polish Basin provide evidence for high-temperature palaeo-wildfires

Abstract

The distributions and concentrations of polycyclic aromatic hydrocarbons (PAHs) and plant-derived aromatic biomarkers were analysed in Middle Jurassic strata from central Poland. These compounds were abundant in all samples indicating the occurrence of specific palaeoflora families and the intensity of palaeo-wildfires. Moreover, calculated fusinite reflectance values revealed that the temperature of Middle Jurassic wildfires may have exceeded 950 °C, consistent with high oxygen concentrations in the atmosphere during this period. The high content of charred organic matter has an influence on certain biomarker maturity indicators based on terrestrial organic matter such as C29 20S/(S + R). A new indicator was developed, benzo[ghi]perylene/(perylene + benzo[ghi]perylene), abbreviated as BgP/(Per + BgP, high values of which are characteristic of samples in which terrestrial organic matter is mostly charred. The use of this indicator is limited to immature organic matter, since perylene disappears when vitrinite reflectance is higher than 0.7 %Ro. Measurements of vitrinite reflectance indicated that Middle Jurassic organic matter with a reflectance value < 0.51 %Ro was immature or early-mature. Juxtaposition of the modified higher plant index (mHPI) calculated from mHPI = (retene + cadalene + ip-iHMN)/(retene + cadalene + ip-iHMN + 1,3,6,7-TeMN) and BgP/(Per + BgP) showed that samples from the central part of the Polish Basin contained more charred than unburnt terrestrial organic matter, as values of BgP/(Per + BgP) > 0.5 are correlated with lower mHPI values. This is evidence for the transport of wildfire products by rivers, wind, and/or sea currents over large distances. Tracers for conifers, non-vascular plants, and fungi were identified.