PALEOCLIMATE AND PALEOECOLOGY OF SOUTH-CENTRAL ALASKA’S CHICKALOON AND ARKOSE RIDGE FORMATIONS

Late Paleocene and Early Eocene strata in south-central Alaska’s Matanuska Valley preserve a record of terrestrial climate and ecology during a global greenhouse phase. Here we present a synthesis of a multifaceted study of this region’s Chickaloon and Arkose Ridge formations - low paleoelevation, fluvial depositional systems within a deformed forearc basin succession at sub-polar paleolatitudes.

Both units preserve diverse floras within floodplain facies and are geochronologically constrained to the latest Paleocene (Arkose Ridge) and earliest Eocene (upper Chickaloon). Upper Chickaloon leaf floral collections from near Sutton, AK are dominated by Metasequoia foliage and >30 morphotypes of dicot foliage best preserved in crevasse splay deposits. Carbonate permineralized wood occurs within the unit’s backswamp mire facies. The Arkose Ridge Formation flora with ~20 leaf morphotypes is best preserved in isolated ponded water facies within an overall higher energy fluvial setting in comparison with the more basinal Chickaloon Formation. Leaf taxonomic and physiognomic analysis from both units indicate warm (~10-14C MAT) and wet (~150cm MAP) yearly averaged conditions during Late Paleocene-Early Eocene time in southern Alaska. Growth ring analysis of fossil wood from the Chickaloon Formation indicates high seasonal variability in this sub-polar region. Multiple, stratigraphically-tied leaf collections from both the Arkose Ridge and upper Chickaloon formations exhibit unexpectedly little plant-insect association with low leaf damage frequencies in every censused collection. Amber recovered from Chickaloon backswamp facies is from a Metasequoia source and preserves arachnids, insects, and an array of palynomorphs and bryophyte tissue.

Our findings suggest that the Chickaloon and Arkose Ridge paleoenvironments were warm and wet compared to the Recent. We also posit that these forested ecosystems functioned under non-analog environmental conditions with elevated yearly averaged temperatures, abundant precipitation, high seasonal variability, and a high-latitude light regime. Continued work on these and additional records at similarly high paleolatitudes will yield results that provide a greater understanding of this region’s sensitivity to global change.