GROWTH RING ANALYSIS OF METASEQUOIA FROM THE NORTHEAST USA AND THE PALEOCENE/EOCENE CHICKALOON FORMATION, ALASKA

The Cretaceous-Paleogene Matanuska Valley-Talkeetna Mountains basin developed in association with subduction in southern Alaska. The basin is bound to the north by volcanics/volcaniclastics in the southern Talkeetna Mountains and to the south by the uplifted Chugach accretionary sequence. Cretaceous to Paleogene sediments indicate a shift in depositional environments from marine to terrestrial conditions. The Chickaloon Fm. records floodbasin deposition during the late Paleocene and early Eocene. Previous studies of fossil foliage from the Chickaloon Fm. indicate a warm, wet temperate seasonal climate in the basin. These paleoclimatic conditions are comparable to modern climates of the lower-latitude Mid-Atlantic region in the northeast United States.

Fossil Taxodioxylon (Cupressaceae) wood was collected from early Eocene strata in the Chickaloon Fm. at the Evan Jones Mine near Sutton, Alaska. Modern Metasequoia wood samples were collected as cores from this analogous, living taxon within the Mid-Atlantic Delaware River valley. We conducted a series of growth ring analyses on modern and ancient wood to determine whether respective growing environments are analogous and how high-latitude light regimes may affect tree ring morphology. Modern and fossilized wood was measured for earlywood, latewood, and total ring width as well as earlywood-latewood cell number. We noted false rings, determined earlywood-latewood transitions and ratios, and calculated wood mean sensitivity (MS). Modern wood data are compared to known contemporaneous climatic conditions obtained from nearby weather stations. This comparison may provide the ability to infer annual variability in climatic conditions during the deposition of the Chickaloon Fm.

MS has been used as a method to estimate the variability of an individual tree’s growing environment; MS greater than 0.3 indicates “sensitive” tree growth in an annually variable environment while less than 0.3 indicates a “complacent” tree growth history with a low degree of annual variation. A higher average MS calculated for modern Metasequoia samples (~0.48) than that from well-preserved Alaskan fossil samples (~0.33) suggests that the early Eocene Chickaloon growing season was more consistent from year to year than that of sample sites of the temperate Mid-Atlantic.