ALASKAN CLIMATE OF THE LATE PALEOCENE-EARLY EOCENE AS TOLD BY THE FOSSIL LEAVES OF THE CHICKALOON FORMATION

The Paleocene-Eocene Thermal Maximum (PETM) is one of the most intensively studied intervals of warming in the geologic record. Evidence from ocean sediments suggests rapid warming, and a negative carbon-isotope anomaly implicates a large release of methane, possibly from marine clathrate deposits. This catastrophic release of methane is believed to have triggered feedback mechanisms such as a shift in the heat transport of the oceans that could have increased global sea-surface temperatures as much as 4-5º C for a period of 210,000 years. This study examines the fossil flora of the Late Paleocene-Early Eocene Chickaloon Formation of south-central Alaska. Higher-latitude regions are thought to be more sensitive to climate change than lower latitudes and the excellent exposure and high fossil content at the potentially high paleolatitude study site make it an ideal place to look for major shifts in the distribution of the Early Tertiary flora and fauna. We examined four beds in the uppermost Chickaloon Formation, in a section that Triplehorn (1984) identified as spanning the Paleocene-Eocene boundary. In the three beds thus far examined, 35 morphotypes have been described. Between one half and one third of the leaf morphotypes found in each bed are unique to that bed. Morphotype overlap is greater between sets of adjacent beds than between the upper and lower beds. All morphotypes that are found in both the upper and lower beds are also found in the middle bed. This suggests that there was a shift in the floral assemblage over the period represented. This shift may have been due to either lateral migration of the stream system or a change in climate. Leaf margin analysis, however, shows that paleotemperature was relatively stable between beds and significantly higher than current Alaskan conditions.