Eemian snails from Gorzów Wielkopolski palaeolake (NW Poland): malacological and isotopic data and their ecological implications

Abstract The results of isotope and malacological investigations of mollusc shells from the Eemian Interglacial (MIS 5e) and Vistulian (Weichselian, MIS 5a-d - MIS 4) lake sediments in Gorzów Wielkopolski (NW Poland) are presented. The palaeolake in Gorzów Wielkoposki is the newest paleontological site with the skeleton of the Eemian rhinoceros. The analyzed sediments contained numerous mollusc shells representing 29 species. The dominant species in the malacofauna are Valvata piscinalis and Bithynia tentaculata . Two types of mollusc assemblages have been distinguished, indicating the changes in the lake environment. Carbon and oxygen isotope compositions (δ 13 C and δ 18 O) were measured in the shells of Valvata piscinalis and in the opercula of Bithynia tentaculata . The δ 18 O values of the Eemian snail shells changed from − 6.0 to − 3.8‰, and are higher than the bulk carbonate data. The δ 13 C values oscillated between − 8.6 and − 1.8‰ in the Eemian shells and are lower than the bulk carbonate data. Environmental changes occurring in the immediate vicinity of the rhinoceros skeleton likely influenced the isotope composition of shells, resulting in different δ 13 C and δ 18 O values. The malacological and isotope data aided in distinguishing six phases of lake development. In the first phase (M-1, Early Eemian), a progressive increase in the water level was noted. Then (M-2, Middle Eemian), a significant shallowing of the lake took place. The next phase (M-3, Middle Eemian) was characterized by high water levels and stable hydrological conditions. In the M-4 phase (Late Eemian), a gradual lowering of the water level was observed, leading to the complete disappearance of the lake at the end of the Eemian. During the Vistulian, the lake reemerged. Initially (M-5), it was a permanent lake with a relatively high water level. In the highest part of the profile (M-6, Vistulian), its significant shallowing occurred, and this lake disappeared.