Charring and non-additive chemical reactions during ramped pyrolysis: Applications to the characterization of sedimentary and soil organic material

Ramped pyrolysis is a useful thermal analysis technique for determining the stability, chronology and age distribution of sedimentary/soil organic matter (SOM), but may be limited in its potential applications by uncharacterized chemical reactions. We studied ramped pyrolysis thermal decomposition chemistry and kinetics using 13C nuclear magnetic resonance (NMR) and radiocarbon analyses, employing first order reaction kinetics. We show that ramped pyrolysis results in charring of organic materials by 425 °C, increasing the thermal stability of young and reactive carbon compounds. While the charring reactions observed affected neither sediment chronology interpretations, which normally depend on isotopic data from lower temperatures, nor determinations of relative thermal stability, they do affect interpretations of age distribution and source apportionment that use isotopic data from higher temperature intervals. We developed a charring correction from pyrolysis decomposition kinetics. This tool improves the ramped pyrolysis characterization of the age distribution of SOM and allows for further application of ramped pyrolysis to systems with petrogenic and/or naturally charred carbon sources.