New insights into the geochemistry of charcoals produced by modern wildfires: Implications for palaeowildfire studies

Wildfires, as a vital component of many terrestrial ecosystems, play a crucial role in vegetation succession and climate change. Charcoal records from modern wildfires can provide a unique research perspective for understanding deep-time wildfire events and accurate reconstruction of palaeowildfire. In this study, charcoal residuals, originating from a natural stand of Larix gmelinii , were collected from modern natural wildfires in the Greater Khingan Mountains area (Heilongjiang Province, China), and the botanical characteristics, reflectance characteristics, biomarker compositions, and stable carbon isotope of samples with varying degrees of charring were systematically analyzed. The results showed that with increasing degree of charring of the samples, the cell walls of charcoal gradually homogenized, and the thickness continuously decreased until the cell walls became mechanically disintegrated. These observations provide an important reference for studying the formation mechanisms of fusinite and inertodetrinite in coal. Specifically, the higher the degree of charring of charcoal, the higher reflectance values have been detected. However, the reflectance of different cell components showed a differential increase, with a discontinuous jump during the combustion process. The types, abundances, and contents of pyrogenic polycyclic aromatic hydrocarbons (PAHs) all increased with increasing charring degree, and there is a trend for aromatic ring numbers to shift from being dominated by two-ring compounds to three-ring and higher-ring compounds. The new findings show that the charring temperature has a significant control effect on a variety of PAH parameters used to distinguish combustion characteristics and fuel sources, and will affects the determination of combustion characteristics of deep-time wildfires using PAH parameters. These new findings from a multidimensional study of modern wildfire charcoal based on different degrees of combustion are of great significance for the study of coal geology and evidence of deep-time wildfires, and can provide more accurate evidence for the interpretation of the residues of deep-time wildfires.