STABLE OXYGEN ISOTOPE METHOD RESTRICTIONS FOR PALEOTEMPERATURE RECONSTRUCTIONS IN THE CASPIAN SEA

Closed basins or lake systems in general and the Caspian Sea, in particular, are important paleoclimate archives that preserve paleogeographic and hydrologic responses to critical periods in Quaternary history, such as glacial-interglacial cycles. Stable isotope measurements of δ¹⁸O biogenic carbonate are indicators of paleogeographic variability in such systems throughout the geologic record. It is a common misconception that these measurements can be easily applied to create accurate estimates of past temperatures.

The stable isotope composition (SIC) of waters in different reservoirs is not formed under ideal (model) conditions but is influenced by many factors that vary in the oceans and seas, thereby complicating the relationship with the SIC. Moreover, the SIC changes of closed basins obey much more complex trends than the ocean and require a detailed study of all factors that can influence them. Accordingly, the use of any quantitative method requires corrections, coefficients. It is necessary to empirically establish the relationship between the SIC and hydrometeorological parameters in different parts of the water basin, depending on specific conditions. Unfortunately, no special studies for the Caspian Sea have been carried out currently, so the method is not applicable for the Caspian for several reasons described in this study circumstantially (not only water SIC changes throughout geological history but basin water volume changes more than 10% within each time step as well as configuration and sea-level, salinity, pH and [CO₃^2-]/[DIC] ratios, etc.) where the importance and necessity of special corrections are shown.

For illustrative purposes, we measure the δ¹⁸O of Caspian ostracod shells and use the data in a number of variations of the paleotemperature scale equations. The resulting values are abnormally high. The calculated paleotemperatures cannot be considered reliable. Stable oxygen isotope analysis for calculating past temperatures in the Caspian region currently is not applicable without additional special research.

This research was funded by grant from the Russian Science Foundation № 21-44-04401.