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

Stable isotope ratios (reported as delta C-13) 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 C-13/C-12 in an open system results in concentration of C-13 in the residual organic matter preserved in the soil and paleosol. This paper summarizes studies of modem soils which show that this 13 C-enrichment follows a Rayleigh distillation process whereby the C-13/C-12 ratio is a function of the natural logarithm of the fraction of remaining soil organic carbon, leaving soil organic carbon 13 C-enriched by up to similar 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 13 C-enriched by several per mil, and thus not a good paleoecological indicator of the delta C-13 value of the original biomass. Therefore, effective paleoecological studies of delta C-13 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 13 C-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. (c) 2007 Elsevier B.V All rights reserved.