High frequency floral changes at the Paleocene-Eocene boundary revealed by comparative biomarker and palynological studies

The Cap d'Ailly area (Upper Normandy, France) shows several terrestrial-lagoonal sections recording the negative carbon isotope excursion (CIE) associated with the Paleocene-Eocene Thermal Maximum (PETM; 55.8 Ma). A study of the biomarkers and spore/pollen content of the Vasterival section gave complementary information on paleofloral changes that occurred around the Paleocene-Eocene (P-E) boundary. Aliphatic hydrocarbon fractions revealed a high abundance of vascular plant biomarkers, including tricyclic diterpanes derived from conifers, fernenes derived from ferns and des-A-triterpenes derived from angiosperms. Whereas the vegetation of the depositional environment surroundings seemed alternately dominated by ferns and angiosperms (revealed from biomarkers), the regional flora seemed dominated strictly by angiosperms such as Juglandaceae (revealed by pollen). Spore/pollen assemblages revealed no major turnover in the paleovegetation, but slight variation in vegetation composition. However, both biomarkers and spores/pollen exhibited two intervals of rapid floral change: the first occurred around the P-E boundary and the second at the top of the strictly terrestrial unit that coincides with a marine transgression. Comparison of the data with published paleoclimatic results, obtained from the same samples, indicated that the first interval of floral change correlates with strong paleohydrological perturbations, whereas the second is linked to an increasing marine influence.