Distribution of pyrolytic PAHs across the Triassic-Jurassic boundary in the Sichuan Basin, southwestern China: Evidence of wildfire outside the Central Atlantic Magmatic Province

Abstract

Emplacement of the Central Atlantic Magmatic Province (CAMP) during the Triassic-Jurassic (Tr-J) transition is believed to have indirectly increased wildfire frequency through climatic warming and drying. To date, evidence for wildfires has come mainly from within the CAMP region. Here, we present biomarker evidence for wildfires in the form of polycyclic aromatic hydrocarbons (PAHs) in two terrestrial Triassic-Jurassic boundary sections in the Sichuan Basin, southwestern China. Pyrolytic PAHs of probable wildfire origin identified in these sections include pyrene, benzofluoranthene, benzo[e]pyrene, indeno[1,2,3-cd]pyrene, benzo[ghi]perylene, and coronene. Sharp increases in the abundances of pyrolytic PAHs normalized to total organic carbon were found during the Rhaetian Stage (R1 and R2) and at the Tr-J boundary. The ratios of pyrolytic PAHs (PPAHs) to methylated homologues document the combustion origin of PPAHs from methylated PAHs during these intervals of increased wildfire frequency. The abundances of oxygenated PAHs with furan structure show the same stratigraphic trend, testifying to enhanced soil erosion or weathering that may have resulted from a reduction of vegetative cover. Although CAMP-induced climate changes probably set the stage for more frequent wildfires in South China, the immediate trigger was probably a southward shift of the paleo-Intertropical Convergence Zone (ITCZ). These observations suggest a complex feedback involving climatic warming, precipitation and evapo-transpiration, vegetation density, and wildfire prevalence during the Tr-J transition.