Pedosedimentary environments in the Caspian Lowland during MIS5 (Srednaya Akhtuba reference section, Russia)

Abstract

Srednaya Akhtuba outcrop provides a detailed record of the Late Pleistocene continental and marine deposits and paleosols from MIS5 to MIS1. The MIS5 chronozone is presented by a continuous pedosedimentary sequence with three well-developed paleosols. The paper is based on the study of depositional environment and pedostratigraphy, with OSL chronological framework and precise altitude references. Field data are supported by chemical analyses, micromorphology, and clay mineralogy. A detailed study of MIS5 paleosols was first conducted for the Lower Volga area. Pedosedimentary sequence at that time developed on the river terrace in a backswamp and/or wetland influenced by extremely slow-moving stream after long seasonal floods. Thin loess layers were deposited during the brief episodes of low flood activity, while fluvial heavy loams – at the time of more intensive flooding. Mollic Gleysols and Fluvic Chernozems have been formed in the arid or semi-arid climate with seasonal freezing under productive wet meadows. Changes in the depositional environment resulted in the formation of welded paleosols, marked by textural difference, cryogenic levels, and accretionary humus horizons. Despite the complicated depositional pattern and the influence of seasonal floods, the MIS5 pedosedimentary sequence may serve as a good stratigraphic marker for the onset of Late Pleistocene allowing correlation with loess-paleosol sequences of the whole Ponto-Caspian region and linking it with the global stratigraphic schemes.

Introduction

The Caspian Lowland is a marine and fluvial plain with a surface varying from −28 m below to +50 m above sea level. Its leveled surface is influenced by erosional and aeolian landforms. The Caspian Lowland is a part of the extensive Ponto-Caspian basin that reflects fluctuations in the level of the Caspian and Black sea, glacial-interglacial cycling, and related fluctuations in fluvial activity and aeolian sedimentation. During the deglaciations and degradation of permafrost, the Volga River basin collected meltwater and acted as a final trap for fine-grained sediments from the southern margin of the Scandinavian ice-sheet (Yanina, 2014; Svitoch, 2009, 2010; Tudryn et al., 2016). Sediments of the Late Khazarian (MIS5), the Khvalynian (MIS2), and the New Caspian (MIS1) transgressions are presented in the outcrops of the Caspian Lowland (Fig. 1c). Dynamic changes in sedimentation environments resulted in detailed pedosedimentary sequences with marine, aeolian, and fluvial deposits intermixed with 7 interglacial and interstadial paleosols (Yanina et al., 2017). For tens of kilometers, the coastline of the Volga River and its tributaries is represented by a continuous 15–20 m high bluff of the second Early Khvalynian terrace exposing the Middle-Upper Pleistocene strata. The flat surface of the extensive terrace ensures good preservation of paleosols and sedimentary layers. The pedosedimentary sequences of the Volga-Akhtuba area have been closely investigated in terms of Quaternary stratigraphy and paleogeography (Pravoslavlev, 1926; Fedorov, 1957; Moskvitin, 1962; Goretsky, 1966; Shkatova, 1975; Svitoch and Yanina, 1997; Lavrushin et al., 2014). The chronology of the palaeogeographical events recorded in these sections remains controversial. Existing late Quaternary age estimates for various transgressive/regressive events have been obtained using electron paramagnetic resonance spectroscopy (Molodkov 1989, 2012; Molodkov and Bolikhovskaya 2009), thermoluminescence (Shakhovets 1987; Rychagov 1997), uranium-thorium (Arslanov, 2016), and radiocarbon (Kaplin et al., 1993; Svitoch et al., 1994; Badyukova 2007; Tudryn et al., 2013; Kurbanov et al., 2014); These various methods often provided contradictory results, as shown by Kurbanov et al. (2020). The reliable chronology for the Late Pleistocene was obtained using luminescence dating for the Srednaya Akhtuba section (Yanina et al., 2017). It was recently complimented by luminescence dating of 16 samples from the three reference sections, including the Srednaya Akhtuba for the period from the Early Khvalynian to the Holocene (Kurbanov et al., 2020).

Most of the published data relate to marine successions, while the continental environment during regressive stages was studied much less. This is especially true for the study of paleosols. Paleosols have been recognized but never studied in detail in sense of pedogenesis and as paleolandscape archives. In many outcrops of the study area the MIS5 chronozone is presented by three well-developed paleosols, in some cases separated from each other by sediment layers and in some cases welded, depending on the rate of deposition. The first short descriptions of the three MIS5 paleosols at the Srednaya Akhtuba outcrop were carried out by Moskvitin (1962). He was also the first to describe frost wedges from Atelian suite (MIS4), penetrating deep into MIS5 chronozone. The study of the Late Pleistocene cryogenic features was recently done for the Srednaya Akhtuba reference section in the paper of Taratunina et al. (2020). Four cryogenic horizons, including those described by Moskvitin (1962) were thoroughly studied based on cryolithological, micromorphological analyzes, and particle size distribution of the mineral matter. The environment during the Atelian regression (27–80 ka, MIS 4–3) was reconstructed based on a detailed magnetic mineralogical analysis made for the Srednaya Akhtuba, Leninsk, and Raigorod sections, located on both banks of the Volga River (Költringer et al., 2020). Magnetic properties showed clear similarities to loess, deposited in a dry and cool climate. The continental environment during MIS5 is much less known. The area of the Srednaya Akhtuba reference section was outside of the Late Khazarian transgression, and the aerial extent of its surface area is estimated to have been slightly larger than the present Caspian Sea (Yanina, 2014). The triplet with three paleosols is widely presented in the outcrops of the Lower Volga area. Nevertheless, they have never been studied before in sense of pedogenesis, depositional environment, and paleolandscape reconstructions. Needless to say, that paleosols of the Lower Volga area never used for inter-regional correlations.

The goal of the paper is to fill the gap in our knowledge of the continental environments of the Lower Volga area during the MIS5 based on the study of paleosols. The detailed record of MIS5 chronozone with the triple paleosols is important for a general understanding of the landscape dynamics within the broader area and may serve as a basis for inter-regional correlation for the whole Ponto-Caspian region, Central Asia, and the Russian Plain.