Paleoclimate-induced wildfires in a paleomire in the Ordos Basin, Northern China during the Middle Jurassic greenhouse period

Paleo-wildfires are of great significance in the study of paleomire ecosystems. The products of such fires preserved in paleomires can provide detailed information for the reconstruction of wildfire regimes. This study analyzed the distribution of charcoal and aromatic biomarkers in a coal seam from the Middle Jurassic Yan'an Formation in the Ordos Basin, China. The results showed very high content of pyrogenic inertinite (38%-62%) and common occurrence of pyrogenic polycyclic aromatic hydrocarbons (PAHs) in the studied coal seam, suggesting that wildfires occurred frequently during the period of peat accumulation. The mean reflectance of pyrogenic inertinite (1.50%) in all samples indicates a dominance of low-temperature wildfires in the paleomire, which is also supported by the high abundance of 2- to 3-ring PAHs. Furthermore, the common observations of high-reflectance pyrogenic inertinite (similar to 4.47%RIo) and 4- to 6-ring PAHs suggest the repeated occurrence of high-temperature wildfires. Various parameters of 3- to 4-ring PAHs indicate a relatively high proportion of charred material contributing to the organic matter in the paleomire. The dimethylphenanthrene ratios of the studied samples were inconsistent with those of modern experimental results, which can be probably attributed to the non-analogous vegetation composition in the Jurassic paleomire, and caution should thus be exercised when applying dimethylphenanthrene ratios to ancient sediments. The ratio of retene to 3-ring PAHs implies the contribution of conifers to the fires in the paleomire. The pyrogenic PAHs originated from both, combustion residues and smokes produced by in situ and local wildfires. The high O-2 level during the Middle Jurassic promoted the frequent occurrence of wildfires in the studied paleomire. In addition, the occurrence of wildfires was collectively affected by the wet/humid climates, seasonality, and depositional environment.