O. S. Khokhlova

Records of climatic changes in the carbonate profiles of Russian Chernozems

Abstract The carbonate profiles of Chernozems bear important information on soil processes and can be successfully used for paleoenvironmental reconstruction. In the Northern Caucasus region, Russia, carbonate profiles of Chernozems were compared under anthropogenic (irrigation) and natural changes of moisture regime. The results for irrigation served as the basis for understanding the response to natural climatic changes. A soil chronosequence, consisting of soils buried under archaeological mounds dated to >5000, 3800–4000 and 1600–1700 BP and modern surface soils, was studied in a similar way. The soils buried >5000 and 3800–4000 BP had distinctive migrational and segregational carbonate accumulations (CAs). The migrational forms occurred in the surface horizons and contained 89–92% calcite with the highest dissociation temperatures. In the soils buried 1600–1700 BP the carbonate profile was clearly defined in terms of migrational CAs; they occurred only in the deeper horizons, had no clear boundaries and were diffused throughout the soil mass. In the modern surface soils the migrational CAs have almost disappeared, and the segregational CAs have the largest halos of recrystallised carbonates. The values of δ13C for CAs in the soils buried >5000 and 3800–4000 BP were lighter than in the soils buried 1600–1700 BP and the modern surface soils (−10.6‰ to −9.9‰ and −9.6‰ to −8.8‰, respectively). We conclude that the climate of the region during the second half of the Holocene changed from relatively dry and warm in the Atlantic period (>5000 BP) to more humid and cooler in the early Subboreal (5000–4000 BP). Since 4000 BP the climatic conditions have remained relatively stable with some changes in moisture regime resulting from human activities in recent centuries.

Paleosols buried under kurgans of the Pit-grave culture in the steppe zone of the Cis-Ural region

The analysis of soil chronosequences developed from loamy and loamy sandy substrates and buried under kurgans dating back to the Pit-grave archaeological culture in the steppe zone of the Cis-Ural region demonstrated that it is possible to trace the dynamics of the soil properties and to perform paleoclimatic reconstructions for different intervals within the studied period. The properties of sandy soils changed more rapidly than the properties of loamy soils. For sandy soils, notable changes in their properties were revealed in the soil chronosequences dating back to the particular stages of the Pit-grave culture; for loamy soils, such changes could only be traced for the soils buried under the kurgans dating back to different stages of the Pit-grave culture.

Distribution of Stable Carbon Isotopes in an Agrochernozem during the Transition from C 3 Vegetation to a Corn Monoculture

The distribution of carbon in an agrochernozem’s profile was studied by the natural 13C abundance method during the C3-C4 vegetation transition and the analysis of the soil phytolith complex under a continuous corn monoculture. A young pool of soil organic matter (SOM) formed during 43 years of monoculture growing was detected by the isotope analysis in the 0-to 60-cm layer, while the analysis of the phytolith complex identified this pool deeper: corn phytoliths were detected in the 0- to 80-cm layer. The maximum size of the young pool was found in the upper soil horizon; it reached 6.4% of the SOM in the 0- to 20-cm layer. The apparent time of the SOM turnover was 635 and 2225 years in the 0- to 20- and 40- to 60-cm layers, respectively. The high values of the mean residence time were related to the low input of plant residues to the soil at the growing of corn for silage and the high initial content of organic carbon in the chernozem. The changes in the isotope composition after the decalcification of the soil to remove carbonates and the variation of the δ13C in the corn biomass during the vegetation period significantly affected the calculated value of the mean residence time.

Kurgans and nomads: new investigations of mound burials in the southern Urals

A new study of the group of kurgans (burial mounds) which stands near Orenburg at the south end of the Ural mountains has revealed a sequence that began in the early Bronze Age and continued intermittently until the era of the Golden Horde in the Middle Ages. The application of modern techniques of cultural and environmental investigation has thrown new light on the different circumstances and contexts in which mound burial was practised, and confirmed the association between investment in burial and nomadism.

Polygenesis of volcanic paleosols in Armenia and Mexico: Micromorphological records of climate variations in the quaternary period

The Pleistocene volcanic paleosols of Armenia and Mexico combine the features of both arid and humid pedogeneses. They were studied in order to reconstruct the environmental conditions during the period of the initial settlement of humans in these areas. The main attention was paid to the micromorphological analysis of the pedogenic features. The pedocomplexes studied in Armenia have been forming since the Early Pleistocene. They bear evidences of humid (weathering, clay illuviation, and gleying) and arid (calcification) pedogeneses alternating in different soil layers and pointing to climatic cycles. The nonuniform lithology of the soil profiles is related to pulsating volcanic activity. Mexican paleosols of the Late Pleistocene period are similar to the lower layers of the soils in Armenia with respect to the features characteristic of the humid and arid phases of pedogenesis. This allows us to suggest that the early stages of the formation of the Armenian pedocomplex proceeded under warmer paleoclimatic conditions similar to those of a tropical climate. It is shown that the arid pedogenesis may erase the features produced during the previous stage of humid pedogenesis. The studied paleosols are characterized by the destruction of the features inherited from the humid phase (the clayey groundmass and illuviation coatings) by the newly forming calcite. Paleopedological records point to the fact that the paleoenvironmental conditions during the periods of the initial settlement of humans in both regions differed significantly from the modern environmental conditions in these regions.

Evolution of the carbonate state of agrogenically transformed dark gray forest soils in the central forest-steppe

The integrated study of the carbonate state of soils, which involves all the forms of soil carbonates at different levels of the soil organization, allows exactly assessing the degree and the rate of the soil transformation due to agricultural development. The evolution of the carbonate state in agrogenically transformed soils was assessed on a site with a known land-use history and with remaining natural soils. The direction, rate, and stages of the carbonate state transformation upon the agrogenic use of dark gray forest soils were determined on the basis of the morphogenetic analysis. Agricultural development entails a significant reorganization of the carbonate profile: a dark gray forest soil evolves into a medium-thick slightly podzolic chernozem with a greater pool of carbonates. The calcareous pedofeatures and horizons evolve stepwise: in the soil of a 100-year-old plowland, the transition to another organization of the carbonate state is observed: carbonates migrate more actively, and the dissolution-precipitation conditions are most dynamic. In the upper calcareous horizons, the structure of the calcitic pedofeatures becomes more collomorphic, and this mass fills almost all the voids; in the lower calcareous horizons, calcite is predominantly segregated into calcareous pedofeatures.

The pool of pedogenic carbon in the soils of different types and durations of use as croplands in the forest-steppe of the Central Russian Upland

Based on studying five agrochronoseries, including recent forest (dark) gray soils and soils plowed for 100, 150, and 200–240 and more years in the forest-steppe zone of the Central Russian Upland, the dynamics of the pedogenic carbon pool, including the Corg and Ccarb, are considered. In the 2-m-thick layer of the agrogenic soils studied, the pedogenic carbon pool was shown to increase by 15–30% (up to 50%) mainly due to the changes in the Ccarb content. The insignificant (by ∼10%) growth of the Corg content was found in the soils that were plowed for more than 200–250 years. As the hydrothermal regime changed when passing from the forest to croplands, the Ccarb reserves increased due to the ascending of carbonates from the parent rock through the capillary pores, probably, in colloid solution-suspensions. This process proceeded without exchange with the soil CO2, since the 14C age and the content of the newly formed carbonates became higher. These carbonates may be called pedogenic-lithogenic agrocarbonates, since they appear in soils as a result of the (agro-) pedogenesis. In this case, their additional source is the lithogenic carbonates, which bring in the “old” carbon. The process of carbonates ascending could be referred to the rapid soil-forming ones with their implementation time being close to ≤50 years.

Soils of cryogenic subarid steppe landscapes in the Terekhol intermontane Depression of the Sangilen Upland

The soil cover of the Terekhol intermontane Depression in the Tyva Region is poorly studied. The data obtained in the 1950s do not reflect the real genetic diversity of soils and their specific character. According to these data, the soil cover was dominated by meadow-chernozemic soils under virgin steppe-like meadows. The investigations of 2007–2009 show that the disperse-carbonate chernozems, which are often solonetzic and weakly saline; the specific polygenetic dark soda solonchaks-solonetzes; and the postagrogenic chernozems, solonetzes, and agrozems are the main components of the soil cover of the depression at the present time. The described chernozems correspond mainly to the central image of southern Siberia chernozems. The investigated solonetzes are characterized by a number of evolution-genetic features that distinguish them from previously described solonetzes of Tyva and from all the solonetzic soils in the current interpretation. The chernozems and solonetzes have cryogenic features due to the cold extremely continental climate and relatively shallow continuous permafrost.

Short chronosequences of paleosols from the Skvortsovka kurgans in the Buzuluk River valley of Orenburg oblast

Paleosols buried under kurgans of the Yamnaya (the third millennium BC) and Srubnaya (the 18th–17th centuries BC) cultures within the Skvortsovka group of kurgans in the Buzuluk River valley in Orenburg oblast have been studied. This is the area of the dry steppe zone. The changes in the paleosol properties make it possible to distinguish between relatively short-term chronointervals separately for the Yamnaya and Srubnaya cultures. The paleosol data and radiocarbon dating have been used for revealing the relative order of the construction of the kurgans within these chronointervals and for the paleoclimatic reconstruction. Quite definite changes took place in the soil formation from the beginning to the end of the particular chronointervals: the contents of humus and exchangeable bases and the portion of calcium ions in the composition of the exchangeable cations increased in parallel with the enhanced leaching of carbonates and the enhanced biological activity. These changes attest to the increasing amount of precipitation. In general, the climate of the Srubnaya cultural epoch was less continental, whereas the climate of the Yamnaya cultural epoch was relatively close to the modern climate. The 14C dates of the pedogenic carbonates in the upper meter of the paleosol profiles have been used as auxiliary data to confirm the sequence of the soil’s burial during the short chronointervals corresponding to the humid stages of the paleosol’s evolution, and the 14C dates of the diagenetic carbonates have been used to determine the dates of construction of the kurgans with due account for some time gap between the construction of the kurgans and the appearance of these carbonates in the buried soil profiles.

Evolution of forest pedogenesis in the south of the forest-steppe of the Central Russian Upland in the Late Holocene

The Late Holocene stage of forest pedogenesis has been studied on the interfluves along river valleys in the forest-steppe zone of the Central Russian Upland. The development of gray forest soils from the former chernozems as a result of the Late Holocene advance of forest vegetation over steppes is discussed. It is argued that the climatic conditions of the Subatlantic period were unstable, so that multiple alternation of forest and steppe vegetation communities took place. This specified a complex character of soil evolution upon contrasting substitution of forest pedogenesis for steppe pedogenesis. On the interfluves near the natural drainage network (balkas, ravines, and steep slopes of river valleys), the climate-driven dynamics of forest and steppe vegetation with corresponding changes in the character of pedogenesis could take place during the entire Holocene, which is reflected in a lower thickness of humus profiles and deeper leaching of carbonates from chernozems of the Early Iron Age in comparison with their analogues formed under steppe cenoses in central parts of the interfluves. Two variants of the evolution of gray forest soils can be suggested: the pulsating evolution typical of balkas and interfluves near river valleys and the continuous progressive evolution typical of automorphic (plakor) positions in central parts of the interfluves.