ИСТИНА

Pedogenic carbonates are very specific and still underestimated compartment of paleoenvironmental memory/record of semiarid and arid soils. These pedofeatures are especially attractive because ones can be potentially 14C dated. There were relatively numerous publications dealing with radiocarbon dating and isotopic signatures of pedogenic carbonates though the interpreting of these data is still contradictory as the formation of isotopic record of soil carbonates are not completely understood. A comparative study was conducted on the radiocarbon age and isotopic record of soil carbonates in two different semiarid environments: one with a cold climate (MAT -3.8°C) in South Siberia and the other with a hot climate (MAT +17°C) in the Chihuahuan Desert. Carbonate coatings on rock clasts and carbonates of fine earth of the surrounding matrix were conducted for pre-dated (Late Holocene) anthropogenically deposited/affected substrates. Climatic characteristics of the studied sites differ mostly in part of temperatures while the sum of annual precipitation is about the same 200-300 mm. Cold semiarid site has pure C3 vegetation never intervened by C4 plants even in a long-term perspective. Hot semiarid site now is also dominated by C3 plants (creosote bush) but was dominated by C4 grasses 150 years ago. In both cases soils do not contain detrital calcite but contain old (Pleistocene) pedogenic carbonates. In both sites, coatings on rock clasts within anthropogenically accumulated parent material have been radiocarbon dated to a later period compared to deposition of the material itself. Carbonate coatings penetrate up to the base of the human-constructed 3 m embankment in the cold semiarid site. Carbonates of the matrix in the loamy layers of the embankments and carbonates of fragmented ex-situ coatings imbedded into the matrix pre-date the embankment construction. Both are inherited from soil material used for the construction. Fine earth matrix carbonates of the horizons above and below the hearth (hot semiarid conditions) have compatible 14C ages with the carbonates of coatings in these horizons; at the same time matrix carbonates of the surface crust are surprisingly older than the hearth. This fact implies that carbonates above the hearth are multigenerational and along with newly formed pedogenic carbonates may contain older ex-situ pedogenic calcite. In both sites presence of old (Late Pleistocene) pedogenic carbonates in the system (within the zone of contemporary soil processes acting) didn’t have a significant influence on pedogenic carbonates of the younger generations. δ13C of pedogenic carbonates in soils from both hot (Chihuahuan Desert) and cold (S. Siberia) sites have similar values varying in a range between -0.9 and -6.7‰. At the same time intra-profile variability of δ13C in soil of the cold site reveals clear trend in enrichment of δ13C related to Late Pleistocene colder environment. We suppose that relatively high δ13C values of soils carbonates in Chihuahua Desert and S. Siberia are governed by different key factors. The participation of C4 plants plays a crucial role in the formation of the isotopic signature in the desert. In the case of the cold site, δ13C enrichment results from prevailing abiotic processes at low temperatures, particularly cryogenic carbonate formation. δ18O values show significant depletion in carbonates formed in cold environments (S. Siberia) compared to those formed in hot climates. The δ18O values of 'warmer' Holocene carbonates are higher compared to the 'colder' Late Pleistocene carbonates at the cold South Siberian site. All the data clearly demonstrate the complexity and multigenerational character and morphological variety of the pool of pedogenic carbonates in both profiles. All that stresses that interpreting of stable isotopic and 14C data for soil carbonates demands great awareness and cautious.