Effect of burning on the distribution pattern and isotopic composition of plant biomolecules: Implications for paleoecological studies

Abstract

In fire-prone biomes, the interpretation of paleoecological proxies is subjected to uncertainties since the plant biomarkers/molecules are thermally modified during vegetation fires, and the extent of alterations is yet to be well-constrained, particularly for oxic conditions. Towards this, we performed a series of controlled experiments where leaf samples from C3 (tree and shrub) and C4 (grass) plants were burned under ambient oxygen at temperatures between 200 °C and 500 °C. A topsoil sample was also heated to understand the effect of thermal degradation on organic matter (OM) already present in the soil. Our results show a reduction in the total organic carbon content and leaf wax concentrations which is consistent with the previous studies. We also observed a shift from predominantly long-chain homologues (with odd-over-even in n-alkanes or even-over-odd predominance in n-alkanoic acids; FAs) to balanced distributions with increased mid and short-chain homologues. We observed that the short, mid, and long-chain n-alkanes were mainly formed at the expense of FAs (with losses up to 100%), possibly due to oxygen-rich burning conditions. Burning of plant leaves also affected their stable isotopic compositions. In burned C3 plant leaves (tree and shrub), the bulk carbon isotope values (δ13COM) increased by 1.0–1.4‰, while in C4 grasses, it was 0.4–1.6‰ lower than their unburned counterparts. The changes in the δ13COM values are suggested to be a cumulative product of the kinetic isotope effect and source-driven isotopic fractionation. The carbon isotope values in long-chain n-alkanes (δ13Cn-alk) mostly decreased (up to 4.6‰) in burned C3 and C4 plant leaves due to the isotopic modifications associated with the generation of secondary n-alkanes from FAs. In contrast, the hydrogen isotope values of n-alkanes (δ2Hn-alk) in the burned samples were up to 86‰ higher than their initial values, mainly due to the kinetic isotope effect. Therefore, in biomes susceptible to frequent canopy and litter layer fires, unusually high δ2Hn-alk values coupled with a disproportionate change between δ13Cn-alk and δ2Hn-alk values might indicate pyrogenic OM presence within the soil. We also observed that changes in the n-alkane characteristics due to heating of soil samples were substantially lower than those in plant leaves due to the protected lipid component within organomineral complexes. In summary, we recommend careful consideration and identification of vegetation fire history before using stable isotope-enabled organic proxies for generating paleoecological records.