FOSSIL PLANTS IN THE EOCENE GREENHOUSE: DECIPHERING AN ALASKAN PALEOCLIMATE WITH PETRIFIED LEAVES

Excellent exposure and high fossil content of the Late Paleocene-Early Eocene Chickaloon Formation of south-central Alaska make it an ideal place to look for major shifts in the distribution of Paleogene flora and fauna. The fossil plants provide a record of paleoclimate near or during a period of rapid climate change at the Paleocene-Eocene boundary. I examine the distributions of morphotypes across five beds in the uppermost Chickaloon Formation in a section that conformably sits on the beds Triplehorn et al. (1984) identified as spanning the Paleocene-Eocene boundary. I use Leaf Margin Analysis, Provisional Leaf Margin Analysis, CLAMP, and Leaf Area Analysis to estimate climatic conditions of the beds. These techniques predict that the site had a mean annual temperature of about 13.7±1.2º C and received ~155 cm of precipitation without a severe dry season. Although these temperatures are higher than those found at the site today (~1ºC), it is unlikely that they represent the Paleocene-Eocene thermal maximum because studies such as Weijers et al. (2007) have reported terrestrial arctic temperatures of 25º C during the PETM with a baseline temperature of 17º C before and after the event. Put in this context, the temperature estimates of this study seem to be anomalously cool and may be equivalent to a temperature minimum 1.5 My after the Paleocene-Eocene boundary found in Wyoming. The estimated temperatures in this study lend weight to the hypothesis that the Chickaloon Fm. was deposited close to its present latitude because it would be difficult to explain how lower latitudes could have had such low temperatures. The results of this study add to a growing body of knowledge on the paleoenvironment of the Chickaloon Formation and yield valuable information about the earth’s thermal gradient and plant and animal biogeography in a very interesting period in earth’s history.