Paleoenvironmental changes and vegetation of the Transylvanian Basin in the early stages of Lake Pannon (late Miocene, Tortonian)

Abstract

New palynological data from the deep-water Gușterița section in the Transylvanian Basin depict the late Miocene evolution of Lake Pannon between 11.0 and 10.5 Ma. Spores and pollen were used to analyze vegetation and climate while dinoflagellate cysts were used to reconstruct lake ecology. The section hosts primarily endemic brackish-water dinoflagellate cysts related to those in the Ponto-Caspian realm. The lake was fringed by swamps of taxodioid gymnosperms and riparian forests. The proximity of the Carpathians and Apuseni Mts. allowed the presence of montane vegetation with Picea and Abies. The azonal vegetation narrowed down the extent of the zonal mesophytic forests. Plants were growing under warm-temperate, humid climate with mean annual temperature of 13.3–21.3 °C and mean annual precipitation of 705–1613 mm. The decline of the thermophilous Taxodioideae is linked to the uplift of the mountains and volcanism, but the expansion of Lake Pannon contributed to the swamp-retreat too. The lake expansion was stepwise with the alternation of minor floodings and stabilization of the lake-level. This pattern is inferred from the oscillation of “Virgodinium”-dominated dinoflagellate cyst assemblages characteristic of transgressive periods and assemblages with Spiniferites indicating the slowing-down of the flooding and higher nutrient input from rivers. Active sediment transport into the deep basin via turbidity currents is inferred by the large amount of plant debris in the sedimentary organic matter. The introduction of new dinoflagellate cyst morphotypes with more pronounced ornamentation and thicker wall is possibly a phenotypic response of the dinoflagellates to increasing freshwater influence.