PALEOCENE/EOCENE CLIMATE AND ECOLOGY IN A HIGH-LATITUDE FLOODBASIN: THE CHICKALOON AND ARKOSE RIDGE FORMATIONS, SOUTH-CENTRAL ALASKA

Alaska’s Matanuska Valley–Talkeetna Mountains Basin contains a paleoenvironmental record of Late Paleocene and Early Eocene hothouse climate conditions in the fluvial-lacustrine sediments of the Chickaloon and Arkose Ridge formations. Chickaloon coals containing large Metasequoia trees are preserved in mire deposits while lacustrine, floodplain, and crevasse splay deposits preserve dicot and monocot leaves, shoots of conifers and equisetaleans, angiosperm fruits, and cupressaceous cones. Arkose Ridge sandstones and lacustrine mudrocks preserve a similar diversity of floral remains in more proximal depositional conditions. Recent analysis of stratigraphic successions, fossil plant and insect collections, and sedimentary geochemistry in these units reveal that a diverse forest existed at sub-polar latitudes under temperate paleoclimatic conditions.

Leaf physiognomy-based paleoclimate estimates indicate mean annual temperatures between 11 and 14°C across various lithofacies; much higher than at present (~2°C). Annual paleoprecipitation estimates are high as well (120–160 cm/yr vs. 40–60 cm/yr in the Recent). The occurrence of palmetto fronds (Sabalites) and evidence of high seasonal biomass production support the notion of a yearly-averaged ameliorated climate at the time of Chickaloon and Arkose Ridge deposition.

Ancient interactions between insect herbivores and dicot leaves are unexpectedly rare in the study assemblages. Leaf damage frequency (~7-13%) is low when compared to similar studies of coeval assemblages on North America with marginally higher paleotemperatures. We propose that the non-analogous situation of warm climate conditions under a high-latitude light regime rather than climatic or leaf economic traits may explain these results.