Alina Tkach

In the present study, sedimentation conditions and rates in Lake Pleshcheyevo (Yaroslavl region, Central part of the East European Plain) over the past 3600 years were reconstructed based on lithological, geochronological, isotopic, and paleobiological studies of bottom sediments from the deep part of the lake. C/N ratio analysis was used to identify the sources of organic carbon in the lake bottom sediments. Studies of anthropogenic indicators in pollen spectra provided a qualitative comparison of the two age–depth models. Deepwater mesotrophic to eutrophic sedimentation conditions were established during the epoch of cattle farming, 3600–2700 cal yr BP (1600–750 BCE), with relatively high (up to 0.04–0.05 g cm⁻² year⁻¹) mass accumulation rates (MAR) corresponding to period of high fluvial activity on the East European Plain. About 1900 cal yr BP (since the beginning of the 1st century CE), with the spread of slash-and-burn agriculture, lake eutrophication occurred, with an increase in sedimentation rates associated with deforestation and ploughing of territories. From 1300 to 200 cal yr BP (7th to the 19th century CE), sedimentation rates decreased. A sharp (up to 0.07 g cm⁻² year⁻¹) increase in MAR from the end of the 19th century to the middle of the 20th century was established. Similar trends are typical of lakes within the developed areas and are associated with expansion of cultivated agricultural land.

Fluctuations in the level of the Caspian Sea played a decisive role in the development of the settlement complexes’ infrastructure in the Volga River delta during the Holocene. In turn, the transgressive-regressive cycle had a significant impact on the restructuring of the hydrographic network and sedimentation conditions in the Volga River delta. During periods of transgression, part of the delta was flooded, leading to the development of extensive bays in which a distinctive complex of sediments and malacofaunal assemblages formed. Traces of the transgressive stages are clearly visible in the form of a series of ancient shorelines developed on the Baer knolls in the central part of the delta. During regressive phases, the deltaic territory expanded, the hydrographic network was transformed, and extensive alluvial plains were formed. This paper presents the results of comprehensive (archaeological and geological-geomorphological) investigations in the central part of the Volga River delta, in the vicinity of the Semibugorinsky archaeological complex, which includes settlements from the Khazar period. Archaeological and geological-geomorphological data have made it possible to identify areas corresponding to the flooding phase that occurred during the existence of Volga Khazaria in the Volga River delta. The geomorphological setting, the lithological structure of the deposits, and the presence of archaeological artefacts have made it possible to identify layers that were subject to fluvial processes. It was also possible to trace the position of the shoreline and establish the maximum level of the Caspian Sea in the second half of the X century, the absolute height of which reached elevations of -24 to -23.8 m. It is likely that during the period of rising Caspian Sea levels, the population of Volga Khazaria migrated to the northern part of the Volga River delta, which was not affected by flooding.

Mollusk fauna is an important component of the Caspian Sea ecosystem alongside ostracods and diatoms. These faunal proxies are essential indicators of hydrological shifts reflecting global and regional climate changes. Adding lithological, geochemical, and geochronological (radiocarbon) data, we revealed paleogeographic events of different scales recorded in the sequence of the Rybachya core from the North Caspian Sea. Here, we present the reconstruction of Mangyshlak paleovalley sediments during the Holocene multi-stage Neocaspian transgression, reflecting global and regional climate changes varying in scale and direction. The determined age of paleovalley-fill sediments, 8070 ± 110 cal yr BP and 7020 ± 140 cal yr BP, suggests that sedimentation processes with extended warming and humidification started later and lasted longer than was assumed earlier. Biological proxies indicate quasi-cyclic variability and shifts from brackish to freshwater conditions throughout the studied interval. Rybachya core was obtained from the early Khvalynian deposits. The Mangyshlak flow formed the depression and eroded the late Khvalynian deposits, which we did not observe in the core structure. It possibly collapsed into paleodepression and acted as a host material for the freshwater lentic faunal association. During the Holocene, we detected a transition from a tranquil water regime to a more dynamic one during the paleovalley gradual filling, followed by marine conditions typical for the modern Caspian Sea.