Co-occurrence of charcoal, polycyclic aromatic hydrocarbons and terrestrial biomarkers in an early Permian swamp to lagoonal depositional system, Paraná Basin, Rio Grande do Sul, Brazil

Abstract

The Permian period had the most oxygen-rich atmosphere in Earth's history. With the presence of large amounts of fuel from vegetation, there was therefore a high probability of igniting fires during this time. This study presents the geochemical characterization of charcoal-bearing Asselian sediments, represented by terrestrial to lagoonal facies, collected from the Quitéria and Curva do Belvedere outcrops, Paraná Basin, Rio Grande do Sul in Brazil. Lower Permian sedimentary rock organic extracts were studied using gas chromatography–mass spectrometry, to determine their molecular composition. Associations were found between biomarkers of unburned terrestrial gymnosperm plants and organic matter (OM) of pyrolytic origin. The predominance of odd-carbon n-alkanes with a high molecular weight (C25–C31), indicates an origin from higher plants. The same pattern was observed for high molecular weight even-carbon n-carboxylic acids, but was less distinctive. Between other polar compounds, aromatic carboxylic acids, and aldehydes associated with decomposition of lignin, were found as important polar fraction constituents. Aromatic carboxylic acids were also the main components of a copper oxide (CuO) oxidation experiment, which imply preservation of lignin remnants in OM-rich sedimentary rocks. Other higher plant biomarkers were represented by cadalene, simonellite, retene and perylene. Aromatic hydrocarbon series such as alkylnaphthalenes and alkylphenanthrenes are very common in the OM-rich mudstones, and rare or absent in alluvial fan or lagoon deposits, which may be associated with secondary processes like water washing and OM oxidation. Unsubstituted polycyclic aromatic hydrocarbons (PAH), common in pyrolytic processes, were widespread in the investigated material. The PAHs with the highest concentrations in the rock samples were benzo[e]pyrene, benzo[b]fluoranthene, chrysene/triphenylene, fluoranthene and pyrene. Less abundant but almost always present were phenanthrene, benzo[a]pyrene, benzo[a]anthracene, anthracene and 13H-indeno[1,2-e]pyrene. The temperatures and types of wildfires were assessed based on total unsubstituted PAH concentrations and fusinite reflectance measurements. The estimated temperatures were around 400–500 °C, which correspond to surface fires. These findings suggest that wildfires occurred more commonly during dry seasons when the swamps were periodically desiccated.