Record of Middle Jurassic wildfire and its incidental mercury emissions in northern Qaidam Basin, China: Evidence from the inertinite and mercury anomalies in coal

Abstract

Inertinite in coals contains important information regarding the palaeo-wildfire history and can provide insights into its triggers and relationship with the evolution of local ecosystems. However, such information in the Middle Jurassic coal is often overlooked, and the impacts of incidental mercury emission from wildfires on the mercury enrichment in coal are also underestimation. In this study, the organic petrology, geochemistry and palynology of coal seams from the coal-bearing member of Shimengou Formation in northern Qaidam Basin (NQB) were investigated. Results show that these coal seams generally contain abundant records of palaeo-wildfire: inertinite and combustion-derived polycyclic aromatic hydrocarbons (PAHs). The high consistency of these two evidences confirms the existence of widespread wildfires during the late Middle Jurassic (Bathonian) in NQB. These widespread wildfires also indicate a high level of atmospheric oxygen concentration during this period. The overall low inertinite reflectance (avg. 1.27%) and low content of coronene in coal seams indicate that these wildfires were dominated by ground and surface fire with some small-scale crown fires. These frequent wildfires under the overall humid climate are attributed to the sufficient atmospheric oxygen, fuel and intermittent dry season. The co-occurrence of wildfires records and mercury anomaly in coal seams suggests that incidental mercury emissions from wildfires enhanced mercury input in the peatlands, which is the direct trigger for mercury anomaly in the coal from the Shimengou Formation in NQB. This study not only revealed the characteristics and causes of the widespread wildfires during the late Middle Jurassic in NQB, but also provides a new insight into the abnormal enrichment of mercury in coal.