Carbon isotopic subsets of soil carbonate—A particle size comparison of limestone and igneous parent materials

The C isotopic analysis of pedogenic carbonate is an important method for investigating paleoclimatic conditions and carbon sequestration. However, a calcic soil sample may contain multiple types of carbonate, especially in soils formed in limestone parent material. To evaluate isotopic values of the different soil carbonate types, stable C isotopic analysis was applied to an array of particle size classes in bulk soil that was collected from soils formed in limestone parent material and neighboring soils formed in igneous parent material. A pronounced trend of decreasing δ13C with progressively decreasing soil particle size was revealed in soils of both parent materials, ranging from about + 2 to − 6‰ for limestone soils and − 1 to − 5‰ for igneous soils. Thin section microscopy and radiocarbon dating also indicated multiple subsets of calcium carbonate types in soils. Thin section analysis disclosed the presence of limestone detritus, calcitans, aggregates of carbonate crystals, and calcified root hairs. Radiocarbon dating revealed different δ13C values of carbonate crystals of different ages, ranging from about 2620 to 40,290 years for limestone soils and 1660 to 5830 years for igneous soils. These results indicate (1) limestone particles in soil may contribute to a complex mixture of δ13C values that is absent from soils formed in igneous parent material and (2) it is necessary to recognize that isotopic values of bulk soil represent a mixture of different δ13C values from different size fractions and ages in both limestone and igneous soils.