Diatom assemblages in the surface layer of bottom sediments from the Volga river delta and their importance for palaeoreconstructions

The study of the Caspian Sea level changes during the Holocene predetermines the interest to palaeogeographic reconstructions of sedimentary environments in the Volga River Delta. Diatom analysis has been for a long time used for the reconstruction of coastline positions of the seas and lakes. In our study diatom assemblages from the surface layer of bottom sediments in the lower delta water bodies with different hydrological characteristics are compared with fossil ones taken from the boreholes in the upper and lower delta. In order to identify diatom indicator species of recent habitats and improve the palaeoreconstructions of earlier sedimentary environments using the actualism method we applied the following statistical methods to samples of recent and fossil diatoms: principal component analysis (PCA) and non-metric multidimensional scaling (NMDS). Statistical analysis of diatoms from recent bottom sediments and sediments from two cores in the Volga River delta let us create a database for reliable palaeoreconstructions of the sedimentation stages in deltaic channels and low flow waters within the study area. In the upper part of both cores, which corresponds to the latest stages of sedimentation, species non-common for recent diatom habitats were identified, namely Hantzschia amphioxys and Craticula cuspidata, which are probably indicators of subaerial conditions. The environments of freshwater lakes (il’mens) having no analogues among the studied modern habitats were also identified in the boreholes’ sediments, which was confirmed by their lithological composition. Species of Epithemia and Eunotia genera are indicators of these conditions. Compared with fossil material, we observed a sharp increase in the proportion of the planktonic species Aulacoseira granulata (up to 90%) in some samples of the surface layer of bottom sediments from the Bystraya River, the Sazaniy Kultuk, and the Lotosny and Dubnoy eriks. It might be a consequence of mass sedimentation of the species valves during the period of surface sampling.