Freshwater mollusks from Neogene-Quaternary Dniester and Prut riverine deposits as indicator paleoenvironments: chemical composition of shells and its palaeoecological interpretation

Abstract

The respective environments in two ancient rivers were studied using geochemical methods with paleogeographic reconstructions of fossil material represented by shells of freshwater bivalve mollusks. The studied outcrops are located in the basins of the Dniester and Prut rivers. Materials were collected from two Pliocene (Brînza, Giurgiuleşti) outcrops and the ages of the second group of localities (Sucleia, Slobozia Mare, Gura Bîcului) are from Middle to Late Pleistocene. The determination of the taxonomic position was carried out using standard malacological methods. Geochemical data were used for the environmental reconstruction and included stable isotope ratio and trace element compositions in subfossil freshwater bivalve shell (Bivalvia: Unionoida). Key indicators of paleoenvironments show changes in water temperature. Changes in the taxonomic composition of bivalve mollusk assemblages also occurred. In the present study, increasing the water temperature caused a change of stenobiont species of bivalve mollusks to eurybiont species. Eutrophication of watercourses, caused by anthropogenic pollution and climate change in the direction of warming, has led to the replacement of pearl mussels of the family Margaritiferidae by more eurybiont species of the family Unionidae in the benthic communities of European oligotrophic rivers. These processes are similar to those deduced for the Neogene-Quaternary watercourses of the Prut and Dniester basins, but they proceed at a much faster pace. They are processes of eutrophication of watercourses – the main factor leading to the catastrophically rapid modern reduction of the ranges of pearl mussel.