Multi-proxy records of wildfires and climate-vegetation-wildfire mechanisms during the Middle Jurassic of the Yili Basin, Northwest China

Abstract

Wildfires are an important source of disturbances in the Earth's system and are of great significance for understanding the interactions between environmental, atmospheric and vegetation changes in deep time. The Early-Middle Jurassic was a fire-prone stratigraphic interval, and the Yili Basin has Middle Jurassic coals that contain high-quality information about palaeowildfires. In this study, a multi-proxy study was undertaken to analyze coal macerals, inertinite reflectance, and polycyclic aromatic hydrocarbons (PAHs) in coal and mudstone samples from the Aalenian coal-forming Yili Basin to characterize palaeowildfire events. In addition, normal alkanes (n-alkanes) were used as biomarkers of alterations in coal-forming vegetation, and natural char was used as complementary coal petrological evidence for palaeowildfires. The presence of high levels of inertinite, high natural char contents, and high abundances of potentially combustion-derived PAHs demonstrate that multiple, widespread wildfires occurred during the Aalenian in the Yili Basin. Inertinite reflectance values ranging from 0.65 % to 3.81 % show that palaeowildfires, based on modern combustion experiments, were dominated by lower temperature ground fires, with a small proportion of surface fires and almost no high temperature crown fires. It is believed that the total amount of PAHs can be used as good evidence of palaeowildfires, but the ratios of high to low ring PAHs in the molecules are not necessarily indicative of combustion temperatures of palaeowildfires due to the high solubility of low-ring PAHs. Based on our findings and integration with previous studies, we elucidate the likely mechanisms of climate-vegetation-wildfire feedback.