Laboratory tests indicate that 15% O2, instead of 12%, is required for the propagation of a widespread forest fire, a 3% increase from what was previously assumed. The presence of widespread wildfire records in the Upper Triassic and Lower Jurassic of Central Europe suggests that the lower limit for O2 during this time was at least 15%. Wildfire records are based on the co-occurrence of charcoal fragments and elevated concentrations of pyrolytic polycyclic aromatic hydrocarbons (PAHs). In all samples charcoal fragments are large to medium-sized and angular, suggesting that they were transported by rivers only short distances after charcoalification. Calculated combustion temperatures vary with stratigraphic position and average 295–377 °C, which is characteristic for ground or near-surface wildfires. The most extensive wildfires occurred in the earliest Jurassic and their intensities successively decreased with time. Average concentrations of the sum of pyrolytic PAHs for the lowermost Jurassic Zagaje Formation reached ∼1253 µg/g total organic carbon (TOC), whereas for the Upper Triassic–Lower Jurassic Skłoby Formation they did not exceed ∼16 µg/g TOC. Charcoal-bearing sequences were also characterized by the presence of phenyl-PAHs (Ph-PAHs) and oxygen-containing aromatic compounds. The dominance of the more stable Ph-PAH isomers in these immature to low-maturity sedimentary rocks supports their pyrolytic origin. The oxygenated PAHs may also be derived from combustion processes.

The Frasnian-Famennian (F-F) global event, one of the five largest biotic crises of the Phanerozoic, has been inconclusively linked to rapid climatic perturbations promoted in turn by volcanic cataclysm, especially in the Viluy large igneous province (LIP) of Siberia. Conversely, the triggers of four other Phanerozoic mass extinction intervals have decisively been linked to LIPs, owing to documented mercury anomalies, shown as the diagnostic proxy. Here, we report multiple Hg enrichments in the two-step late Frasnian (Kellwasser) crisis interval from paleogeographically distant successions in Morocco, Germany, and northern Russia. The distinguishing signal, >1 ppm Hg in the domain of closing Rheic Ocean, was identified in different lithologies immediately below the F-F boundary and approximately correlated with the onset of the main extinction pulse. This key Hg anomaly, comparable only with an extreme spike known from the end-Ordovician extinction, was not controlled by increased bioproductivity in an anoxic setting. We suggest, therefore, that the global chemostratigraphic pattern near the F-F boundary records a greatly increased worldwide Hg input, controlled by the Center Hill eruptive pulse of the Eovariscan volcanic acme, but likely not manifested exclusively by LIP(s). Consequently, all five major biotic crises of the Phanerozoic have now been more reliably linked to volcanic cataclysms.

A palynological study of the uppermost Famennian section from Kowala Quarry (Holy Cross Mountains, central Poland) allowed recognition of two miospore zones: LV (Retispora lepidophyta–Apiculiretusispora verrucosa) and LN (Retispora lepidophyta–Verrucosisporites nitidus). Based on palynology and sedimentology, the black shale within the upper part of the section is identified as equivalent to the Hangenberg Black Shale, which is known globally. This black shale contains compounds characteristic of photic zone euxinia, including isorenieratane and its derivatives. Such compounds are absent in the organic-poor marls and shales occurring below the LN Zone, and are present only as traces in the layers just above the black shale, indicating fluctuations in the oxygen minimum zone during uppermost Famennian sedimentation. Palynofacies show high amounts of amorphous organic matter and prasinophyte concentrations in the black shale, and a subsequent significant decrease of amorphous organic matter concomitant with a rapid increase of terrestrial input (mainly miospores with common tetrads) in the layers above the black shale. This supports the relatively rapid change in the taxonomic composition of phytoplankton caused by fluctuations of the chemocline. The whole succession corresponds to one sea-level rise and fall. The presence of high concentrations of peri-condensed polycyclic aromatic hydrocarbons and large amounts of small charcoal particles at the Hangenberg event horizon indicate the occurrence of wildfires. Such observations suggest that atmospheric O₂2 levels had exceeded the critical threshold of 13 %, above which wildfires may occur, by latest Famennian time.