This paper is the first one to investigate the stable oxygen isotope values of modern ostracods sampled across the whole area of the Caspian Sea. Six taxa were analyzed for δ 18 Oost. The correlation between δ 18 Oost and some parameters of the aquatic environment, such as water temperature, salinity and δ 18 Owater, was examined, providing the basis for reliable paleoenvironmental reconstructions in the future using oxygen isotope data. The results of 105 δ 18 Oost measurements taken from 76 sites are consistent with the temporal and spatial indicators of the bottom water masses, as impacted by regional hydrography and physical processes. It has been proven that the δ 18 O of Caspian ostracods is mainly determined by a species-specific isotopic offset with an average value of +1‰. The corresponding inference allows to successfully identify the properties of the water mass, where the carapace has been formed, and also enables to recognize the climatical signal in the δ 18 O records. The vital offset for the mixed ostracod samples was found to be equivalent to the average species-specific isotopic offset calculated on the mean annual and winter temperatures of the Caspian Sea water; it equals to +1 (±0.5) ‰. The regression analysis showed correlations to temperature and salinity parameters that may also facilitate the prediction of water-mass characteristics when being applied to core sediment records. The predicted temperatures correspond to actual measured temperatures at the site of collection, and the oxygen isotope composition of the studied species can be a reliable indicator of water temperatures corresponding to the period of shell calcification. However, in the context of the Caspian Sea environmental changes during the Quaternary period, such attempts of calculating paleotemperatures may lead to false results, since the paleotemperature equation and its variations are only applicable to a water body in its steady state.
