Hydrological changes in the central part of the Volga River Delta during the VII-X centuries

The environmental history of the Volga River Delta is inextricably linked with the transgressive-regressive rhythm of the Caspian Sea. Over the past two thousand years, the coastlines of the Caspian Sea have experienced a number of fluctuations ranging from -34 to -20 m bsl. This period of time was characterized by repeated flooding of vast areas of the modern delta of the Volga River, caused by the rise in the level of the Caspian Sea. This circumstance is reflected in the hydrodynamic processes, lithological structure and development of ancient states in a vast area of the Volga River Delta. The study of the structure of four archaeological excavations and twelve boreholes in the Semibugry archaeological site was carried out, in which cultural layers with archaeological finds dating back to the era of the Khazarian Khaganate (VIII-X centuries) were investigated. The results of studying the geological and geomorphological structure in the area of the Semibugry archaeological site suggest that the Khazarian settlement in the studied area was located in a relatively intense hydrodynamic environment. This is evidenced by in-situ finds of shells of freshwater and brackish-water mollusks Didacna baeri, Dreissena rostriformis, Unio sp., a group of freshwater planktonic diatoms Aulacoseira granulata and A. mounds of ancient coastlines. Thus, in the Khazarian time, during the period of sea level rise up to +25 m asl and the settlements were concentrated on the island (Baer knolls). The Delta River Volga was located higher, which may have contributed to the resettlement of the population in its upper parts.