Silurian wildfire proxies and atmospheric oxygen

Abstract

The earliest evidence of wildfire is documented from two localities: the early mid-Silurian Pen-y-lan Mudstone, Rumney, Wales (UK), and the late Silurian Winnica Formation, Winnica, Poland. Nematophytes dominate both charcoal assemblages. Reflectance data indicate low-temperature fires with localized intense conditions. Fire temperatures are greater in the older and less evolved assemblage. These charcoal assemblages and others, new and previously documented, from the Silurian and earliest Devonian are compared to box models of atmospheric oxygen concentration (pO2). Based on modern charring experiments, these data indicate pO2 is divergent from the broad trends predicted by the COPSE-revisited and GEOCARBSULFOR models. Sustained burns require a minimum pO2 threshold of 16%, or ~0.75 present atmospheric level. This threshold was first met and, our charcoal data indicate, was exceeded in the mid-Silurian and then, later in the Silurian, attained again repeatedly.

Evolution of Atmospheric O2 Through the Phanerozoic, Revisited

“…Inertinite is a by-product of wildfires which require high levels of oxygen to proliferate. Experiments have determined that fires cannot sustain themselves below 15% O2, setting a clear lower limit for much of the Phanerozoic (Belcher and McElwain, 2008;Belcher et al, 2013;Glasspool and Gastaldo, 2022). There are periods after the charcoal record begins (e.g.…”

“…Cope and Chaloner, 1980;Pyne et al, 1996;Scott and Jones, 1991). The presence of inertinite in the geologic record indicates that there was sufficient oxygen in the atmosphere to sustain wildfires for much of the last ~430 Myr (Glasspool and Gastaldo, 2022), but there have also been attempts to use the fossil charcoal record to generate quantitative estimates of Phanerozoic pO2. A database of the percentage by volume of inertinite in coals (Inert%, e.g.…”

“…This was the approach of Algeo and Ingall (2007), who used records of Corg:Ptotal in sediments as a proxy for Corg:Preactive, and scaled the resulting qualitative O2 curve to ensure it did not violate the constraints imposed by the existence of fossil charcoal (Wildman et al, 2004). More recent work on wildfires and the charcoal record (Belcher and McElwain, 2008;Glasspool et al, 2015;Glasspool and Gastaldo, 2022) have subsequently meant that the predicted O2 curve no longer satisfies these constraints.…”