Palaeoclimate versus vegetation reconstruction – palynological investigations on the Oligocene sequence of the Sava Basin, Slovenia

Abstract

An Oligocene succession from the Slovenian Sava Basin (Zasavje) was analysed to reconstruct climate and vegetation of the interval. Independent quantitative approaches were applied on a detailed palynological data set to reconstruct both parameters separately allowing for a synthesis and interpretation. The reconstruction of palaeoclimate is based on the coexistence approach and documents an equal, warm-temperate, humid climate with low annual variation. Quantitative results indicate values of mean annual temperature of 16–19°C, with winter temperatures of 6–9°C and summer temperatures of 25–28°C, and a mean annual precipitation ranging between 1100 and 1300 mm. In contrast to the very homogeneous climatic signal, the vegetational reconstruction based on multivariate statistical analyses shows distinct changes through time. During intervals of peat accumulation, the environment was a typical swamp vegetation dominated by Taxodiaceae and ferns, in which ephemeral small ponds developed due to changing groundwater levels. With the development of an expanded limnic environment, the Taxodiaceae retreated and a riparian swamp flora dominated. This vegetation, in turn, was displaced by elements of drier mesophytic habitats that increased in abundance during a tectonically forced lowering of the groundwater table. Subsequently, the entire study area was flooded by transgression and overlain by marine sediments. All changes in vegetation can be explained by influences of the basin development on the habitats of the plant communities. The results demonstrate a vegetational change in the Oligocene of the Sava Basin that is independent from climate. Vegetation change dynamics were controlled probably by the regional tectonic–palaeogeographic development within a very equable, warm-temperate, and humid climate.