Carbon isotope fractionation during decomposition of organic matter in soils and paleosols: Implications for paleoecological interpretations of paleosols

Stable isotope ratios (reported as δ13C) of organic carbon preserved in paleosols have potential utility as a valuable paleoecological indicator if the isotopic composition of paleobiomass is preserved in the paleosol organic carbon without fractionation. However, soil organic matter is in a constant state of decomposition in an open system, and frequently decomposes further during alteration after burial of the paleosol. During this decomposition, kinetic fractionation of 13C/12C in an open system results in concentration of 13C in the residual organic matter preserved in the soil and paleosol. This paper summarizes studies of modern soils which show that this 13C-enrichment follows a Rayleigh distillation process whereby the 13C/12C ratio is a function of the natural logarithm of the fraction of remaining soil organic carbon, leaving soil organic carbon 13C-enriched by up to ∼ 6‰ with respect to the original biomass. If the fraction of original biomass carbon remaining during these decomposition processes deviates significantly from 1, the soil organic carbon can be 13C-enriched by several per mil, and thus not a good paleoecological indicator of the δ13C value of the original biomass. Therefore, effective paleoecological studies of δ13C preserved in paleosol organic carbon must therefore demonstrate that the measured concentration of organic carbon in the paleosol is near its maximum concentration during soil formation, or that the paleosol organic matter does not include the 13C-enriched products of microbial decomposition. Suggestions are made for interpreting the effects of carbon isotope fractionation of organic matter, and for materials and methods most useful for paleoecological studies from bulk paleosol organic carbon.