Stable oxygen isotopes in modern ostracods from the Caspian Sea

Abstract

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 most common taxa (shallow-water Euxinocythere virgata and Tyrrhenocythere amnicola donetziensis; deeper-water Candona schweyeri, Paracyprideis sp., Bakunella dorsoarcuata and Camptocypria sp.) were analyzed for δ¹⁸O_ost. We present a specific isotopic offset for each studied taxa relative to the expected equilibrium value of inorganic calcite. An average vital offset of + 1‰ has been determined. The correlation between δ¹⁸O_ost and some parameters of the aquatic environment was examined. The regression analysis showed correlations to temperature, salinity and δ¹⁸O_water, facilitating the prediction of water mass characteristics when being applied to core sediment records and providing the basis for reliable paleoenvironmental reconstructions in the future using oxygen isotope data. The overall results of 105 δ¹⁸O_ost measurements taken from 76 sampling sites are consistent with the temporal and spatial indicators of the bottom water masses, as impacted by regional hydrography and physical processes. The corresponding inference allows to successfully identify average properties of ambient water mass, where a set of carapaces has been formed, and also allows to recognize the climatical signal in the δ¹⁸O_ost records. The predicted temperatures calculated from the measured δ¹⁸O_ost values correspond to actual measured temperatures at the site of collection, and the oxygen isotope composition of the studied common species can be a reliable indicator of water temperatures for the lake-type steady-state of the Caspian Sea 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. Notwithstanding, the potential of the studied ostracod δ¹⁸O_ost records to be used as a paleoenvironmental proxy in future downcore reconstructions in the Caspian Sea has been shown.