Fire activities and their impacts on local ecosystems in the southern Ordos Basin during the Middle Jurassic: Evidence from pyrogenic PAHs and petrography of inertinite-rich coal

Abstract

The high frequency of fires during the formation of some mire systems in the Jurassic is thought to have influenced how terrestrial ecosystems evolved. Ancient mires, now present as coal seams, contain high-resolution paleoenvironmental and paleoclimatic records. Thus, the analysis of coal beds provides valuable, insightful, and practical knowledge on the changes in paleoclimate and, in this case, fire prevalence. Using petrographic, carbon isotopic, mercury content, and polycyclic aromatic hydrocarbon (PAH) determinations, the Middle Jurassic No. 4 coal from the Binchang region in the Ordos Basin was examined in order to understand the nature of fire during the formation of the mires. The results show that the No. 4 coal exhibits a high content of inertinite (average content 70%). Notably, the correlation coefficient (r = 0.79) between the concentration of PAHs (with >3 rings) and the proportion of inertinite indicates that the high content of inertinite was predominantly the result of fire. The presence of natural char and coke in the coals further indicates the frequent incidence of fires during the Middle Jurassic in the southern Ordos Basin. High frequency of fires can be attributed to the availability of fuel (i.e., the buildup of organic material), high atmospheric oxygen levels, and intermittent dry climatic conditions. The random reflectance of inertinite (specifically, fusinite and semifusinite) in the No. 4 coal bed ranges from 1.25% to 2.45%, which, with the presence of pyrogenic PAHs, suggests that their formation may have been predominantly affected by low- and moderate-temperature fires. The high frequency of fires is thought to have significant impacts on terrestrial and hydrological ecosystems. These effects may have included the circulation of atmospheric mercury, as well as the possible occurrence of anoxic events in the lakes.