Climatic oscillations versus environmental changes in the interpretation of Tertiary plant assemblages

Abstract

AbstractZonal plant communities are the best climatic indicators because they thrive under specific climatic conditions. However, palaeoclimatic estimates based on mere comparisons of local fossil plant assemblages, even if assessed by multivariate methods, may be misleading, if the taphonomical bias is neglected. The fossil record expresses only limited parts of the vegetation of a given period. The example of the Lower Miocene section at Bílina, North Bohemian Basin, shows how quickly the composition of the plant megafossil record changes as a result of different environmental factors and sedimentary settings. These changes can imitate climatic oscillation because swamp, fluvial and upland facies represented within this site have different vegetation-palaeoclimatic aspects. On the other hand, if one compares two assemblages originating from the same sedimentary and environmental setting (preferably mesophytic conditions), the resulting palaeoclimatic changes can be estimated more safely. Two local assemblages from the Oligocene volcanic area of the České středohoří Mts in North Bohemia are of mesophytic character. Both are embedded in diatomites deposited in crater-maar lakes. The assemblage of Suletice (20–29 Ma), with prevailing thermophilous elements, indicates more warm-temperate conditions, whereas the assemblage of Bechlejovice (27 Ma), composed mostly of deciduous broad-leaved trees and shrubs, corresponds to a temperate climate. Thus, together, these two floral assemblages suggest that a climatic oscillation occurred within Late Oligocene time in Central Europe.