ISSN 10642293, Eurasian Soil Science, 2013, Vol. 46, No. 4, pp. 341–354. © Pleiades Publishing, Ltd., 2013. Original Russian Text © T.V. Tatyanchenko, T.V. Alekseeva, P.I. Kalinin, 2013, published in Pochvovedenie, 2013, No. 4, pp. 379–392.
GENESIS AND GEOGRAPHY OF SOILS
Mineralogical and Chemical Compositions of the Paleosols of Different Ages Buried under Kurgans in the Southern Ergeni Region and Their Paleoclimatic Interpretation T. V. Tatyanchenko, T. V. Alekseeva, and P. I. Kalinin Institute of Physicochemical and Biological Problems of Soil Science, Russian Academy of Sciences, ul. Institutskaya 2, Pushchino, Moscow oblast, 142290 Russia Email:
[email protected] Received October 25, 2011
Abstract—The chemical and mineralogical compositions and the contents and properties of the organic matter were studied in the paleosols of different ages buried under the kurgan group “Kalmykia” in the south ern part of the Ergeni Upland. The investigated sequence of soils included profiles developed on the given territory about 5100, 4410, 4260, 4120, 3960, and 600 yrs ago. The background light chestnut soil was also examined. The results of our study showed that the earlier established climate changes in this area during the second half of the Holocene are reflected in the chemical and mineralogical compositions of the soils. These characteristics can be used as indicators of the paleoclimatic conditions together with some petrophysical characteristics, such as the magnetic susceptibility of the soil samples. The study of the mineralogical com position of the clay fraction of the paleosols attests to the transformation of the smectitic phase, accumulation of illites, and destruction of chlorites manifested at different degrees. It is argued that the geochemical indi ces—CIA, Al2O3/(CaO + MgO + K2O + Na2O), Rb/Sr, and Ba/Sr—are sensitive to climate changes and reflect the transformation of the mineral soil mass and the soil genesis. Keywords: soil science, soil mineralogy, biogeochemistry, clay minerals, organic matter, climate dynamics DOI: 10.1134/S1064229313040145
INTRODUCTION This study continues a series of earlier investiga tions of the patterns of soil evolution in the Lower Volga region in the past 5000 yrs on the basis of the materials obtained during excavation of the kurgan group “Kalmykia” in the southern part of the Ergeni Upland (the Republic of Kalmykia) within the desert steppe zone. The studies of the nature of this territory have a centurylong history and are associated with the names of many wellknown researchers [13, 14, 22, 27]. The paleosol studies in the Lower Volga region have been conducted since the beginning of the 1990s by researchers from the Institute of Physicochemical and Biological Problems of Soil Science of the Rus sian Academy of Sciences in Pushchino (Ivanov, Demkin, Borisov, Alekseev, Alekseeva, Demkina, Khomutova, Khokhlova, and others) [10–12, 17, 19– 21, 29, 31, 45, 46, 48, 49]. The paleosol studies in this area are based on the pedoarchaeological method. In essence, it consists of a comparative analysis of the modern soils and the paleosols buried under archaeological monuments of different ages determined by the methods of archaeol ogy. This approach makes it possible to reconstruct the paleoenvironmental conditions in space and time. A
new conceptual model of pedogenesis in steppe areas of eastern Europe was developed with the use of this method. In particular, it was found that the interval from 4400 to 3900 BP was characterized by a sharp increase in the processes of salinization, dehumifica tion, calcification, and erosion of soils. This led to a general desertification of the landscapes and the wide spread distribution of the eroded saltaffected and cal careous chestnutlike paleosols at the end of the third millennium BC. These paleosols have no analogues in the modern soil cover of the region. As shown by many researchers, their development was due to a catastrophic aridization of the climate [12, 19, 49]. An important stage in the paleosol studies was related to the application of new geophysical methods, including the set of magnetic methods. The first works in this field were performed by Alekseev in the early 1990s [1]. Examination of the magnetic properties of steppe soils showed that the increase in the magnetic susceptibility of the soil material in comparison with the parent material directly depends on the mean annual precipitation [4]. This made it possible to cal culate the mean annual precipitation values for the studied area in the past epochs [2–4, 49]. There are many works, in which various morpho logical characteristics (color, structure, thickness of
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the humus horizon, etc.), the content and composi tion of humus, the degree of solonetzicity, the mag netic susceptibility values, the depth of carbonaterich and gypsum and saltbearing horizons, and the com position of various pedofeatures are analyzed in the buried paleosols [11, 12, 20, 21, 46]. Studies of the mineralogical and chemical compositions of the paleosols are few in number [10, 30, 31, 47, 49]. At the same time, it is known that these characteristics of the buried paleosols are less subjected to the diagenetic transformation and, hence, they may be considered reliable carriers of the soil memory [10, 24, 30, 31, 40]. On the contrary, the content and composition of the soil organic matter are unstable characteristics of the buried soils. The humus content in the buried soils gradually decreases under the impact of mineraliza tion processes. Literature data indicate that the amount of humus in the buried paleosols is by an order of magnitude lower than that in their modern ana logues [14, 24, 38]. It has been shown that the humus content in the buried steppe soils decreases to 60% of the initial value in 170 yrs, to 43% in 1000 yrs, and to 40% in 2000 yrs [16, 23, 29, 35]. The rate of the organic matter mineralization in the buried soils depends not only on the time factor; the initial characteristics of the soil organic matter are also important. Thus, free and loosely bound organic sub stances are more susceptible to mineralization than chemically bound organic substances. Soil organic matter bound with the clay fraction represents a pas sive pool with a much longer mean residence time. According to some estimates, the mean residence time of this pool of the organic matter in the buried soils is about 1000–3000 yrs [51, 54]. The humus preserva tion in the paleosols also depends on the composition of the mineral part of the soil organomineral com plex. For example, the palygorskite mineral matrix ensures preservation of humic substances in the fos sil soils of the Carboniferous period with the age of about 300 M yrs [8, 9]. The aim of our study was to examine the effect of the climate (mean annual precipitation) on the miner alogical and chemical composition of the paleosols of different ages buried under kurgans in the southeast of the Russian Plain, within the southern part of the Ergeni Upland. We also studied the content and prop erties of the organic matter in the bulk samples and in the clay fraction of the paleosols in order to reveal the relationships between mineralization losses of the organic matter, the climate, and the time of the soil burial. OBJECTS AND METHODS Paleosols buried in the second half of the Holocene under archaeological monuments (kurgans) and the modern light chestnut soil were studied in the south ern part of the Ergeni Upland (the IkiBurul district of the Republic of Kalmykia). This territory belongs to
the desertsteppe zone. The modern soil cover is char acterized by the predominance of the complexes of light chestnut soils and solonetzes; the portion of the latter in the complexes reaches about 40%. Light chestnut soils also have solonetzic features that are considered to be typical zonal pedogenetic features in the desertsteppe zone [10, 17, 21]. The parent mate rials are represented by the loesslike calcareous and saline loams of the Quaternary age (QI–III). This area has a moderately continental climate with the mean annual precipitation of 300–350 mm. The studied kurgans were constructed in the early and developed stages of the Bronze Age (IV–III centuries BC) and the developed Middle Ages (XIV century AD) [12]. Overall, the studied pedochronosequence includes the paleosols buried 5100 ± 50, 4410 ± 100, 4260 ± 120, 4120 ± 70, 3960 ± 40 yrs before present and the mod ern surface soil. To simplify the transfer of the material below will be used abbreviated dates 5100, 4410, 4260, 4120, 3960 and 600 years BP. Detailed descriptions of the studied soil profiles were published earlier [4, 11, 12, 17, 21, 30, 31, 49]. We studied the particlesize distribution, the soil pH, the composition of exchangeable cations, the content and properties of the organic matter in the bulk soil samples and in the clay fraction, the mineral ogical composition of the clay fraction, the elemental composition of the bulk soil samples and the clay frac tion, and the magnetic susceptibility values. The clay fraction (
