RADIOCARBON AGE OF VERTISOLS AND ITS INTERPRETATION USING DATA ON GILGAI COMPLEX IN THE NORTH CAUCASUS

Radiocarbon dates were analyzed to assess Vertisols age around the world. They show an increase of radiocarbon age from mainly modern-3000 BP in 0-100 cm layer up to 10,000 BP at a depth 100-200 cm. Older dates reflect the age of parent material. The inversion of C-14 dates seems to be a frequent phenomenon in Vertisols. A series of new dates of Vertisols from gilgai microhigh, microslope and microlow in the North Caucasus was done in order to understand the nature of this inversion. C-14 age in the gilgai soil complex ranges from 70 +/- 45 BP in the microlow to 5610 +/- 180 BP in the microhigh. A trend of similar depths being younger in the microslope and microlow was found. We explain this by intensive humus rejuvenation in the microlows due to water downward flow. The older date in the microhigh represents the old humus horizon sheared laterally close to the surface and preserved by impermeable water regime. We explain inversions of C-14 age-depth curves by the sampling procedures. In a narrow pit, genetically different parts of former gilgai could easily be as a genetically uniform soil profile. Because of this strong microvariability, Vertisols require sampling in a trench accounting for gilgai elements, even when gilgai are not obvious.