HIGH-RESOLUTION STRATIGRAPHY IN HOLOCENE SEDIMENTS FROM THE VOLGA DELTA BASED ON DIATOM ANALYSIS

This paper presents stratigraphy and correlations of Holocene Caspian events recorded in the sediments from the lower part of the Volga Delta. The lower part of the delta sensitively reacts on the Caspian Sea level changes. It is a key area for high-resolution study of the Holocene fluctuations of the sea.

The reconstructions based on diatom analysis according to the radiocarbon dates cover the last 4300 years. We studied samples of the two outcrops and the core from the lower delta (Damchick area).

All studied samples contained predominantly freshwater diatoms from the genera Aulacoseira, Amphora, Cocconeis and Stephanodiscus. Facies of the deltaic channels usually contain planctonic species Aulacoseira granulata, A. italica, Stephanodiscus astraea var. intermedia. Facies with fine-grained material are rich in benthic diatoms Eunotia monodon, Epithemia adnata, Cocconeis placentula, C. pediculus and correspond to shallow-water and stagnant environment.

In the core we distinguish eight diatom zones that probably correspond to three stages of the New-Caspian transgression: Ulluchai stage (when the territory of the delta was a set of freshwater bays and reservoirs), Derbent regression and the latest/modern stage.

In the framework of diatom analysis we also studied a curve of the diatom valves concentration. We assume this parameter to reflect the moisture content for the studied region. We compared this curve with the published data on moisture and temperature curves inferred from detailed pollen analysis in the Damchick area. We note a high coincidence of the curves of the diatom concentration and moisture for the study area.

Two outcrops expose only the upper 90 cm of the Holocene sediments but we collected the samples with a higher frequency. It helped us to reconstruct several fluctuations of the Caspian Sea probably during the last 1100 years. For correlations we used diatom valves concentration parameter. Low diatom concentration corresponds to the dry periods and regressions of the sea; high diatom concentration corresponds to humid periond and the higher levels of the sea. In the outcrops we distinguish one regressional stage (Derbent regression) and tree transgressive-regressive fluctuations within a modern transgression of the Caspian Sea.

This work was supported by the Russian Science Foundation (RSF), project № 20-77-00068.