VARIABILITY IN VEGETATION DENSITY ACROSS LATERALLY COEVAL STRATIGRAPHIC SECTIONS WITHIN THE HANNA BASIN, WY

Vegetation structure influences many important variables of ecosystems such as hydrological and carbon cycles, erosion, animal migration patterns, and plant composition. However, ancient vegetation structure has been exceptionally difficult to quantify. Recent development of a new proxy based on the morphology of leaf epidermal cells, imprinted upon fossilized plant cuticles, offers a unique opportunity to reconstruct past landscapes. Studies of modern systems indicate that the degree of sunlight exposure during leaf development influences epidermal cell shape. Leaves that develop in full sunlight tend to have rounder cells with less undulatory cell walls; whereas leaves that developed in a shade dominated environment tend to have more elliptical cells with undulatory cell walls. Measurements of many epidermal cells from dispersed cuticles preserved in the sediments, then, can be used to reconstruct vegetation structure, quantified as leaf area index.

This study assesses spatial variability in LAI across paleo-landscapes and tests for correlations between reconstructed LAI and specific depositional environments. Samples were collected from early Paleogene strata in the Hanna Basin of Wyoming, a Laramide structural basin in south-central Wyoming, U.S.A., that contains a thick succession of fluvial, deltaic, and lacustrine strata. We measured four stratigraphic sections that are roughly coeval with one another and display a change from brittle anthracite at the base, laminated carbaceous shales toward the middle of the section, and massive claystone at the top. We reconstructed vegetation structure across this 0.35 km transect. We find that LAI values increase up in the section as the depositional environment changes from a more lacustrine to fluvial system. At the base of these sections, the value of LAI is 5.79 (+/- 2.37), indicating a dense forest, while at the top of the section we observe a higher value of 7.66 (+/- 2.95), representing an extremely closed canopy. This signifies an extremely lush forest once covered the Hanna Basin during the Paleocene and early Eocene epochs. Compared to LAI values reconstructed from the late Paleocene and the PETM interval preserved down section, this study demonstrates that post-PETM, early Eocene floras were the densest of the early Paleogene investigated thus far.