Palaeofloristic and palaeovegetational changes across the Paleocene/Eocene boundary in northern South America

Abstract

[EN] A floral change occurring in northern South America at the Paleocene/Eocene boundary is analysed using palynological data. The sequence studied is an outcrop from the Venezuelan Maracaibo basin, deposited in shallow marine to coastal environments without apparent stratigraphic breaks. Significant pollen and spore counts from 237 samples were studied stratigraphically and statistically in order to compare Paleocene and Eocene palaeofloras and palaeoecological trends. The Late Paleocene/Early Eocene transition is the boundary between two floras which differ both qualitatively and quantitatively. However, the change is not sudden but stepped and gradual. Paleocene taxa seem to be of pantropical distribution, whereas Eocene assemblages are more restricted to the Neotropics. The global warming well documented elsewhere is proposed as the major cause for these changes. Trends in sporomorph diversity in this record appear to track changes in temperature documented in isotopic records from temperate regions. The extinct parent plant of Echitriporites trianguliformis is tentatively proposed to be intolerant to high temperatures, because of its absence during the Early Eocene warm phase. Palaeoecologically, although marsh and back-mangrove swamps dominated both Late Paleocene and Early Eocene assemblages, their taxonomic compositions were different, especially in the inland marsh forests. Mangrove components are scarce or absent through the whole sequence studied, suggesting the absence of these communities during the time-interval analysed. A palaeoecological subdivision into assemblage zones was not possible; instead, a recurrent pattern suggesting palynological cycles was observed. However, palynocycles could not be studied in detail due to the lack of knowledge of botanical affinities for many of the taxa involved and the apparent absence of mangrove assemblages. Ó 1999 Elsevier Science B.V. All rights reserved.