Isotope palaeothermometry problems for the Caspian Sea

The oxygen isotope analysis is an important tool for the correlation of palaeogeographic events both within the Caspian region and while considering the Caspian history against the background of global climate changes. At the current level of knowledge, the oxygen isotope application as a palaeotemperature method for the Caspian region cannot be substantiated theoretically; such reconstructions for the Caspian Sea require a special approach. The necessary special studies have not been carried out yet in the region. A practical attempt to calculate palaeotemperatures for the Late Pleistocene – Holocene was carried out on the basis of oxygen isotope data from two boreholes in the northern part of the Caspian Sea. Ostracod shells are one of the most reliable indicators of palaeoecological parameters, so they were chosen as a material for isotope measurements. An attempt to solve several variations of the palaeotemperature equations with a certain degree of convention was made, by applying zero correction for the water background, calculating water background through salinity values and considering taxonomic and metabolic features of ostracod shells. The resulting water temperatures in the Late Pleistocene – Holocene were from 12.8 to 99.7°C in the first case, from 5.4 to 49.9°C in the second, and about 35.4–62.3°C during the Holocene in the third case. The calculated palaeotemperatures values are definitely overestimated and cannot be considered reliable. We conclude that the considered methods are not applicable for calculating palaeohydrological parameters in the Caspian region. The importance and necessity of the selection and introduction of special corrections is shown, since the changes in the isotopic composition of a closed reservoir undergo much more complex trends than in the ocean, and require a detailed study of all factors that can influence them.

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