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The Caspian Sea is the world's largest lake noted for a unique history of its evolution in the past. One of most important events in the Caspian history is the Atelian regression. The purpose of this work is to specify the extent, the time, and environments of the Atelian regression, as well as its correlation with events in the adjacent regions, on the basis of multidisciplinary studies of the Atelian deposits penetrated by engineering boreholes in the Northern Caspian. Processing of two-frequency seismoacoustic profiles permitted to stratify the sequence and to perform a spatial correlation between the determined boundaries of the sedimentary series. The laboratory studies included lithological and geotechnical analysis of the cores, floristic and faunal studies of the organic remains extracted from the cores, as well as analyses of pollen and spores, the isotope and radiocarbon (14C) analysis. In the seismic-acoustic profiles Atelian regression is represented by erosional landforms (incisions) under the reflecting horizon at the base of Khvalynian sediments. The regressive series is noted for inhomogeneous lithology. In depressions they are represented by interlayered clays and clay loams. The elevations between the depressions are composed of loamy sands in association with loams. Plant detritus occur as separate inclusions, or forms layered concentrations; there are also shells of freshwater and terrestrial mollusks. The paleontological material suggests aquatic or wetland environments with freshwater or freshened brackish-water shallow water bodies. The palynological materials provide evidence of diversified landscapes in the northern Caspian Lowland at the time of the Atelian regression – from forests dominated by conifers to periglacial forest-steppe and tundra-forest-steppe, mostly resulting from the climate fluctuations. The redeposited pollen is present in abundance, often well-preserved, indicative of highly active erosion at the time of regression. The Atelian regression maximum and the erosional processes on the northern Caspian Lowland correspond to the maximum cooling of the Kalinin (MIS 4) glacial time. The final stages of the regression fell on the initial phases of the MIS 3 interstadial. As follows from materials obtained by seismic-acoustic profiling and drilling, the Caspian level dropped to −100 m. We attribute the sea level lowering to the Caspian (Hyrcanian) water discharge through the Manych into the Pontian basin and to the specific glacial environment during the Valday (MIS 4) time. The Atelian regression maximum is correlatable with the maximum of the post-Karangatian regression of the Black Sea. It was at the same time that the lacustrine Burtass (Gudilo) series were deposited in the Manych depression. In the stratigraphic scheme of the Caspian Sea the Atelian regressive horizon (as a stratigraphic unit) is positioned between Hyrcanian and the base of Lower Khvalynian deposits. In the Northern Caspian the continental Atelian formation spans a greater chronological interval and should be considered as a suite (or formation) in the regional stratigraphic scheme. A comparison between the position of Atelian deposits in the Pleistocene sequence within the sea basin and on the dry land supports the assumption that the geological records exposed in the coastal zone are incomplete. Only a thorough analysis of the materials obtained from the offshore drilling and from the sequences studied in exposures on land can give a comprehensive picture of the history of regional environments and present them correctly in a stratigraphic scheme.

For the first time, the Upper Pleistocene Girkanian sediment subhorizon was recorded in the submarine boreholes of the northern Caspian Sea and its radiocarbon age was estimated. Girkanian beds lying at a depth of 25 m beneath the seafloor were studied by lithological, biostratigraphical, seismic and radiocarbon dating methods. They were distinguished based on the characteristic assemblage of Mollusca (Didacna subcatillus, D. cristata, Corbicula fluminalis, etc.). This assemblage is similar to its onshore analogue and differs from the fauna of the underlying Upper Khazarian (D. surahanica, D. nalivkini, etc.) and overlying Lower Khvalynian (D. ebersini, D. parallella, D. protracta, etc.) layers. Acoustic profiling indicates the ubiquity of the horizon in the northern Caspian Sea, as seen from the clear correlation between boreholes. AMS 14C dating has shown that the age of the Girkanian deposits exceeds 55 ka. Their real age haze to be about 80 ka. The obtained results on identification and age estimation of Girkanian beds are an important contribution to the verification of the Upper Pleistocene stratigraphy of the Caspian Sea region.

This paper reports data on the Atelian horizon identified in the Caspian Sea bottom. Seismoacoustic profiling has made it possible to determine its area, position, and setting in the Upper Quaternary sequence and the relation to the host deposits. According to the drillhole core data, the Atelian horizon is composed of continental clay loam and sandy loam containing peatlike organic remains. They are depleted in biogenic residues containing rare freshwater mollusk species. The Atelian deposits were accumulated in lake reservoirs filling the depressions developed in the strata of the Upper Khazarian horizon. The radiocarbon age determined for the first time by humic acids has been used to estimate the Atelian deposition time in the range of 40 000–45 000 calibrated years BP.

Comprehensive analysis of the data of high-precision seismoacoustic profiling, drilling and sampling of deposits using seabed corers, biostratigraphic studies, and radiocarbon age data was performed for the first time for Mangyshlak sediments in several bottom sites of the North Caspian. It was found that the Mangyshlak sediments comprise numerous linearly stretched depressions of 5–10 m in depth (morphologically similar to modern substeppe ilmen areas in the Volga River delta), which are covered by the Novocaspian sedimentary cover, and river incisions (among them the largest Volga River valley). In addition, the Mangyshlak sediments comprise the deltaic alluvial fans of different sizes along the shelf zone of the North Caspian. Analysis of mollusks and biogenic remains indicates that accumulation of the Mangyshlak sediments occurred in freshwater and slightly salty water environments under various hydrodynamic and hydrochemical conditions. According to radiocarbon dating of organic matter, the Mangyshlak sediments formed during sea regression in the range of 10-8 ka (isotopic age) or 11.5-8.5 ka (calendar age). Several types of sediments are distinguished: clayey-carbonate sediments, enriched with organic matter up to the formation of sapropel and peat, accumulated at the lowest sea level; weakly calcareous silty-clayey silts, formed during the subsequent intense filling of paleodepressions with terrigenous material. The features of the mineral composition of sediments are as follows: polymineral composition of clayey material with a high proportion of hydromica and disordered mixed-layered formations, a high content of minerals of the epidote group, amphiboles, and other accessory minerals. All of this indicates a genetic relationship between the Mangyshlak sediments and the Volga terrigenous material.

The evolution of the Black and Caspian seas is considered based on the analysis of new stratigraphic and paleogeographic data. Three transgression stages (Karangatian, Surozh, Black-Sea) and two regression stages (Post-Karangatian and New-Euxinian) were characterized for the Black Sea, as well as four transgression stages (late Khazarian, early Khvalynian, late Khvalynian, and New-Caspian) and three regression stages (Atelian, Enotaevkan, Mangyshlakian), for the Caspian Sea. The analysis of data on the absolute age of deposits allowed correlation of paleogeographic events for the basins, between them and with the stages of the Last (Valdai) Glaciation: the Karangatian and late Khazarian transgressions were correlated with the Mikulinian Interglacial; the post-Karangatian and Atelian regressions, with the Kalininan glaciation; the early Khvalynian and Surozh transgressions, with the middle Valdai Interstadial; the New-Euxinian and Enotaevkan regressions, with the Ostashkovian glaciation; the Black Sea, late Khvalynian, and New-Caspian transgression, with the late glaciation — post-glaciation periods; the Mangyshlakian regression, with the Older Dryas (?). The last connections between the Caspian and Black seas are dated to the middle Valdai time when waters of the early Khvalynian basin drained down to the Surozh basin.

The analysis of paleontological remains in many samples from the Lower Quaternary Chauda sediments drilled and cored on the Bulgarian shelf of the Black Sea revealed widespread mollusks of the genera Didacna and Dreissena (Didacna tschaudae guriana, D. tschaudae, D. pleistopleura, D. crassa guriensis, Dreissena rostriformis tschaudae, D. rostriformis abchasica) accompanied by reworked Neogene representatives of the genus Digressodacna. In numerous places of a continental slope and an adjacent deep-sea depressions near Crimea and Caucasus the similar mollusc assemblage is described in the redeposited state for the first time. The composition of palynological spectra and diatom assemblages in shelf sediments indicates climate changes during the Chaudan period. The Chaudan mollusk fauna from the Black Sea sediments, which is lacking Caspian Bakuan species characteristic of the Chauda stratotype, is compositionally close to the mollusk assemblage from basal layers of the Chauda Horizon in the Guriya area of Georgia (“Gurian” Chauda). These data imply repeated changes in the level of the Chaudan basin between present-day isobaths of −30...−50 to −140 m.

Based on data of high-resolution acoustic profiling performed in the upper part of the sedimentary succession at the western continental slope of the Middle Caspian Sea (at a depth range from 100 to 400 m), we revealed specific accumulative forms of a running wave type. Examination of their morphology and inner structure allowed us to suggest two mechanisms of their formations: (a) by means of flank growth due to migration of channels of sedimentary flow associated with the Samur and other rivers and (b) due to flows of sedimentary material forming sedimentary waves.

Structural specific features of the sedimentary succession on the continental margin of the South Crimea in the Black Sea have been examined in the Yalta Area using geomorphologic, acoustic, and lithologic methods. Alternation of regressive and transgressive sequences has been ascertained in sediment sections formed on the shelf and in the Yalta deep-sea fan due to Quaternary oscillations of the basin level. The analysis of sediment sections resulted in revealing a leading role of sedimentary material transference from upper hypsometric levels to lower ones in the sediment accumulation.

The character of changes in the mollusk fauna in the Late Pleistocene and Holocene sediments within the Bulgarian, Northwestern, Crimean, and Kerch segments of the Black Sea shelf has been examined. A bed containing fossil remains of brackish-water and marine mollusks was recognized in these sediments; the bed accumulated, based on results of radiocarbon datings, 500–1000 years ago. The obtained data do not support the existing notion of a catastrophic fill-up of the Black Sea Basin by marine Mediterranean water [19].

[1] The coupling between the atmosphere and lower ionosphere (due to either the DC electric field or atmospheric gravity waves) may lead to the formation of horizontal irregularities of ionospheric conductivity over an earthquake preparation region. Such interaction of these irregularities with the background electromagnetic noise leads to occurrence of periodic polarization electric currents. These currents are considered to be a coherent source of gyrotropic waves propagating in the ionospheric conducting layer with finite thickness. The theoretical estimation has been performed on the generation of those ionospheric micropulsations in the frequency range of ULF to ELF in the equatorial latitude with horizontal Earth's magnetic field. Then it is found that those waves appear as a narrow-banded line spectrum in the frequency range from 1 to 30 Hz, and these waves are expected to be observed on Earth's surface.