Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record?

Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios (δ13C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO2), and thereby provide a source of low 13C CO2 when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO2 from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ13C excursion in the atmosphere (∼ 2.4‰), vegetation (∼ 2.4‰) and the surface ocean (∼ 1.2‰), but not for the deep ocean (∼ 0.9‰). Release of CO2 initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ13C excursions in the rock record, even during times of elevated pCO2.