S. A. Oleinik

Prospects and problems of using the methods of geochemistry of stable carbon isotopes in soil studies

A review of world investigations into the geochemistry of stable isotopes—a new area for soil science—is presented. Studies of the behavior of stable isotopes in soils are being developed in two major directions: reconstruction of the environmental conditions and indication of the soil processes. Investigations into the reconstruction of the environmental conditions are based on the assumption that the isotopic composition of carbon in the soil humus is inherited from and reflects the isotopic composition of carbon in the corresponding vegetation (the source of soil humus). At the same time, studies into the indication of soil processes are based on the effects of fractionation of carbon isotopes in the course of transformation of carbon compounds. In this case, the isotopic composition of carbon in the soil humus should differ from that in the initial plant material. The analysis of available data suggests that the current state of research does not allow assessing the entire diversity of the effects of isotopic fractionation in soils. New data on the isotopic composition of carbon of soil organic matter and carbonates are being actively accumulated at present. However, it is obvious that the effect of isotopic fractionation should be taken into account in the paleoenvironmental and paleoclimatic reconstructions.

Transformation of carbonate pedofeatures in paleosols buried under kurgans in the North Caucasus region

Carbonate concentrations in a chronosequence of paleosols buried under kurgans in the North Caucasus region at the end of the 4th century and the first half of the 5th century AD have been studied with the use of a set of morphological and isotopic methods. It is demonstrated that morphologically different phases of calcite—the collomorphic phase and the crystalline phase—in carbonate pedofeatures (calcareous pseudomycelium) and in the calcareous horizon have different elemental compositions and different isotopic compositions of carbon. Hence, these forms of calcite should have different origins. An addition of colloidal carbonates migrating in colloidal solutions from the lower soil horizons to the surface horizons during the periods of climatic aridization to the acicular calcite may be responsible for a sharp and irregular increase in the radiocarbon age of the newly formed carbonate pseudomycelium.

Reconstruction of the paleotemperature and precipitation in the Pleistocene according to the isotope composition of humus and carbonates in loess on the Russian Plain

The isotope composition of carbon and oxygen in humus and carbonates has been studied in the Pleistocene loess and soil of the Russian Plain in order to reconstruct the paleoclimatic parameters during the time of their formation. It is established that most of the buried soils were formed upon the mean annual temperature exceeding the modern temperature by 1–2°C. The climate aridity (the portion of C4 plants in the ecosystem) varied from 0 to 25%. It is shown that the isotope composition of carbon and oxygen in the humus and carbonates may be a reliable indicator of the paleoclimate (i.e., the paleotemperature, paleoprecipitation, and aridity dynamics). An isotope procedure of culling soil samples unsuitable for paleoclimatic reconstruction is proposed.

Specific features of paleosols of burial mounds involved in current farming practices and their contemporary pedogenesis (using the early Alanian burial mounds in the North Caucasus region as an example)

Paleosols of three burial sites—Brut 1, Brut 2, and Beslan—in the Northern Osetia Republic (Alania) of the North Caucasus region were studied. A part of the Brut 2 burial site was subjected to intensive agricultural use (more than 30-year-long irrigation and regular tillage). This led to the leveling of the burial mounds and changes in some properties of the buried paleosols. A comparative analysis of paleosols of the same age under the burial mounds involved not included in the intensive agricultural use made it possible to determine the degree of preservation of the initial properties in the irrigated paleosols and to use them for reconstruction of the paleoclimates. Some specific features of the soil evolution were revealed, and paleoclimate reconstructions were performed for the period of the Early Alan civilization in the North Caucasus region (from the second half of the 2nd century AD to the middle of the 5th century AD).

Isotopic composition of sulfur from sulfates in loess sediments of the North Caucasus and adjacent regions as an indicator of the genesis of salts

Isotopic composition of sulfur from sulfates in loess sediments of the North Caucasus region was studied. It proved to be very homogenous within the vertical soil-loess sequences and in the lateral direction: the δ34S values in the studied samples varied from 7.8 to 8.3‰. The origin of sulfates in the soil-loess sequences is related to their deposition from different sources in solonchaks of the Black Sea-Caspian region with further mixing of sulfur and its eolian transport toward the areas of loess deposition. The gypsiferous horizons of studied soils were formed under the impact of the eluvial-illuvial redistribution of salts deposited from the atmosphere onto the soil surface.

Identification of carbonate pedofeatures of different ages in modern chernozems

Carbonate pedofeatures of three chernozemic soils developed from loesslike loams in the foreststeppe zone of Lipetsk oblast under fallow plot (Luvic Chernozem (Clayic, Pachic)) and under forest (Calcic Chernozem (Clayic, Pachic)) and in the steppe zone of Dnepropetrovsk oblast (Calcic Chernozem (Episiltic, Endoclayic, Pachic)) were studied in the field and laboratory with the use of a set of methods, including the radiocarbon method, mass spectrometry, and micro- and submicromorphology. The morphological diversity of carbonate pedofeatures in these soils was represented by carbonate veins, coatings, disperse carbonates (carbonate impregnations), soft masses (beloglazka), and concretions. In the forest-steppe soils, disperse carbonates and soft masses were absent. The radiocarbon age of carbonate pedofeatures in the forest-steppe soils varied within a relatively narrow range of 3–4.3 ka cal BP with a tendency for a younger age of carbonate concretions subjected to destruction (geodes). In the steppe chernozem, this range was larger, and the 14C ages of different forms of carbonate pedofeatures were different. Thus, soft masses (beloglazka) had the age of 5.5–6 ka cal BP; disperse carbonates, 17.5–18.5 ka cal BP; and hard carbonate concretions, 26–27 ka cal BP. Data on δ13C demonstrated that the isotopic composition of carbon in virtually all the “nonlabile” carbonate pedofeatures does not correspond to the isotopic composition of carbon of the modern soil organic matter. It was shown that the studied chernozemic soils are polygenetic formations containing carbonate pedofeatures of different ages: (a) recent (currently growing), (b) relict, and (c) inherited pedofeatures. The latter group represents complex pedofeatures that include ancient fragments integrated in younger pedofeatures, e.g., the Holocene soft carbonate nodules with inclusions of fragments of the ancient microcodium.

The origin of sulfate salts in soils developed on Maikop clays in the North Caucasus region (According to data of the isotope composition of sulfur)

The ratio of 32S/34S stable isotopes was determined for soils developed on the Maikop clay sediments in the North Caucasus region in order to study the salt geochemistry in the soils. The soil sulfates were proved to be polygenetic. Three groups (generations) of sulfates of different geneses were distinguished according to their isotopic composition (34S). In the underlying deposits and, partially, in the salt-bearing horizons of the soils, secondary sulfates originated from the oxidation of sulfides contained in the marine clay predominate. In the soil profiles, the sulfates inherited from the parent material (marine clay) are mixed with sulfates arriving with the atmospheric precipitation. The portion of the latter reaches about 30% of the total amount of sulfates in the soils. The salt (gypsum)-bearing horizons were formed in the course of the eluvial-illuvial redistribution of salts to the depth of seasonal wetting. The mobilization of sulfate salts from the underlying deposits and their transport to the upper parts of the soil profile with ascending water flows in the course of evaporation from the soil surface did not play a significant role in the accumulation of sulfates in the studied soils.