FILLING A PALEOLATITUDINAL GAP IN THE RECORD OF PALEOCENE/EOCENE FORESTED ECOSYSTEMS: THE CHICKALOON AND ARKOSE RIDGE FORMATIONS, ALASKA

Extensive new field collections of fossil plants from south-central Alaska’s terrestrial Paleocene/Eocene strata are providing an increasingly clear picture of northern North American Cordilleran sub-polar forests during hothouse climate conditions. Rich assemblages of leaf, wood, amber, and reproductive organ fossils have been stratigraphically tied into age-constrained facies analyses in the Paleocene/Eocene Chickaloon and Arkose Ridge formations within Matanuska Valley-Talkeetna Mountains Basin.

Geological evidence from exposures of the Chickaloon and Arkose Ridge and associated volcanic rocks points toward minimal paleolatitudinal displacement of this Paleogene terrestrial record. A lateral, down-stream facies shift toward coastal/marginal marine deposition indicates that the study assemblages were preserved at low elevation, at least within the Chickaloon depositional system. These two depositional constraints suggest that these datasets have the potential to provide a suite of paleoecological information regarding the non-analog conditions of yearly-averaged warmth in a high-latitude light regime. They also may be used to test existing and new paleobotanical approaches for estimating paleoclimate and paleoecology in sub-polar latitudes and fill an important paleolatitudinal gap in the plant fossil record.

Existing leaf physiognomy-based paleoclimate estimation methods, as applied to the Chickaloon and Arkose Ridge paleofloras, indicate warm and wet temperate biome conditions across various lithofacies associations. These results are in stark contrast to the modern climate in the region. Evidence of high seasonal biomass production in cupressaceous wood and leaf mats together with the presence of palmetto fronds (Sabalites) support the notion of a yearly-averaged ameliorated climate at the time of Chickaloon and Arkose Ridge deposition. This multifaceted record is ripe for the application of new approaches for reconstructing paleoclimate and paleoecology using fossil plants and these paleofloras provide a way to test hypotheses concerning the latitudinal distribution of fossil plant assemblages during warm global climate phases.