Enhanced wildfire at the Frasnian-Famennian boundary and its implications for O2 level and F-F mass extinction: evidence from organic petrology and Os isotope stratigraphy

In this study, organic petrology and Osmium isotope (187Os/188Os) stratigraphy, major and trace element, and programmed pyrolysis analysis were performed on five outcrop samples from western New York, USA. Seawater Os isotope composition is controlled by radiogenic input from weathering of the ancient land and nonradiogenic input from extraterrestrial and hydrothermal sources (Peucker-Ehrenbrink and Ravizza, 2000). Os is complexed by the organic matter present at the time of deposition without isotope fractionation. Seawater Os isotope composition is reconstructed by analysing the Os isotope composition of the organic-rich sedimentary rock (Turgeon et al., 2007). The short residence time of Os (10 – 50 kyrs) in seawater makes it homogenised in the ocean, thus the Os isotope record from an open marine setting is representative of the global signal (Peucker-Ehrenbrink and Ravizza, 2000; Rooney et al., 2016). Here, we report increased inertinite as evidence for enhanced wildfire events at the F-F boundary and give implications on the F-F mass extinction. O2 level is estimated to be around (24.8%) with the average inertinite abundance data. Unlike previously thought, our findings support a model with higher atmospheric oxygen level (Berner et al., 2003) during the Late Frasnian to early Famennian (Late Devonian). The atmospheric oxygen level may have reached the present level (21 %) at late Frasnian (25 Myrs earlier than previous thought). Our Osi record excludes extra-terrestrial impact or hydrothermal event as a trigger for the mass extinction. Also, we give implications on the mechanism of Re-Os enrichment and fractionation in the organic-rich sedimentary rocks.