Polycyclic aromatic hydrocarbons in the Upper Cretaceous lacustrine deposits from the Songliao Basin (NE China): Implications for wildfires and paleoclimate

Changes in wildfires and paleoclimate during the Late Cretaceous have been widely discussed in the literature, yet terrestrial evidence for these changes is temporally limited. This study presents biomarker evidence for wildfires and paleoclimate using polycyclic aromatic hydrocarbons (PAHs) from drill core J79 of the Nenjiang Formation (K2n1+2, boundary at about 83.3 Ma) deposited during the Late Santonian in the Songliao Basin, NE China. A variety of di-ring to hepta-ring aromatic hydrocarbons, including pyrogenic-derived, petrogenic-derived and mixed PAHs, was identified in the lacustrine deposits using gas-chromatography mass-spectrometry. According to fluorene and its derivates, the PAHs in the K2n1 were dominantly preserved under an anoxic water column, with a shift to preservation under a weakly oxidizing water column (around 83.3 Ma) in the lower part of K2n2. The occurrence of higher-plant-derived PAHs (e.g., retene, simonellite, cadalene, perylene and 6-isopropy-1-isohexyl-2-methylnaphthalene) is as probable plant fingerprint, representing a relatively humid condition and a semi-humid to arid paleoclimate condition during the lower part of K2n1 and lower part of K2n2 and during the upper part of K2n1, respectively. Sharp increases in the abundance of pyrogenic PAHs compounds (e.g., benzo[h]fluoranthenes, benzo[e]pyrene, and coronene) are interpreted as indicators of intense wildfires in the K2n. The scale of fire events in the lower part of K2n1 and lower part of K2n2 was larger than that in the upper part of K2n1. This change in wildfire extent may have been caused by the wetter climate stimulating the growth of sufficient biomass to fuel large wildfires. Therefore, the availability of fuel likely played an important role in controlling fire frequency during the Late Cretaceous.