Late Quaternary Caspian Sea environment: Late Khazarian and Early Khvalynian transgressions from the lower reaches of the Volga River

This magnetic, clay minerals and palynological study provides a record of two Late Pleistocene transgressions (Late Khazarian and Early Khvalynian) in the Caspian Sea, through the analysis of present-day lower Volga terraces. The results allow the clarification of some points about the Caspian Sea evolution during the considered periods. Early diagenetic iron sulphides (greigite and pyrite) are present in sediments of the lower part of the Late Khazarian and reflect restricted water circulation in the basin. The change in iron sulphide mineralogy is most probably related to the change of the water level and its salinity. Low magnetic mineral contents with magnetite, if any, recorded in the upper part of the Late Khazarian, Atelian and Early Khvalynian, reflect a well-oxygenated basin and detrital material sources poor in magnetic minerals. Smectite is the dominant clay mineral in the Late Khazarian, reflecting chemical weathering in its source area due to the wet and warm climate. The upper part of this transgression is nevertheless marked by a stepwise relative decrease of smectite and an increase of illite, attributed to a cooling climate. During the Atelian and Early Khvalynian periods, phases of sandy deposits are characterised by equal proportions of illite and smectite, similar to Ukrainian loess. “Chocolate” clays are clearly dominated by illite. Both clay mineralogies are attributed to cool climate, which favours physical weathering in the catchment area. The conditions of “chocolate” clays deposition are unclear. The recorded cooling during the last phase of the Late Khazarian and the ¹⁴C age on a well-preserved bivalve shell, suggest that it is probably much younger than the Eemian interglacial, and could correspond to the Girkan transgression. The ¹⁴C age for the Early Khvalynian sample suggests approximately a Last Glacial Maximum age for this transgression.