Environmental interpretations of pollen assemblages from the middle Eocene Clarno Formation of Oregon, USA

Fossil fruits, seeds and wood are documented from the Nut Beds of the middle Eocene Clarno Formation (central Oregon, USA) but little is known about the palynoflora. Here, we perform a pollen survey of three sites (Red Gap, White Cliffs Jr. and Nut Beds) using light microscopy. We used scanning electron microscopy to obtain better taxonomic resolution for pollen grains from each site and performed a probability-based Bioclimatic Analysis to obtain paleoclimate estimates. Overall, we identified 1 lycopod, 5 fern, 4 conifer, 1 cycad, 1 Ephedra, 65 angiosperms, 54 pollen or spores of unknown affinity, 2 fungal structures, and 1 additional non-pollen palynomorph taxon. Red Gap had the highest species richness (S = 53) followed by Nut Beds (S = 29) and White Cliffs Jr (S = 27). While unidentifiable pollen grains were a notable percentage of palynomorphs from each site ranging between 9–25.6% of an assemblage, Betulaceae and Juglandaceae were also common. Onagraceae is now recognized from the Nut Beds where no macrofossils had been recorded before. Bioclimatic Analysis revealed a mean annual temperature (MAT) of 12.4–14.1 °C and mean annual precipitation (MAP) of 955–1072 mm. The climatic estimates for the Nut Beds (12.4 °C and 955 mm) are likely underestimates of both temperature and precipitation since an analysis of combined macro- and microfossils yields a MAT of 18.5 °C and MAP of 1276 mm. This discrepancy may be explained by few grains being assigned to modern taxa and/or mesothermal plants tending to be more zoophilous and less likely to get preserved in the microfossil record. The Nut Beds are interpreted as a lacustrine delta due to the large amounts of dispersed cuticle, whereas White Cliffs Jr. and Red Gap are interpreted as stream fed lake deposits. The results highlight differences in biodiversity between each locality and differences in preservational bias among different plant organs.