PALEOSALINITY AND PALEOPRODUCTIVITY FLUCTUATIONS RECORDED IN THE CRETACEOUS LACUSTRINE STROMATOLITES, GYEONGSANG BASIN, KOREA

Lacustrine stromatolites formed in the geologic past have provided valuable information on depositional environments and microbial communities as well as physicochemical conditions of paleolakes. Textural and stable isotopic analyses revealed that the Middle Cretaceous lacustrine stromatolites in the Sinyangdong Formation of the Gyeongsang Supergroup, Korea, clearly demonstrate the paleosalinity and paleoproductivity fluctuations recorded during their growth. The stromatolites contain 2-5mm thick, repetitive cycles, which are characterized by lower fibrous calcite layer gradually changing to upper microbial micrite layer. Consistent d18O enrichment from fibrous calcite to micrite towards the top of the cycle indicates that paleosalinity of the lake-water increased due to evaporation as the stromatolite grew. Also, the coeval depletion in d13C suggests that the paleoproductivity decrease was accompanied by the paleosalinity change. Coordinated textural and stable isotopic trends of the lacustrine stromatolites reflected by paleosalinity and paleoproductivity variations in this study could well be applied to other lacustrine stromatolites elsewhere.