Stable isotope compositions of pedogenic carbonates and soil organic matter in a temperate climate Vertisol with gilgai, southern Russia

A Vertisol gilgai complex from the North Caucasus, Russia, was studied to evaluate the climatic and environmental conditions governing pedogenesis. The stable isotope compositions of soil organic matter (SOM) and carbonate pedofeatures in the Vertisol record a complex, but interpretable, pedogenic history. Variability in the isotopic composition of pedogenic carbonate as a function of morphology, position in gilgai microrelief, and soil depth emphasizes the importance of these parameters to interpreting pedogenic conditions. The isotopic compositions and age of SOM record evidence of pedogenesis in an earlier (> 5000 yr BP) environment that was warmer and drier than modern conditions, with a significant component of C4 flora (up to 30%). This portion of the soil history is preserved only in the deepest portions of the microlow (> 140 cm) or in the central portion of the microhigh chimney. A change to cooler and wetter conditions was accompanied by a shift to an ecosystem dominated by C3 vegetation (> 95%) and resulted in a wetter soil environment, particularly in gilgai microlow positions. Most pedogenic carbonate preserves isotopic compositions reflecting only the more recent climate history. The wetter environment contributed to recrystallization of pedogenic carbonate nodules and formation of soft carbonate masses. There are systematic differences in the isotopic composition of hard nodules and soft masses that indicate the influence of greater soil water evaporation, a greater proportion of xerophytic plants, and/or lower rates of soil respiration in the gilgai microhighs. Self-consistent interpretation of the pedogenic and paleoclimate history of these Vertisols requires consideration of a full suite of information, including carbonate pedofeature micromorphology, the stable isotope composition and age (absolute and relative) of these features, and other soil characteristics.