Stable isotopic composition of carbonates in Quaternary sediments of the Skala Podil'ska sequence (Ukraine)

Stable isotopes of carbon and oxygen in carbonates from Quaternary sediments sequence of Skala Podil'ska in Podolia were studied. Samples were collected from two loess with fossil soils packages, from lacustrine loams separated them, and from the underlying alluvium. The isotopic analysis was carried out on the bulk loess, fossil soils and loam samples, calcitic concretions and other detrital and auhigenic forms of carbonates (rhizocretions). These are thin tubes lined by micrite, empty or filled by root fragments with calcified cells and calcified cortex of the roots. Variation in carbon isotopic composition of soil and loess samples depends on the 13C/12C ratios of authigenic and detrital CaCO3-containing components. Therefore, the carbon isotopic composition of rhizocretions affected bulk isotopic composition of loess and soils. Likewise, the isotopic composition of lithoclasts and bioclasts influence the isotopic composition of carbonates from the loams of lacustrine-like subsequence. The δ13C as well as δ18O values for concretions from all the analysed horizons in the sequence fell within a very narrow range. This suggests that carbonate concretions and rhizocretions were formed under roughly similar conditions, with considerable supply of soil CO2 to the groundwater bicarbonate. The smallest δ13C value obtained for calcified root cortex, for calcite precipitated directly inside the root structure, may indicate mineralization of C3 plants.