Holocene stable oxygen isotope record from the North Caspian Rybachya core

Abstract

The first results of 22 δ¹⁸O_ost measurements on ostracod shells from the Rybachya core in the North Caspian Sea are presented. The stable oxygen isotope record here confirms previous studies of Upper Quaternary and Holocene sediments of the Rybachya core area, where it was suggested that a paleovalley was formed during the Mangyshlak regressive stage and then gradually filled during the Holocene transgressive stage. We recorded three local isotopic stages, each characterized by δ¹⁸O_ost values. The hydrological variability and sea level changes associated with the transgressive-regressive rhythm of the Caspian Basin and the climatic changes in the Northern Hemisphere led to the δ¹⁸O_ost changes obtained in this work. Relatively depleted δ¹⁸O_ost values from sediments filling the paleovalley allowed to interpret the isotope record in terms of freshwater influx. The average δ¹⁸O enrichment of ostracod shells is taken as evidence for the isolation of the paleovalley during the Caspian Sea level retreat. However, the higher δ¹⁸O_ost values obtained for the last (modern) stage of Caspian history are under the strong control of host water salinity, which is higher than during the paleovalley filling episode.