INFLUENCE OF WILDFIRE EMISSIONS ON TERRESTRIAL CYCLING OF CARBON ISOTOPES DURING THE LATE CRETACEOUS

Abundances of pyrolytic-polycyclic aromatic hydrocarbons, and fusinite combined with fusinite reflectance values for the Fort Crittenden Formation in southeastern Arizona substantiate the prevalence of wildfires or peat fires associated with seasonally dry conditions in the paleo-watershed.  Enhanced CO₂ levels in the Cretaceous atmosphere probably increased global lightning activity and served as an ignition source for wildfires.  A simplistic, equilibrium model was constructed in order to evaluate the role wildfire emissions in the terrestrial cycle of carbon isotopes.  Three reservoirs (atmosphere, vegetation, and soil), and three fluxes (productivity, respiration, and leaf litter) were assumed to represent an equilibrium (i.e., closed) system. Burning efficiency and post-fire growth rates are important initial variables that were used to characterize each experimental run.  Initial atmospheric pCO₂ was assumed to be 2x present atmospheric levels (PAL) and isotopic composition of different reservoirs were assumed to be: d¹³C=-7‰ (atmosphere), -26‰ (vegetation), and –26‰ (soil).  An experimental run with 1% of the biomass being burned each year for a ten-year period produced an increase in the atmospheric CO₂ (2.11x PAL) and resulted in a negative isotope excursion (-0.73‰) of both atmosphere and new growth.  Post-fire equilibration of pCO₂ required 57 years and post-fire equilibration of d¹³C of the atmosphere, vegetation, and soil was 83, 103 and 117 years, respectively.  Emissions from wildfires represent an under-studied mechanism for substantial negative isotope excursions at time scales of decades to millennia.