Climate and Diversity of Early and Middle Eocene Floras From Middle Latitudes of the Northern Hemisphere

Abstract

I examined the paleoclimate and floral biodiversity of the early to middle Eocene with an emphasis on the greater northwest of the US. Looking at pollen assemblages from the Clarno Formation (Oregon) I found using a Mantel test that geographic distance does not explain the differences in pollen distribution between three sites: the Nut Beds, White Cliffs Jr. and Red Gap. Each site had a different dominant taxon: monosulcate psilate at Nut Beds, Unidentifiable Betulaceae at White Cliffs Jr. and Juglans at Red Gap. Using a scanning electron microscope, I also found grains from Onagraceae, which was not previously known from the Nut Beds. These differences may be explained by time or community assemblage. Regardless, they show several habitats for this space and time. Using 119 dicot leaves, I inferred the mean annual temperature for the Nut Beds using the Climate Leaf Analysis Program and leaf margin analysis to be 19.8 and 20.00 degrees Celsius, respectively. These temperatures provide evidence that the locality is closer to 48 Ma rather than the commonly cited 44 Ma date. Precipitation estimates were too low to be considered valid. During the process, I tried to identify all leaves and found representatives of Thelypteridaceae, Elaeagnaceae and Rhamnaceae, which had previously not been recognized from the Nut Beds. I looked at the Kishenehn flora (Montana), for which I inferred a mean annual temperature between 8.91-12.10 degrees Celsius using the Bioclimatic Analysis/Mutual Climate Range Technique, (coexistence approach). This is the most diverse Lutetian (47.8-41.2 Ma) lacustrine deposit and contains the oldest Thlaspi and exceptionally well preserved Spirodela. Lastly, I found that beta-diversity was lower during the Lutetian compared to modern times by comparing the mean and range of Sorensen-Dice coefficients from the Lutetian to 1000 randomly generated modern coefficients, which shows there is overall more heterogeneity in the modern world.